[PATCH v3 00/12] Fix LKDTM for PPC64/IA64/PARISC

[PATCH v3 00/12] Fix LKDTM for PPC64/IA64/PARISC

[Christophe Leroy]

PPC64/IA64/PARISC have function descriptors. LKDTM doesn't work
on those three architectures because LKDTM messes up function
descriptors with functions.

This series does some cleanup in the three architectures and
refactors function descriptors so that it can then easily use it
in a generic way in LKDTM.

Patch 8 is not absolutely necessary but it is a good trivial cleanup.

Changes in v3:
- Addressed received comments
- Swapped some of the powerpc patches to keep func_descr_t renamed as struct func_desc and remove 'struct ppc64_opd_entry'
- Changed HAVE_FUNCTION_DESCRIPTORS macro to a config item CONFIG_HAVE_FUNCTION_DESCRIPTORS
- Dropped patch 11 ("Fix lkdtm_EXEC_RODATA()")

Changes in v2:
- Addressed received comments
- Moved dereference_[kernel]_function_descriptor() out of line
- Added patches to remove func_descr_t and func_desc_t in powerpc
- Using func_desc_t instead of funct_descr_t
- Renamed HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR to HAVE_FUNCTION_DESCRIPTORS
- Added a new lkdtm test to check protection of function descriptors

Christophe Leroy (12):
  powerpc: Move and rename func_descr_t
  powerpc: Use 'struct func_desc' instead of 'struct ppc64_opd_entry'
  powerpc: Remove 'struct ppc64_opd_entry'
  powerpc: Prepare func_desc_t for refactorisation
  ia64: Rename 'ip' to 'addr' in 'struct fdesc'
  asm-generic: Define CONFIG_HAVE_FUNCTION_DESCRIPTORS
  asm-generic: Define 'func_desc_t' to commonly describe function
    descriptors
  asm-generic: Refactor dereference_[kernel]_function_descriptor()
  lkdtm: Force do_nothing() out of line
  lkdtm: Really write into kernel text in WRITE_KERN
  lkdtm: Fix execute_[user]_location()
  lkdtm: Add a test for function descriptors protection

 arch/Kconfig                             |  3 +
 arch/ia64/Kconfig                        |  1 +
 arch/ia64/include/asm/elf.h              |  2 +-
 arch/ia64/include/asm/sections.h         | 24 +-------
 arch/ia64/kernel/module.c                |  6 +-
 arch/parisc/Kconfig                      |  1 +
 arch/parisc/include/asm/sections.h       | 16 ++----
 arch/parisc/kernel/process.c             | 21 -------
 arch/powerpc/Kconfig                     |  1 +
 arch/powerpc/include/asm/code-patching.h |  2 +-
 arch/powerpc/include/asm/elf.h           |  6 ++
 arch/powerpc/include/asm/sections.h      | 29 ++--------
 arch/powerpc/include/asm/types.h         |  6 --
 arch/powerpc/include/uapi/asm/elf.h      |  8 ---
 arch/powerpc/kernel/module_64.c          | 38 +++++--------
 arch/powerpc/kernel/ptrace/ptrace.c      |  6 ++
 arch/powerpc/kernel/signal_64.c          |  8 +--
 drivers/misc/lkdtm/core.c                |  1 +
 drivers/misc/lkdtm/lkdtm.h               |  1 +
 drivers/misc/lkdtm/perms.c               | 71 +++++++++++++++++++-----
 include/asm-generic/sections.h           | 13 ++++-
 include/linux/kallsyms.h                 |  2 +-
 kernel/extable.c                         | 23 +++++++-
 23 files changed, 146 insertions(+), 143 deletions(-)

-- 
2.31.1

[PATCH v3 01/12] powerpc: Move and rename func_descr_t

[Christophe Leroy]

There are three architectures with function descriptors, try to
have common names for the address they contain in order to
refactor some functions into generic functions later.

powerpc has 'entry'
ia64 has 'ip'
parisc has 'addr'

Vote for 'addr' and update 'func_descr_t' accordingly.

Move it in asm/elf.h to have it at the same place on all
three architectures, remove the typedef which hides its real
type, and change it to a smoother name 'struct func_desc'.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/powerpc/include/asm/code-patching.h | 2 +-
 arch/powerpc/include/asm/elf.h           | 6 ++++++
 arch/powerpc/include/asm/types.h         | 6 ------
 arch/powerpc/kernel/signal_64.c          | 8 ++++----
 4 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h
index 4ba834599c4d..c6e805976e6f 100644
--- a/arch/powerpc/include/asm/code-patching.h
+++ b/arch/powerpc/include/asm/code-patching.h
@@ -110,7 +110,7 @@ static inline unsigned long ppc_function_entry(void *func)
 	 * function's descriptor. The first entry in the descriptor is the
 	 * address of the function text.
 	 */
-	return ((func_descr_t *)func)->entry;
+	return ((struct func_desc *)func)->addr;
 #else
 	return (unsigned long)func;
 #endif
diff --git a/arch/powerpc/include/asm/elf.h b/arch/powerpc/include/asm/elf.h
index b8425e3cfd81..971589a21bc0 100644
--- a/arch/powerpc/include/asm/elf.h
+++ b/arch/powerpc/include/asm/elf.h
@@ -176,4 +176,10 @@ do {									\
 /* Relocate the kernel image to @final_address */
 void relocate(unsigned long final_address);
 
+struct func_desc {
+	unsigned long addr;
+	unsigned long toc;
+	unsigned long env;
+};
+
 #endif /* _ASM_POWERPC_ELF_H */
diff --git a/arch/powerpc/include/asm/types.h b/arch/powerpc/include/asm/types.h
index f1630c553efe..97da77bc48c9 100644
--- a/arch/powerpc/include/asm/types.h
+++ b/arch/powerpc/include/asm/types.h
@@ -23,12 +23,6 @@
 
 typedef __vector128 vector128;
 
-typedef struct {
-	unsigned long entry;
-	unsigned long toc;
-	unsigned long env;
-} func_descr_t;
-
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_POWERPC_TYPES_H */
diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
index 1831bba0582e..36537d7d5191 100644
--- a/arch/powerpc/kernel/signal_64.c
+++ b/arch/powerpc/kernel/signal_64.c
@@ -933,11 +933,11 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
 		 * descriptor is the entry address of signal and the second
 		 * entry is the TOC value we need to use.
 		 */
-		func_descr_t __user *funct_desc_ptr =
-			(func_descr_t __user *) ksig->ka.sa.sa_handler;
+		struct func_desc __user *ptr =
+			(struct func_desc __user *)ksig->ka.sa.sa_handler;
 
-		err |= get_user(regs->ctr, &funct_desc_ptr->entry);
-		err |= get_user(regs->gpr[2], &funct_desc_ptr->toc);
+		err |= get_user(regs->ctr, &ptr->addr);
+		err |= get_user(regs->gpr[2], &ptr->toc);
 	}
 
 	/* enter the signal handler in native-endian mode */
-- 
2.31.1

Re: [PATCH v3 01/12] powerpc: Move and rename func_descr_t

[Nicholas Piggin]

Excerpts from Christophe Leroy's message of October 17, 2021 10:38 pm:
> There are three architectures with function descriptors, try to
> have common names for the address they contain in order to
> refactor some functions into generic functions later.
> 
> powerpc has 'entry'
> ia64 has 'ip'
> parisc has 'addr'
> 
> Vote for 'addr' and update 'func_descr_t' accordingly.
> 
> Move it in asm/elf.h to have it at the same place on all
> three architectures, remove the typedef which hides its real
> type, and change it to a smoother name 'struct func_desc'.
> 

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>

> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
>  arch/powerpc/include/asm/code-patching.h | 2 +-
>  arch/powerpc/include/asm/elf.h           | 6 ++++++
>  arch/powerpc/include/asm/types.h         | 6 ------
>  arch/powerpc/kernel/signal_64.c          | 8 ++++----
>  4 files changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h
> index 4ba834599c4d..c6e805976e6f 100644
> --- a/arch/powerpc/include/asm/code-patching.h
> +++ b/arch/powerpc/include/asm/code-patching.h
> @@ -110,7 +110,7 @@ static inline unsigned long ppc_function_entry(void *func)
>  	 * function's descriptor. The first entry in the descriptor is the
>  	 * address of the function text.
>  	 */
> -	return ((func_descr_t *)func)->entry;
> +	return ((struct func_desc *)func)->addr;
>  #else
>  	return (unsigned long)func;
>  #endif
> diff --git a/arch/powerpc/include/asm/elf.h b/arch/powerpc/include/asm/elf.h
> index b8425e3cfd81..971589a21bc0 100644
> --- a/arch/powerpc/include/asm/elf.h
> +++ b/arch/powerpc/include/asm/elf.h
> @@ -176,4 +176,10 @@ do {									\
>  /* Relocate the kernel image to @final_address */
>  void relocate(unsigned long final_address);
>  
> +struct func_desc {
> +	unsigned long addr;
> +	unsigned long toc;
> +	unsigned long env;
> +};
> +
>  #endif /* _ASM_POWERPC_ELF_H */
> diff --git a/arch/powerpc/include/asm/types.h b/arch/powerpc/include/asm/types.h
> index f1630c553efe..97da77bc48c9 100644
> --- a/arch/powerpc/include/asm/types.h
> +++ b/arch/powerpc/include/asm/types.h
> @@ -23,12 +23,6 @@
>  
>  typedef __vector128 vector128;
>  
> -typedef struct {
> -	unsigned long entry;
> -	unsigned long toc;
> -	unsigned long env;
> -} func_descr_t;
> -
>  #endif /* __ASSEMBLY__ */
>  
>  #endif /* _ASM_POWERPC_TYPES_H */
> diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
> index 1831bba0582e..36537d7d5191 100644
> --- a/arch/powerpc/kernel/signal_64.c
> +++ b/arch/powerpc/kernel/signal_64.c
> @@ -933,11 +933,11 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
>  		 * descriptor is the entry address of signal and the second
>  		 * entry is the TOC value we need to use.
>  		 */
> -		func_descr_t __user *funct_desc_ptr =
> -			(func_descr_t __user *) ksig->ka.sa.sa_handler;
> +		struct func_desc __user *ptr =
> +			(struct func_desc __user *)ksig->ka.sa.sa_handler;
>  
> -		err |= get_user(regs->ctr, &funct_desc_ptr->entry);
> -		err |= get_user(regs->gpr[2], &funct_desc_ptr->toc);
> +		err |= get_user(regs->ctr, &ptr->addr);
> +		err |= get_user(regs->gpr[2], &ptr->toc);
>  	}
>  
>  	/* enter the signal handler in native-endian mode */
> -- 
> 2.31.1
> 
> 
> 

Re: [PATCH v3 01/12] powerpc: Move and rename func_descr_t

[Kees Cook]

On Sun, Oct 17, 2021 at 02:38:14PM +0200, Christophe Leroy wrote:
> There are three architectures with function descriptors, try to
> have common names for the address they contain in order to
> refactor some functions into generic functions later.
> 
> powerpc has 'entry'
> ia64 has 'ip'
> parisc has 'addr'
> 
> Vote for 'addr' and update 'func_descr_t' accordingly.
> 
> Move it in asm/elf.h to have it at the same place on all
> three architectures, remove the typedef which hides its real
> type, and change it to a smoother name 'struct func_desc'.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

I like the name. :)

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook

[PATCH v3 02/12] powerpc: Use 'struct func_desc' instead of 'struct ppc64_opd_entry'

[Christophe Leroy]

'struct ppc64_opd_entry' is somehow redundant with 'struct func_desc',
the later is more correct/complete as it includes the third
field which is unused.

So use 'struct func_desc' instead of 'struct ppc64_opd_entry'

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/powerpc/include/asm/sections.h |  4 ++--
 arch/powerpc/kernel/module_64.c     | 10 +++++-----
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/include/asm/sections.h b/arch/powerpc/include/asm/sections.h
index 6e4af4492a14..abd2e5213197 100644
--- a/arch/powerpc/include/asm/sections.h
+++ b/arch/powerpc/include/asm/sections.h
@@ -74,10 +74,10 @@ static inline int overlaps_kernel_text(unsigned long start, unsigned long end)
 #undef dereference_function_descriptor
 static inline void *dereference_function_descriptor(void *ptr)
 {
-	struct ppc64_opd_entry *desc = ptr;
+	struct func_desc *desc = ptr;
 	void *p;
 
-	if (!get_kernel_nofault(p, (void *)&desc->funcaddr))
+	if (!get_kernel_nofault(p, (void *)&desc->addr))
 		ptr = p;
 	return ptr;
 }
diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 6baa676e7cb6..a89da0ee25e2 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -64,19 +64,19 @@ static unsigned int local_entry_offset(const Elf64_Sym *sym)
 #else
 
 /* An address is address of the OPD entry, which contains address of fn. */
-typedef struct ppc64_opd_entry func_desc_t;
+typedef struct func_desc func_desc_t;
 
 static func_desc_t func_desc(unsigned long addr)
 {
-	return *(struct ppc64_opd_entry *)addr;
+	return *(struct func_desc *)addr;
 }
 static unsigned long func_addr(unsigned long addr)
 {
-	return func_desc(addr).funcaddr;
+	return func_desc(addr).addr;
 }
 static unsigned long stub_func_addr(func_desc_t func)
 {
-	return func.funcaddr;
+	return func.addr;
 }
 static unsigned int local_entry_offset(const Elf64_Sym *sym)
 {
@@ -187,7 +187,7 @@ static int relacmp(const void *_x, const void *_y)
 static unsigned long get_stubs_size(const Elf64_Ehdr *hdr,
 				    const Elf64_Shdr *sechdrs)
 {
-	/* One extra reloc so it's always 0-funcaddr terminated */
+	/* One extra reloc so it's always 0-addr terminated */
 	unsigned long relocs = 1;
 	unsigned i;
 
-- 
2.31.1

[PATCH v3 03/12] powerpc: Remove 'struct ppc64_opd_entry'

[Christophe Leroy]

'struct ppc64_opd_entry' doesn't belong to uapi/asm/elf.h

It was initially in module_64.c and commit 2d291e902791 ("Fix compile
failure with non modular builds") moved it into asm/elf.h

But it was by mistake added outside of __KERNEL__ section,
therefore commit c3617f72036c ("UAPI: (Scripted) Disintegrate
arch/powerpc/include/asm") moved it to uapi/asm/elf.h

Now that it is not used anymore by the kernel, remove it.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/powerpc/include/uapi/asm/elf.h | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/arch/powerpc/include/uapi/asm/elf.h b/arch/powerpc/include/uapi/asm/elf.h
index 860c59291bfc..308857123a08 100644
--- a/arch/powerpc/include/uapi/asm/elf.h
+++ b/arch/powerpc/include/uapi/asm/elf.h
@@ -289,12 +289,4 @@ typedef elf_fpreg_t elf_vsrreghalf_t32[ELF_NVSRHALFREG];
 /* Keep this the last entry.  */
 #define R_PPC64_NUM		253
 
-/* There's actually a third entry here, but it's unused */
-struct ppc64_opd_entry
-{
-	unsigned long funcaddr;
-	unsigned long r2;
-};
-
-
 #endif /* _UAPI_ASM_POWERPC_ELF_H */
-- 
2.31.1

[PATCH v3 04/12] powerpc: Prepare func_desc_t for refactorisation

[Christophe Leroy]

In preparation of making func_desc_t generic, change the ELFv2
version to a struct containing 'addr' element.

This allows using single helpers common to ELFv1 and ELFv2.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/powerpc/kernel/module_64.c | 32 ++++++++++++++------------------
 1 file changed, 14 insertions(+), 18 deletions(-)

diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index a89da0ee25e2..b687ef88c4c4 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -33,19 +33,13 @@
 #ifdef PPC64_ELF_ABI_v2
 
 /* An address is simply the address of the function. */
-typedef unsigned long func_desc_t;
+typedef struct {
+	unsigned long addr;
+} func_desc_t;
 
 static func_desc_t func_desc(unsigned long addr)
 {
-	return addr;
-}
-static unsigned long func_addr(unsigned long addr)
-{
-	return addr;
-}
-static unsigned long stub_func_addr(func_desc_t func)
-{
-	return func;
+	return (func_desc_t){addr};
 }
 
 /* PowerPC64 specific values for the Elf64_Sym st_other field.  */
@@ -70,14 +64,6 @@ static func_desc_t func_desc(unsigned long addr)
 {
 	return *(struct func_desc *)addr;
 }
-static unsigned long func_addr(unsigned long addr)
-{
-	return func_desc(addr).addr;
-}
-static unsigned long stub_func_addr(func_desc_t func)
-{
-	return func.addr;
-}
 static unsigned int local_entry_offset(const Elf64_Sym *sym)
 {
 	return 0;
@@ -93,6 +79,16 @@ void *dereference_module_function_descriptor(struct module *mod, void *ptr)
 }
 #endif
 
+static unsigned long func_addr(unsigned long addr)
+{
+	return func_desc(addr).addr;
+}
+
+static unsigned long stub_func_addr(func_desc_t func)
+{
+	return func.addr;
+}
+
 #define STUB_MAGIC 0x73747562 /* stub */
 
 /* Like PPC32, we need little trampolines to do > 24-bit jumps (into
-- 
2.31.1

Re: [PATCH v3 04/12] powerpc: Prepare func_desc_t for refactorisation

[Nicholas Piggin]

Excerpts from Christophe Leroy's message of October 17, 2021 10:38 pm:
> In preparation of making func_desc_t generic, change the ELFv2
> version to a struct containing 'addr' element.
> 
> This allows using single helpers common to ELFv1 and ELFv2.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

> ---
>  arch/powerpc/kernel/module_64.c | 32 ++++++++++++++------------------
>  1 file changed, 14 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index a89da0ee25e2..b687ef88c4c4 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -33,19 +33,13 @@
>  #ifdef PPC64_ELF_ABI_v2
>  
>  /* An address is simply the address of the function. */
> -typedef unsigned long func_desc_t;
> +typedef struct {
> +	unsigned long addr;
> +} func_desc_t;

I'm not quite following why this change is done. I guess it is so you 
can move this func_desc_t type into core code, but why do that? Is it
just to avoid using the preprocessor?

On its own this patch looks okay.

Acked-by: Nicholas Piggin <npiggin@gmail.com>

Re: [PATCH v3 04/12] powerpc: Prepare func_desc_t for refactorisation

[Christophe Leroy]

Le 18/10/2021 à 08:27, Nicholas Piggin a écrit :
> Excerpts from Christophe Leroy's message of October 17, 2021 10:38 pm:
>> In preparation of making func_desc_t generic, change the ELFv2
>> version to a struct containing 'addr' element.
>>
>> This allows using single helpers common to ELFv1 and ELFv2.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> 
>> ---
>>   arch/powerpc/kernel/module_64.c | 32 ++++++++++++++------------------
>>   1 file changed, 14 insertions(+), 18 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
>> index a89da0ee25e2..b687ef88c4c4 100644
>> --- a/arch/powerpc/kernel/module_64.c
>> +++ b/arch/powerpc/kernel/module_64.c
>> @@ -33,19 +33,13 @@
>>   #ifdef PPC64_ELF_ABI_v2
>>   
>>   /* An address is simply the address of the function. */
>> -typedef unsigned long func_desc_t;
>> +typedef struct {
>> +	unsigned long addr;
>> +} func_desc_t;
> 
> I'm not quite following why this change is done. I guess it is so you
> can move this func_desc_t type into core code, but why do that? Is it
> just to avoid using the preprocessor?

I explained it in patch 7 but yes it probably also deserves some more 
explanation here as well.

That's right, it's to avoid having to spread #ifdefs everywhere.

> 
> On its own this patch looks okay.
> 
> Acked-by: Nicholas Piggin <npiggin@gmail.com>
> 

Re: [PATCH v3 04/12] powerpc: Prepare func_desc_t for refactorisation

[Kees Cook]

On Sun, Oct 17, 2021 at 02:38:17PM +0200, Christophe Leroy wrote:
> In preparation of making func_desc_t generic, change the ELFv2
> version to a struct containing 'addr' element.
> 
> This allows using single helpers common to ELFv1 and ELFv2.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
>  arch/powerpc/kernel/module_64.c | 32 ++++++++++++++------------------
>  1 file changed, 14 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
> index a89da0ee25e2..b687ef88c4c4 100644
> --- a/arch/powerpc/kernel/module_64.c
> +++ b/arch/powerpc/kernel/module_64.c
> @@ -33,19 +33,13 @@
>  #ifdef PPC64_ELF_ABI_v2
>  
>  /* An address is simply the address of the function. */
> -typedef unsigned long func_desc_t;
> +typedef struct {
> +	unsigned long addr;
> +} func_desc_t;
>  
>  static func_desc_t func_desc(unsigned long addr)
>  {
> -	return addr;
> -}
> -static unsigned long func_addr(unsigned long addr)
> -{
> -	return addr;
> -}
> -static unsigned long stub_func_addr(func_desc_t func)
> -{
> -	return func;
> +	return (func_desc_t){addr};

There's only 1 element in the struct, so okay, but it hurt my eyes a
little. I would have been happier with:

	return (func_desc_t){ .addr = addr; };

But of course that also looks bonkers because it starts with "return".
So no matter what I do my eyes bug out. ;)

So it's fine either way. :)

Reviewed-by: Kees Cook <keescook@chromium.org>


>  }
>  
>  /* PowerPC64 specific values for the Elf64_Sym st_other field.  */
> @@ -70,14 +64,6 @@ static func_desc_t func_desc(unsigned long addr)
>  {
>  	return *(struct func_desc *)addr;
>  }
> -static unsigned long func_addr(unsigned long addr)
> -{
> -	return func_desc(addr).addr;
> -}
> -static unsigned long stub_func_addr(func_desc_t func)
> -{
> -	return func.addr;
> -}
>  static unsigned int local_entry_offset(const Elf64_Sym *sym)
>  {
>  	return 0;
> @@ -93,6 +79,16 @@ void *dereference_module_function_descriptor(struct module *mod, void *ptr)
>  }
>  #endif
>  
> +static unsigned long func_addr(unsigned long addr)
> +{
> +	return func_desc(addr).addr;
> +}
> +
> +static unsigned long stub_func_addr(func_desc_t func)
> +{
> +	return func.addr;
> +}
> +
>  #define STUB_MAGIC 0x73747562 /* stub */
>  
>  /* Like PPC32, we need little trampolines to do > 24-bit jumps (into
> -- 
> 2.31.1
> 

-- 
Kees Cook

Re: [PATCH v3 04/12] powerpc: Prepare func_desc_t for refactorisation

[Segher Boessenkool]

On Thu, Feb 10, 2022 at 04:54:52PM -0800, Kees Cook wrote:
> On Sun, Oct 17, 2021 at 02:38:17PM +0200, Christophe Leroy wrote:
(edited:)
> > +typedef struct {
> > +	unsigned long addr;
> > +} func_desc_t;
> >  
> >  static func_desc_t func_desc(unsigned long addr)
> >  {
> > +	return (func_desc_t){addr};

> There's only 1 element in the struct, so okay, but it hurt my eyes a
> little. I would have been happier with:
> 
> 	return (func_desc_t){ .addr = addr; };
> 
> But of course that also looks bonkers because it starts with "return".
> So no matter what I do my eyes bug out. ;)

The usual way to avoid convoluted constructs is to name more factors.
So:

static func_desc_t func_desc(unsigned long addr)
{
	func_desc_t desc = {};
	desc.addr = addr;
	return desc;
}


Segher

Re: [PATCH v3 04/12] powerpc: Prepare func_desc_t for refactorisation

[Christophe Leroy]

Le 11/02/2022 à 01:54, Kees Cook a écrit :
> On Sun, Oct 17, 2021 at 02:38:17PM +0200, Christophe Leroy wrote:
>> In preparation of making func_desc_t generic, change the ELFv2
>> version to a struct containing 'addr' element.
>>
>> This allows using single helpers common to ELFv1 and ELFv2.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>> ---
>>   arch/powerpc/kernel/module_64.c | 32 ++++++++++++++------------------
>>   1 file changed, 14 insertions(+), 18 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
>> index a89da0ee25e2..b687ef88c4c4 100644
>> --- a/arch/powerpc/kernel/module_64.c
>> +++ b/arch/powerpc/kernel/module_64.c
>> @@ -33,19 +33,13 @@
>>   #ifdef PPC64_ELF_ABI_v2
>>   
>>   /* An address is simply the address of the function. */
>> -typedef unsigned long func_desc_t;
>> +typedef struct {
>> +	unsigned long addr;
>> +} func_desc_t;
>>   
>>   static func_desc_t func_desc(unsigned long addr)
>>   {
>> -	return addr;
>> -}
>> -static unsigned long func_addr(unsigned long addr)
>> -{
>> -	return addr;
>> -}
>> -static unsigned long stub_func_addr(func_desc_t func)
>> -{
>> -	return func;
>> +	return (func_desc_t){addr};
> 
> There's only 1 element in the struct, so okay, but it hurt my eyes a
> little. I would have been happier with:
> 
> 	return (func_desc_t){ .addr = addr; };
> 
> But of course that also looks bonkers because it starts with "return".
> So no matter what I do my eyes bug out. ;)
> 
> So it's fine either way. :)
> 
> Reviewed-by: Kees Cook <keescook@chromium.org>

I am going for:

  static func_desc_t func_desc(unsigned long addr)
  {
+       func_desc_t desc = {
+               .addr = addr,
+       };
+
+       return desc;
  }


Thanks
Christophe

[PATCH v3 05/12] ia64: Rename 'ip' to 'addr' in 'struct fdesc'

[Christophe Leroy]

There are three architectures with function descriptors, try to
have common names for the address they contain in order to
refactor some functions into generic functions later.

powerpc has 'entry'
ia64 has 'ip'
parisc has 'addr'

Vote for 'addr' and update 'struct fdesc' accordingly.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/ia64/include/asm/elf.h      | 2 +-
 arch/ia64/include/asm/sections.h | 2 +-
 arch/ia64/kernel/module.c        | 6 +++---
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/ia64/include/asm/elf.h b/arch/ia64/include/asm/elf.h
index 6629301a2620..2ef5f9966ad1 100644
--- a/arch/ia64/include/asm/elf.h
+++ b/arch/ia64/include/asm/elf.h
@@ -226,7 +226,7 @@ struct got_entry {
  * Layout of the Function Descriptor
  */
 struct fdesc {
-	uint64_t ip;
+	uint64_t addr;
 	uint64_t gp;
 };
 
diff --git a/arch/ia64/include/asm/sections.h b/arch/ia64/include/asm/sections.h
index 3a033d2008b3..35f24e52149a 100644
--- a/arch/ia64/include/asm/sections.h
+++ b/arch/ia64/include/asm/sections.h
@@ -35,7 +35,7 @@ static inline void *dereference_function_descriptor(void *ptr)
 	struct fdesc *desc = ptr;
 	void *p;
 
-	if (!get_kernel_nofault(p, (void *)&desc->ip))
+	if (!get_kernel_nofault(p, (void *)&desc->addr))
 		ptr = p;
 	return ptr;
 }
diff --git a/arch/ia64/kernel/module.c b/arch/ia64/kernel/module.c
index 2cba53c1da82..4f6400cbf79e 100644
--- a/arch/ia64/kernel/module.c
+++ b/arch/ia64/kernel/module.c
@@ -602,15 +602,15 @@ get_fdesc (struct module *mod, uint64_t value, int *okp)
 		return value;
 
 	/* Look for existing function descriptor. */
-	while (fdesc->ip) {
-		if (fdesc->ip == value)
+	while (fdesc->addr) {
+		if (fdesc->addr == value)
 			return (uint64_t)fdesc;
 		if ((uint64_t) ++fdesc >= mod->arch.opd->sh_addr + mod->arch.opd->sh_size)
 			BUG();
 	}
 
 	/* Create new one */
-	fdesc->ip = value;
+	fdesc->addr = value;
 	fdesc->gp = mod->arch.gp;
 	return (uint64_t) fdesc;
 }
-- 
2.31.1

[PATCH v3 06/12] asm-generic: Define CONFIG_HAVE_FUNCTION_DESCRIPTORS

[Christophe Leroy]

Replace HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR by a config option
named CONFIG_HAVE_FUNCTION_DESCRIPTORS and use it instead of
'dereference_function_descriptor' macro to know whether an
arch has function descriptors.

To limit churn in one of the following patches, use
an #ifdef/#else construct with empty first part
instead of an #ifndef in asm-generic/sections.h

On powerpc, make sure the config option matches the ABI used
by the compiler with a BUILD_BUG_ON().

And include a helper to check whether an arch has function
descriptors or not : have_function_descriptors()

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/Kconfig                        | 3 +++
 arch/ia64/Kconfig                   | 1 +
 arch/ia64/include/asm/sections.h    | 2 --
 arch/parisc/Kconfig                 | 1 +
 arch/parisc/include/asm/sections.h  | 2 --
 arch/powerpc/Kconfig                | 1 +
 arch/powerpc/include/asm/sections.h | 2 --
 arch/powerpc/kernel/ptrace/ptrace.c | 6 ++++++
 include/asm-generic/sections.h      | 8 +++++++-
 include/linux/kallsyms.h            | 2 +-
 10 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 8df1c7102643..6e610a53d832 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -197,6 +197,9 @@ config HAVE_FUNCTION_ERROR_INJECTION
 config HAVE_NMI
 	bool
 
+config HAVE_FUNCTION_DESCRIPTORS
+	bool
+
 config TRACE_IRQFLAGS_SUPPORT
 	bool
 
diff --git a/arch/ia64/Kconfig b/arch/ia64/Kconfig
index 1e33666fa679..97cf02951610 100644
--- a/arch/ia64/Kconfig
+++ b/arch/ia64/Kconfig
@@ -34,6 +34,7 @@ config IA64
 	select HAVE_FUNCTION_TRACER
 	select TTY
 	select HAVE_ARCH_TRACEHOOK
+	select HAVE_FUNCTION_DESCRIPTORS
 	select HAVE_VIRT_CPU_ACCOUNTING
 	select HUGETLB_PAGE_SIZE_VARIABLE if HUGETLB_PAGE
 	select VIRT_TO_BUS
diff --git a/arch/ia64/include/asm/sections.h b/arch/ia64/include/asm/sections.h
index 35f24e52149a..2460d365a057 100644
--- a/arch/ia64/include/asm/sections.h
+++ b/arch/ia64/include/asm/sections.h
@@ -27,8 +27,6 @@ extern char __start_gate_brl_fsys_bubble_down_patchlist[], __end_gate_brl_fsys_b
 extern char __start_unwind[], __end_unwind[];
 extern char __start_ivt_text[], __end_ivt_text[];
 
-#define HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR 1
-
 #undef dereference_function_descriptor
 static inline void *dereference_function_descriptor(void *ptr)
 {
diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
index 27a8b49af11f..01d46ca32119 100644
--- a/arch/parisc/Kconfig
+++ b/arch/parisc/Kconfig
@@ -65,6 +65,7 @@ config PARISC
 	select HAVE_DYNAMIC_FTRACE_WITH_REGS
 	select HAVE_SOFTIRQ_ON_OWN_STACK if IRQSTACKS
 	select TRACE_IRQFLAGS_SUPPORT
+	select HAVE_FUNCTION_DESCRIPTORS if 64BIT
 
 	help
 	  The PA-RISC microprocessor is designed by Hewlett-Packard and used
diff --git a/arch/parisc/include/asm/sections.h b/arch/parisc/include/asm/sections.h
index bb52aea0cb21..c8092e4d94de 100644
--- a/arch/parisc/include/asm/sections.h
+++ b/arch/parisc/include/asm/sections.h
@@ -9,8 +9,6 @@ extern char __alt_instructions[], __alt_instructions_end[];
 
 #ifdef CONFIG_64BIT
 
-#define HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR 1
-
 #undef dereference_function_descriptor
 void *dereference_function_descriptor(void *);
 
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index ba5b66189358..0effedba082f 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -208,6 +208,7 @@ config PPC
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS	if !(CPU_LITTLE_ENDIAN && POWER7_CPU)
 	select HAVE_FAST_GUP
 	select HAVE_FTRACE_MCOUNT_RECORD
+	select HAVE_FUNCTION_DESCRIPTORS	if PPC64 && !CPU_LITTLE_ENDIAN
 	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER
diff --git a/arch/powerpc/include/asm/sections.h b/arch/powerpc/include/asm/sections.h
index abd2e5213197..fb11544d7e6a 100644
--- a/arch/powerpc/include/asm/sections.h
+++ b/arch/powerpc/include/asm/sections.h
@@ -69,8 +69,6 @@ static inline int overlaps_kernel_text(unsigned long start, unsigned long end)
 
 #ifdef PPC64_ELF_ABI_v1
 
-#define HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR 1
-
 #undef dereference_function_descriptor
 static inline void *dereference_function_descriptor(void *ptr)
 {
diff --git a/arch/powerpc/kernel/ptrace/ptrace.c b/arch/powerpc/kernel/ptrace/ptrace.c
index 7c7093c17c45..740b682caa73 100644
--- a/arch/powerpc/kernel/ptrace/ptrace.c
+++ b/arch/powerpc/kernel/ptrace/ptrace.c
@@ -446,4 +446,10 @@ void __init pt_regs_check(void)
 	 * real registers.
 	 */
 	BUILD_BUG_ON(PT_DSCR < sizeof(struct user_pt_regs) / sizeof(unsigned long));
+
+#ifdef PPC64_ELF_ABI_v1
+	BUILD_BUG_ON(!IS_ENABLED(CONFIG_HAVE_FUNCTION_DESCRIPTORS));
+#else
+	BUILD_BUG_ON(IS_ENABLED(CONFIG_HAVE_FUNCTION_DESCRIPTORS));
+#endif
 }
diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
index d16302d3eb59..a918388d9bf6 100644
--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h
@@ -59,11 +59,17 @@ extern char __noinstr_text_start[], __noinstr_text_end[];
 extern __visible const void __nosave_begin, __nosave_end;
 
 /* Function descriptor handling (if any).  Override in asm/sections.h */
-#ifndef dereference_function_descriptor
+#ifdef CONFIG_HAVE_FUNCTION_DESCRIPTORS
+#else
 #define dereference_function_descriptor(p) ((void *)(p))
 #define dereference_kernel_function_descriptor(p) ((void *)(p))
 #endif
 
+static inline bool have_function_descriptors(void)
+{
+	return IS_ENABLED(CONFIG_HAVE_FUNCTION_DESCRIPTORS);
+}
+
 /* random extra sections (if any).  Override
  * in asm/sections.h */
 #ifndef arch_is_kernel_text
diff --git a/include/linux/kallsyms.h b/include/linux/kallsyms.h
index a1d6fc82d7f0..18799df0c9bf 100644
--- a/include/linux/kallsyms.h
+++ b/include/linux/kallsyms.h
@@ -57,7 +57,7 @@ static inline int is_ksym_addr(unsigned long addr)
 
 static inline void *dereference_symbol_descriptor(void *ptr)
 {
-#ifdef HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR
+#ifdef CONFIG_HAVE_FUNCTION_DESCRIPTORS
 	struct module *mod;
 
 	ptr = dereference_kernel_function_descriptor(ptr);
-- 
2.31.1

[PATCH v3 07/12] asm-generic: Define 'func_desc_t' to commonly describe function descriptors

[Christophe Leroy]

We have three architectures using function descriptors, each with its
own type and name.

Add a common typedef that can be used in generic code.

Also add a stub typedef for architecture without function descriptors,
to avoid a forest of #ifdefs.

It replaces the similar 'func_desc_t' previously defined in
arch/powerpc/kernel/module_64.c

Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/ia64/include/asm/sections.h    | 3 +++
 arch/parisc/include/asm/sections.h  | 5 +++++
 arch/powerpc/include/asm/sections.h | 4 ++++
 arch/powerpc/kernel/module_64.c     | 8 --------
 include/asm-generic/sections.h      | 3 +++
 5 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/arch/ia64/include/asm/sections.h b/arch/ia64/include/asm/sections.h
index 2460d365a057..3abe0562b01a 100644
--- a/arch/ia64/include/asm/sections.h
+++ b/arch/ia64/include/asm/sections.h
@@ -9,6 +9,9 @@
 
 #include <linux/elf.h>
 #include <linux/uaccess.h>
+
+typedef struct fdesc func_desc_t;
+
 #include <asm-generic/sections.h>
 
 extern char __phys_per_cpu_start[];
diff --git a/arch/parisc/include/asm/sections.h b/arch/parisc/include/asm/sections.h
index c8092e4d94de..ace1d4047a0b 100644
--- a/arch/parisc/include/asm/sections.h
+++ b/arch/parisc/include/asm/sections.h
@@ -2,6 +2,11 @@
 #ifndef _PARISC_SECTIONS_H
 #define _PARISC_SECTIONS_H
 
+#ifdef CONFIG_HAVE_FUNCTION_DESCRIPTORS
+#include <asm/elf.h>
+typedef Elf64_Fdesc func_desc_t;
+#endif
+
 /* nothing to see, move along */
 #include <asm-generic/sections.h>
 
diff --git a/arch/powerpc/include/asm/sections.h b/arch/powerpc/include/asm/sections.h
index fb11544d7e6a..1e6b6e732fb3 100644
--- a/arch/powerpc/include/asm/sections.h
+++ b/arch/powerpc/include/asm/sections.h
@@ -8,6 +8,10 @@
 
 #define arch_is_kernel_initmem_freed arch_is_kernel_initmem_freed
 
+#ifdef CONFIG_HAVE_FUNCTION_DESCRIPTORS
+typedef struct func_desc func_desc_t;
+#endif
+
 #include <asm-generic/sections.h>
 
 extern bool init_mem_is_free;
diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index b687ef88c4c4..3d06b996d504 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -32,11 +32,6 @@
 
 #ifdef PPC64_ELF_ABI_v2
 
-/* An address is simply the address of the function. */
-typedef struct {
-	unsigned long addr;
-} func_desc_t;
-
 static func_desc_t func_desc(unsigned long addr)
 {
 	return (func_desc_t){addr};
@@ -57,9 +52,6 @@ static unsigned int local_entry_offset(const Elf64_Sym *sym)
 }
 #else
 
-/* An address is address of the OPD entry, which contains address of fn. */
-typedef struct func_desc func_desc_t;
-
 static func_desc_t func_desc(unsigned long addr)
 {
 	return *(struct func_desc *)addr;
diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
index a918388d9bf6..33b51efe3a24 100644
--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h
@@ -63,6 +63,9 @@ extern __visible const void __nosave_begin, __nosave_end;
 #else
 #define dereference_function_descriptor(p) ((void *)(p))
 #define dereference_kernel_function_descriptor(p) ((void *)(p))
+typedef struct {
+	unsigned long addr;
+} func_desc_t;
 #endif
 
 static inline bool have_function_descriptors(void)
-- 
2.31.1

Re: [PATCH v3 07/12] asm-generic: Define 'func_desc_t' to commonly describe function descriptors

[Nicholas Piggin]

Excerpts from Christophe Leroy's message of October 17, 2021 10:38 pm:
> We have three architectures using function descriptors, each with its
> own type and name.
> 
> Add a common typedef that can be used in generic code.
> 
> Also add a stub typedef for architecture without function descriptors,
> to avoid a forest of #ifdefs.
> 
> It replaces the similar 'func_desc_t' previously defined in
> arch/powerpc/kernel/module_64.c
> 
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Acked-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---

[...]

> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
> index a918388d9bf6..33b51efe3a24 100644
> --- a/include/asm-generic/sections.h
> +++ b/include/asm-generic/sections.h
> @@ -63,6 +63,9 @@ extern __visible const void __nosave_begin, __nosave_end;
>  #else
>  #define dereference_function_descriptor(p) ((void *)(p))
>  #define dereference_kernel_function_descriptor(p) ((void *)(p))
> +typedef struct {
> +	unsigned long addr;
> +} func_desc_t;
>  #endif
>  

I think that deserves a comment. If it's just to allow ifdef to be 
avoided, I guess that's okay with a comment. Would be nice if you could 
cause it to generate a link time error if it was ever used like
undefined functions, but I guess you can't. It's not a necessity though.

Thanks,
Nick

Re: [PATCH v3 07/12] asm-generic: Define 'func_desc_t' to commonly describe function descriptors

[Christophe Leroy]

Le 18/10/2021 à 08:29, Nicholas Piggin a écrit :
> Excerpts from Christophe Leroy's message of October 17, 2021 10:38 pm:
>> We have three architectures using function descriptors, each with its
>> own type and name.
>>
>> Add a common typedef that can be used in generic code.
>>
>> Also add a stub typedef for architecture without function descriptors,
>> to avoid a forest of #ifdefs.
>>
>> It replaces the similar 'func_desc_t' previously defined in
>> arch/powerpc/kernel/module_64.c
>>
>> Reviewed-by: Kees Cook <keescook@chromium.org>
>> Acked-by: Arnd Bergmann <arnd@arndb.de>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>> ---
> 
> [...]
> 
>> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
>> index a918388d9bf6..33b51efe3a24 100644
>> --- a/include/asm-generic/sections.h
>> +++ b/include/asm-generic/sections.h
>> @@ -63,6 +63,9 @@ extern __visible const void __nosave_begin, __nosave_end;
>>   #else
>>   #define dereference_function_descriptor(p) ((void *)(p))
>>   #define dereference_kernel_function_descriptor(p) ((void *)(p))
>> +typedef struct {
>> +	unsigned long addr;
>> +} func_desc_t;
>>   #endif
>>   
> 
> I think that deserves a comment. If it's just to allow ifdef to be
> avoided, I guess that's okay with a comment. Would be nice if you could
> cause it to generate a link time error if it was ever used like
> undefined functions, but I guess you can't. It's not a necessity though.
> 

I tried to explain it in the commit message, but I can add a comment 
here in addition for sure.

By the way, it IS used in powerpc's module_64.c:

static func_desc_t func_desc(unsigned long addr)
{
	return (func_desc_t){addr};
}

static unsigned long func_addr(unsigned long addr)
{
	return func_desc(addr).addr;
}

Re: [PATCH v3 07/12] asm-generic: Define 'func_desc_t' to commonly describe function descriptors

[Nicholas Piggin]

Excerpts from Christophe Leroy's message of October 18, 2021 5:07 pm:
> 
> 
> Le 18/10/2021 à 08:29, Nicholas Piggin a écrit :
>> Excerpts from Christophe Leroy's message of October 17, 2021 10:38 pm:
>>> We have three architectures using function descriptors, each with its
>>> own type and name.
>>>
>>> Add a common typedef that can be used in generic code.
>>>
>>> Also add a stub typedef for architecture without function descriptors,
>>> to avoid a forest of #ifdefs.
>>>
>>> It replaces the similar 'func_desc_t' previously defined in
>>> arch/powerpc/kernel/module_64.c
>>>
>>> Reviewed-by: Kees Cook <keescook@chromium.org>
>>> Acked-by: Arnd Bergmann <arnd@arndb.de>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>>> ---
>> 
>> [...]
>> 
>>> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
>>> index a918388d9bf6..33b51efe3a24 100644
>>> --- a/include/asm-generic/sections.h
>>> +++ b/include/asm-generic/sections.h
>>> @@ -63,6 +63,9 @@ extern __visible const void __nosave_begin, __nosave_end;
>>>   #else
>>>   #define dereference_function_descriptor(p) ((void *)(p))
>>>   #define dereference_kernel_function_descriptor(p) ((void *)(p))
>>> +typedef struct {
>>> +	unsigned long addr;
>>> +} func_desc_t;
>>>   #endif
>>>   
>> 
>> I think that deserves a comment. If it's just to allow ifdef to be
>> avoided, I guess that's okay with a comment. Would be nice if you could
>> cause it to generate a link time error if it was ever used like
>> undefined functions, but I guess you can't. It's not a necessity though.
>> 
> 
> I tried to explain it in the commit message, but I can add a comment 
> here in addition for sure.

Thanks.

> 
> By the way, it IS used in powerpc's module_64.c:

Ah yes of course. I guess the point is function descriptors don't exist 
so it should not be used (in general). powerpc module code knows what it
is doing, I guess it's okay for it to use it.

Thanks,
Nick

[PATCH v3 08/12] asm-generic: Refactor dereference_[kernel]_function_descriptor()

[Christophe Leroy]

dereference_function_descriptor() and
dereference_kernel_function_descriptor() are identical on the
three architectures implementing them.

Make them common and put them out-of-line in kernel/extable.c
which is one of the users and has similar type of functions.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/ia64/include/asm/sections.h    | 19 -------------------
 arch/parisc/include/asm/sections.h  |  9 ---------
 arch/parisc/kernel/process.c        | 21 ---------------------
 arch/powerpc/include/asm/sections.h | 23 -----------------------
 include/asm-generic/sections.h      |  2 ++
 kernel/extable.c                    | 23 ++++++++++++++++++++++-
 6 files changed, 24 insertions(+), 73 deletions(-)

diff --git a/arch/ia64/include/asm/sections.h b/arch/ia64/include/asm/sections.h
index 3abe0562b01a..8e0875cf6071 100644
--- a/arch/ia64/include/asm/sections.h
+++ b/arch/ia64/include/asm/sections.h
@@ -30,23 +30,4 @@ extern char __start_gate_brl_fsys_bubble_down_patchlist[], __end_gate_brl_fsys_b
 extern char __start_unwind[], __end_unwind[];
 extern char __start_ivt_text[], __end_ivt_text[];
 
-#undef dereference_function_descriptor
-static inline void *dereference_function_descriptor(void *ptr)
-{
-	struct fdesc *desc = ptr;
-	void *p;
-
-	if (!get_kernel_nofault(p, (void *)&desc->addr))
-		ptr = p;
-	return ptr;
-}
-
-#undef dereference_kernel_function_descriptor
-static inline void *dereference_kernel_function_descriptor(void *ptr)
-{
-	if (ptr < (void *)__start_opd || ptr >= (void *)__end_opd)
-		return ptr;
-	return dereference_function_descriptor(ptr);
-}
-
 #endif /* _ASM_IA64_SECTIONS_H */
diff --git a/arch/parisc/include/asm/sections.h b/arch/parisc/include/asm/sections.h
index ace1d4047a0b..33df42b5cc6d 100644
--- a/arch/parisc/include/asm/sections.h
+++ b/arch/parisc/include/asm/sections.h
@@ -12,13 +12,4 @@ typedef Elf64_Fdesc func_desc_t;
 
 extern char __alt_instructions[], __alt_instructions_end[];
 
-#ifdef CONFIG_64BIT
-
-#undef dereference_function_descriptor
-void *dereference_function_descriptor(void *);
-
-#undef dereference_kernel_function_descriptor
-void *dereference_kernel_function_descriptor(void *);
-#endif
-
 #endif
diff --git a/arch/parisc/kernel/process.c b/arch/parisc/kernel/process.c
index 38ec4ae81239..7382576b52a8 100644
--- a/arch/parisc/kernel/process.c
+++ b/arch/parisc/kernel/process.c
@@ -266,27 +266,6 @@ get_wchan(struct task_struct *p)
 	return 0;
 }
 
-#ifdef CONFIG_64BIT
-void *dereference_function_descriptor(void *ptr)
-{
-	Elf64_Fdesc *desc = ptr;
-	void *p;
-
-	if (!get_kernel_nofault(p, (void *)&desc->addr))
-		ptr = p;
-	return ptr;
-}
-
-void *dereference_kernel_function_descriptor(void *ptr)
-{
-	if (ptr < (void *)__start_opd ||
-			ptr >= (void *)__end_opd)
-		return ptr;
-
-	return dereference_function_descriptor(ptr);
-}
-#endif
-
 static inline unsigned long brk_rnd(void)
 {
 	return (get_random_int() & BRK_RND_MASK) << PAGE_SHIFT;
diff --git a/arch/powerpc/include/asm/sections.h b/arch/powerpc/include/asm/sections.h
index 1e6b6e732fb3..2c3de9bd1a90 100644
--- a/arch/powerpc/include/asm/sections.h
+++ b/arch/powerpc/include/asm/sections.h
@@ -71,29 +71,6 @@ static inline int overlaps_kernel_text(unsigned long start, unsigned long end)
 		(unsigned long)_stext < end;
 }
 
-#ifdef PPC64_ELF_ABI_v1
-
-#undef dereference_function_descriptor
-static inline void *dereference_function_descriptor(void *ptr)
-{
-	struct func_desc *desc = ptr;
-	void *p;
-
-	if (!get_kernel_nofault(p, (void *)&desc->addr))
-		ptr = p;
-	return ptr;
-}
-
-#undef dereference_kernel_function_descriptor
-static inline void *dereference_kernel_function_descriptor(void *ptr)
-{
-	if (ptr < (void *)__start_opd || ptr >= (void *)__end_opd)
-		return ptr;
-
-	return dereference_function_descriptor(ptr);
-}
-#endif /* PPC64_ELF_ABI_v1 */
-
 #endif
 
 #endif /* __KERNEL__ */
diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
index 33b51efe3a24..c9f30b6e81f9 100644
--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h
@@ -60,6 +60,8 @@ extern __visible const void __nosave_begin, __nosave_end;
 
 /* Function descriptor handling (if any).  Override in asm/sections.h */
 #ifdef CONFIG_HAVE_FUNCTION_DESCRIPTORS
+void *dereference_function_descriptor(void *ptr);
+void *dereference_kernel_function_descriptor(void *ptr);
 #else
 #define dereference_function_descriptor(p) ((void *)(p))
 #define dereference_kernel_function_descriptor(p) ((void *)(p))
diff --git a/kernel/extable.c b/kernel/extable.c
index b0ea5eb0c3b4..1ef13789bea9 100644
--- a/kernel/extable.c
+++ b/kernel/extable.c
@@ -3,6 +3,7 @@
    Copyright (C) 2001 Rusty Russell, 2002 Rusty Russell IBM.
 
 */
+#include <linux/elf.h>
 #include <linux/ftrace.h>
 #include <linux/memory.h>
 #include <linux/extable.h>
@@ -159,12 +160,32 @@ int kernel_text_address(unsigned long addr)
 }
 
 /*
- * On some architectures (PPC64, IA64) function pointers
+ * On some architectures (PPC64, IA64, PARISC) function pointers
  * are actually only tokens to some data that then holds the
  * real function address. As a result, to find if a function
  * pointer is part of the kernel text, we need to do some
  * special dereferencing first.
  */
+#ifdef CONFIG_HAVE_FUNCTION_DESCRIPTORS
+void *dereference_function_descriptor(void *ptr)
+{
+	func_desc_t *desc = ptr;
+	void *p;
+
+	if (!get_kernel_nofault(p, (void *)&desc->addr))
+		ptr = p;
+	return ptr;
+}
+
+void *dereference_kernel_function_descriptor(void *ptr)
+{
+	if (ptr < (void *)__start_opd || ptr >= (void *)__end_opd)
+		return ptr;
+
+	return dereference_function_descriptor(ptr);
+}
+#endif
+
 int func_ptr_is_kernel_text(void *ptr)
 {
 	unsigned long addr;
-- 
2.31.1

Re: [PATCH v3 08/12] asm-generic: Refactor dereference_[kernel]_function_descriptor()

[Michael Ellerman]

Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> diff --git a/kernel/extable.c b/kernel/extable.c
> index b0ea5eb0c3b4..1ef13789bea9 100644
> --- a/kernel/extable.c
> +++ b/kernel/extable.c
> @@ -159,12 +160,32 @@ int kernel_text_address(unsigned long addr)
>  }
>  
>  /*
> - * On some architectures (PPC64, IA64) function pointers
> + * On some architectures (PPC64, IA64, PARISC) function pointers
>   * are actually only tokens to some data that then holds the
>   * real function address. As a result, to find if a function
>   * pointer is part of the kernel text, we need to do some
>   * special dereferencing first.
>   */
> +#ifdef CONFIG_HAVE_FUNCTION_DESCRIPTORS
> +void *dereference_function_descriptor(void *ptr)
> +{
> +	func_desc_t *desc = ptr;
> +	void *p;
> +
> +	if (!get_kernel_nofault(p, (void *)&desc->addr))
> +		ptr = p;
> +	return ptr;
> +}

This needs an EXPORT_SYMBOL_GPL(), otherwise the build breaks after
patch 10 with CONFIG_LKDTM=m.

cheers

Re: [PATCH v3 08/12] asm-generic: Refactor dereference_[kernel]_function_descriptor()

[Kees Cook]

On Thu, Feb 10, 2022 at 09:30:43PM +1100, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> > diff --git a/kernel/extable.c b/kernel/extable.c
> > index b0ea5eb0c3b4..1ef13789bea9 100644
> > --- a/kernel/extable.c
> > +++ b/kernel/extable.c
> > @@ -159,12 +160,32 @@ int kernel_text_address(unsigned long addr)
> >  }
> >  
> >  /*
> > - * On some architectures (PPC64, IA64) function pointers
> > + * On some architectures (PPC64, IA64, PARISC) function pointers
> >   * are actually only tokens to some data that then holds the
> >   * real function address. As a result, to find if a function
> >   * pointer is part of the kernel text, we need to do some
> >   * special dereferencing first.
> >   */
> > +#ifdef CONFIG_HAVE_FUNCTION_DESCRIPTORS
> > +void *dereference_function_descriptor(void *ptr)
> > +{
> > +	func_desc_t *desc = ptr;
> > +	void *p;
> > +
> > +	if (!get_kernel_nofault(p, (void *)&desc->addr))
> > +		ptr = p;
> > +	return ptr;
> > +}
> 
> This needs an EXPORT_SYMBOL_GPL(), otherwise the build breaks after
> patch 10 with CONFIG_LKDTM=m.

Oh good catch!

(There have been a few cases of LKDTM=m being the only thing needed a
symbol, so I've pondered giving it a namespace or constructing a little
ifdef wrapper... but this seems ok to export...)

-- 
Kees Cook

Re: [PATCH v3 08/12] asm-generic: Refactor dereference_[kernel]_function_descriptor()

[Christophe Leroy]

Le 11/02/2022 à 01:56, Kees Cook a écrit :
> On Thu, Feb 10, 2022 at 09:30:43PM +1100, Michael Ellerman wrote:
>> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
>>> diff --git a/kernel/extable.c b/kernel/extable.c
>>> index b0ea5eb0c3b4..1ef13789bea9 100644
>>> --- a/kernel/extable.c
>>> +++ b/kernel/extable.c
>>> @@ -159,12 +160,32 @@ int kernel_text_address(unsigned long addr)
>>>   }
>>>   
>>>   /*
>>> - * On some architectures (PPC64, IA64) function pointers
>>> + * On some architectures (PPC64, IA64, PARISC) function pointers
>>>    * are actually only tokens to some data that then holds the
>>>    * real function address. As a result, to find if a function
>>>    * pointer is part of the kernel text, we need to do some
>>>    * special dereferencing first.
>>>    */
>>> +#ifdef CONFIG_HAVE_FUNCTION_DESCRIPTORS
>>> +void *dereference_function_descriptor(void *ptr)
>>> +{
>>> +	func_desc_t *desc = ptr;
>>> +	void *p;
>>> +
>>> +	if (!get_kernel_nofault(p, (void *)&desc->addr))
>>> +		ptr = p;
>>> +	return ptr;
>>> +}
>>
>> This needs an EXPORT_SYMBOL_GPL(), otherwise the build breaks after
>> patch 10 with CONFIG_LKDTM=m.
> 
> Oh good catch!
> 
> (There have been a few cases of LKDTM=m being the only thing needed a
> symbol, so I've pondered giving it a namespace or constructing a little
> ifdef wrapper... but this seems ok to export...)
> 

powerpc and ia64 had it as a static inline, but parisc had it as a plain 
function and didn't export it. So I guess the export is not required at 
this point. I will export it in patch 10 when it becomes necessary.

Christophe

[PATCH v3 09/12] lkdtm: Force do_nothing() out of line

[Christophe Leroy]

LKDTM tests display that the run do_nothing() at a given
address, but in reality do_nothing() is inlined into the
caller.

Force it out of line so that it really runs text at the
displayed address.

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 drivers/misc/lkdtm/perms.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 2dede2ef658f..60b3b2fe929d 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -21,7 +21,7 @@
 /* This is non-const, so it will end up in the .data section. */
 static u8 data_area[EXEC_SIZE];
 
-/* This is cost, so it will end up in the .rodata section. */
+/* This is const, so it will end up in the .rodata section. */
 static const unsigned long rodata = 0xAA55AA55;
 
 /* This is marked __ro_after_init, so it should ultimately be .rodata. */
@@ -31,7 +31,7 @@ static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
  * This just returns to the caller. It is designed to be copied into
  * non-executable memory regions.
  */
-static void do_nothing(void)
+static noinline void do_nothing(void)
 {
 	return;
 }
-- 
2.31.1

[PATCH v3 10/12] lkdtm: Really write into kernel text in WRITE_KERN

[Christophe Leroy]

WRITE_KERN is supposed to overwrite some kernel text, namely
do_overwritten() function.

But at the time being it overwrites do_overwritten() function
descriptor, not function text.

Fix it by dereferencing the function descriptor to obtain
function text pointer.

And make do_overwritten() noinline so that it is really
do_overwritten() which is called by lkdtm_WRITE_KERN().

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 drivers/misc/lkdtm/perms.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 60b3b2fe929d..035fcca441f0 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -10,6 +10,7 @@
 #include <linux/mman.h>
 #include <linux/uaccess.h>
 #include <asm/cacheflush.h>
+#include <asm/sections.h>
 
 /* Whether or not to fill the target memory area with do_nothing(). */
 #define CODE_WRITE	true
@@ -37,7 +38,7 @@ static noinline void do_nothing(void)
 }
 
 /* Must immediately follow do_nothing for size calculuations to work out. */
-static void do_overwritten(void)
+static noinline void do_overwritten(void)
 {
 	pr_info("do_overwritten wasn't overwritten!\n");
 	return;
@@ -113,8 +114,9 @@ void lkdtm_WRITE_KERN(void)
 	size_t size;
 	volatile unsigned char *ptr;
 
-	size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
-	ptr = (unsigned char *)do_overwritten;
+	size = (unsigned long)dereference_function_descriptor(do_overwritten) -
+	       (unsigned long)dereference_function_descriptor(do_nothing);
+	ptr = dereference_function_descriptor(do_overwritten);
 
 	pr_info("attempting bad %zu byte write at %px\n", size, ptr);
 	memcpy((void *)ptr, (unsigned char *)do_nothing, size);
-- 
2.31.1

[PATCH v3 11/12] lkdtm: Fix execute_[user]_location()

[Christophe Leroy]

execute_location() and execute_user_location() intent
to copy do_nothing() text and execute it at a new location.
However, at the time being it doesn't copy do_nothing() function
but do_nothing() function descriptor which still points to the
original text. So at the end it still executes do_nothing() at
its original location allthough using a copied function descriptor.

So, fix that by really copying do_nothing() text and build a new
function descriptor by copying do_nothing() function descriptor and
updating the target address with the new location.

Also fix the displayed addresses by dereferencing do_nothing()
function descriptor.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 drivers/misc/lkdtm/perms.c | 37 ++++++++++++++++++++++++++++---------
 1 file changed, 28 insertions(+), 9 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 035fcca441f0..1cf24c4a79e9 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -44,19 +44,34 @@ static noinline void do_overwritten(void)
 	return;
 }
 
+static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
+{
+	if (!have_function_descriptors())
+		return dst;
+
+	memcpy(fdesc, do_nothing, sizeof(*fdesc));
+	fdesc->addr = (unsigned long)dst;
+	barrier();
+
+	return fdesc;
+}
+
 static noinline void execute_location(void *dst, bool write)
 {
-	void (*func)(void) = dst;
+	void (*func)(void);
+	func_desc_t fdesc;
+	void *do_nothing_text = dereference_function_descriptor(do_nothing);
 
-	pr_info("attempting ok execution at %px\n", do_nothing);
+	pr_info("attempting ok execution at %px\n", do_nothing_text);
 	do_nothing();
 
 	if (write == CODE_WRITE) {
-		memcpy(dst, do_nothing, EXEC_SIZE);
+		memcpy(dst, do_nothing_text, EXEC_SIZE);
 		flush_icache_range((unsigned long)dst,
 				   (unsigned long)dst + EXEC_SIZE);
 	}
-	pr_info("attempting bad execution at %px\n", func);
+	pr_info("attempting bad execution at %px\n", dst);
+	func = setup_function_descriptor(&fdesc, dst);
 	func();
 	pr_err("FAIL: func returned\n");
 }
@@ -66,16 +81,19 @@ static void execute_user_location(void *dst)
 	int copied;
 
 	/* Intentionally crossing kernel/user memory boundary. */
-	void (*func)(void) = dst;
+	void (*func)(void);
+	func_desc_t fdesc;
+	void *do_nothing_text = dereference_function_descriptor(do_nothing);
 
-	pr_info("attempting ok execution at %px\n", do_nothing);
+	pr_info("attempting ok execution at %px\n", do_nothing_text);
 	do_nothing();
 
-	copied = access_process_vm(current, (unsigned long)dst, do_nothing,
+	copied = access_process_vm(current, (unsigned long)dst, do_nothing_text,
 				   EXEC_SIZE, FOLL_WRITE);
 	if (copied < EXEC_SIZE)
 		return;
-	pr_info("attempting bad execution at %px\n", func);
+	pr_info("attempting bad execution at %px\n", dst);
+	func = setup_function_descriptor(&fdesc, dst);
 	func();
 	pr_err("FAIL: func returned\n");
 }
@@ -153,7 +171,8 @@ void lkdtm_EXEC_VMALLOC(void)
 
 void lkdtm_EXEC_RODATA(void)
 {
-	execute_location(lkdtm_rodata_do_nothing, CODE_AS_IS);
+	execute_location(dereference_function_descriptor(lkdtm_rodata_do_nothing),
+			 CODE_AS_IS);
 }
 
 void lkdtm_EXEC_USERSPACE(void)
-- 
2.31.1

Re: [PATCH v3 11/12] lkdtm: Fix execute_[user]_location()

[Christophe Leroy]

Hi Kees,

Le 17/10/2021 à 14:38, Christophe Leroy a écrit :
> execute_location() and execute_user_location() intent
> to copy do_nothing() text and execute it at a new location.
> However, at the time being it doesn't copy do_nothing() function
> but do_nothing() function descriptor which still points to the
> original text. So at the end it still executes do_nothing() at
> its original location allthough using a copied function descriptor.
> 
> So, fix that by really copying do_nothing() text and build a new
> function descriptor by copying do_nothing() function descriptor and
> updating the target address with the new location.
> 
> Also fix the displayed addresses by dereferencing do_nothing()
> function descriptor.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

Do you have any comment to this patch and to patch 12 ?

If not, is it ok to get your acked-by ?

Thanks
Christophe

> ---
>   drivers/misc/lkdtm/perms.c | 37 ++++++++++++++++++++++++++++---------
>   1 file changed, 28 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index 035fcca441f0..1cf24c4a79e9 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -44,19 +44,34 @@ static noinline void do_overwritten(void)
>   	return;
>   }
>   
> +static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
> +{
> +	if (!have_function_descriptors())
> +		return dst;
> +
> +	memcpy(fdesc, do_nothing, sizeof(*fdesc));
> +	fdesc->addr = (unsigned long)dst;
> +	barrier();
> +
> +	return fdesc;
> +}
> +
>   static noinline void execute_location(void *dst, bool write)
>   {
> -	void (*func)(void) = dst;
> +	void (*func)(void);
> +	func_desc_t fdesc;
> +	void *do_nothing_text = dereference_function_descriptor(do_nothing);
>   
> -	pr_info("attempting ok execution at %px\n", do_nothing);
> +	pr_info("attempting ok execution at %px\n", do_nothing_text);
>   	do_nothing();
>   
>   	if (write == CODE_WRITE) {
> -		memcpy(dst, do_nothing, EXEC_SIZE);
> +		memcpy(dst, do_nothing_text, EXEC_SIZE);
>   		flush_icache_range((unsigned long)dst,
>   				   (unsigned long)dst + EXEC_SIZE);
>   	}
> -	pr_info("attempting bad execution at %px\n", func);
> +	pr_info("attempting bad execution at %px\n", dst);
> +	func = setup_function_descriptor(&fdesc, dst);
>   	func();
>   	pr_err("FAIL: func returned\n");
>   }
> @@ -66,16 +81,19 @@ static void execute_user_location(void *dst)
>   	int copied;
>   
>   	/* Intentionally crossing kernel/user memory boundary. */
> -	void (*func)(void) = dst;
> +	void (*func)(void);
> +	func_desc_t fdesc;
> +	void *do_nothing_text = dereference_function_descriptor(do_nothing);
>   
> -	pr_info("attempting ok execution at %px\n", do_nothing);
> +	pr_info("attempting ok execution at %px\n", do_nothing_text);
>   	do_nothing();
>   
> -	copied = access_process_vm(current, (unsigned long)dst, do_nothing,
> +	copied = access_process_vm(current, (unsigned long)dst, do_nothing_text,
>   				   EXEC_SIZE, FOLL_WRITE);
>   	if (copied < EXEC_SIZE)
>   		return;
> -	pr_info("attempting bad execution at %px\n", func);
> +	pr_info("attempting bad execution at %px\n", dst);
> +	func = setup_function_descriptor(&fdesc, dst);
>   	func();
>   	pr_err("FAIL: func returned\n");
>   }
> @@ -153,7 +171,8 @@ void lkdtm_EXEC_VMALLOC(void)
>   
>   void lkdtm_EXEC_RODATA(void)
>   {
> -	execute_location(lkdtm_rodata_do_nothing, CODE_AS_IS);
> +	execute_location(dereference_function_descriptor(lkdtm_rodata_do_nothing),
> +			 CODE_AS_IS);
>   }
>   
>   void lkdtm_EXEC_USERSPACE(void)
> 

Re: [PATCH v3 11/12] lkdtm: Fix execute_[user]_location()

[Christophe Leroy]

Hi Kees,


Le 17/12/2021 à 12:49, Christophe Leroy a écrit :
> Hi Kees,
> 
> Le 17/10/2021 à 14:38, Christophe Leroy a écrit :
>> execute_location() and execute_user_location() intent
>> to copy do_nothing() text and execute it at a new location.
>> However, at the time being it doesn't copy do_nothing() function
>> but do_nothing() function descriptor which still points to the
>> original text. So at the end it still executes do_nothing() at
>> its original location allthough using a copied function descriptor.
>>
>> So, fix that by really copying do_nothing() text and build a new
>> function descriptor by copying do_nothing() function descriptor and
>> updating the target address with the new location.
>>
>> Also fix the displayed addresses by dereferencing do_nothing()
>> function descriptor.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> 
> Do you have any comment to this patch and to patch 12 ?
> 
> If not, is it ok to get your acked-by ?

Any feedback please, even if it's to say no feedback ?

Many thanks,
Christophe

> 
> Thanks
> Christophe
> 
>> ---
>>   drivers/misc/lkdtm/perms.c | 37 ++++++++++++++++++++++++++++---------
>>   1 file changed, 28 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
>> index 035fcca441f0..1cf24c4a79e9 100644
>> --- a/drivers/misc/lkdtm/perms.c
>> +++ b/drivers/misc/lkdtm/perms.c
>> @@ -44,19 +44,34 @@ static noinline void do_overwritten(void)
>>       return;
>>   }
>> +static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
>> +{
>> +    if (!have_function_descriptors())
>> +        return dst;
>> +
>> +    memcpy(fdesc, do_nothing, sizeof(*fdesc));
>> +    fdesc->addr = (unsigned long)dst;
>> +    barrier();
>> +
>> +    return fdesc;
>> +}
>> +
>>   static noinline void execute_location(void *dst, bool write)
>>   {
>> -    void (*func)(void) = dst;
>> +    void (*func)(void);
>> +    func_desc_t fdesc;
>> +    void *do_nothing_text = dereference_function_descriptor(do_nothing);
>> -    pr_info("attempting ok execution at %px\n", do_nothing);
>> +    pr_info("attempting ok execution at %px\n", do_nothing_text);
>>       do_nothing();
>>       if (write == CODE_WRITE) {
>> -        memcpy(dst, do_nothing, EXEC_SIZE);
>> +        memcpy(dst, do_nothing_text, EXEC_SIZE);
>>           flush_icache_range((unsigned long)dst,
>>                      (unsigned long)dst + EXEC_SIZE);
>>       }
>> -    pr_info("attempting bad execution at %px\n", func);
>> +    pr_info("attempting bad execution at %px\n", dst);
>> +    func = setup_function_descriptor(&fdesc, dst);
>>       func();
>>       pr_err("FAIL: func returned\n");
>>   }
>> @@ -66,16 +81,19 @@ static void execute_user_location(void *dst)
>>       int copied;
>>       /* Intentionally crossing kernel/user memory boundary. */
>> -    void (*func)(void) = dst;
>> +    void (*func)(void);
>> +    func_desc_t fdesc;
>> +    void *do_nothing_text = dereference_function_descriptor(do_nothing);
>> -    pr_info("attempting ok execution at %px\n", do_nothing);
>> +    pr_info("attempting ok execution at %px\n", do_nothing_text);
>>       do_nothing();
>> -    copied = access_process_vm(current, (unsigned long)dst, do_nothing,
>> +    copied = access_process_vm(current, (unsigned long)dst, 
>> do_nothing_text,
>>                      EXEC_SIZE, FOLL_WRITE);
>>       if (copied < EXEC_SIZE)
>>           return;
>> -    pr_info("attempting bad execution at %px\n", func);
>> +    pr_info("attempting bad execution at %px\n", dst);
>> +    func = setup_function_descriptor(&fdesc, dst);
>>       func();
>>       pr_err("FAIL: func returned\n");
>>   }
>> @@ -153,7 +171,8 @@ void lkdtm_EXEC_VMALLOC(void)
>>   void lkdtm_EXEC_RODATA(void)
>>   {
>> -    execute_location(lkdtm_rodata_do_nothing, CODE_AS_IS);
>> +    
>> execute_location(dereference_function_descriptor(lkdtm_rodata_do_nothing), 
>>
>> +             CODE_AS_IS);
>>   }
>>   void lkdtm_EXEC_USERSPACE(void)
>>

Re: [PATCH v3 11/12] lkdtm: Fix execute_[user]_location()

[Kees Cook]

On Wed, Jan 19, 2022 at 08:28:54PM +0100, Christophe Leroy wrote:
> Hi Kees,
> 
> 
> Le 17/12/2021 à 12:49, Christophe Leroy a écrit :
> > Hi Kees,
> > 
> > Le 17/10/2021 à 14:38, Christophe Leroy a écrit :
> > > execute_location() and execute_user_location() intent
> > > to copy do_nothing() text and execute it at a new location.
> > > However, at the time being it doesn't copy do_nothing() function
> > > but do_nothing() function descriptor which still points to the
> > > original text. So at the end it still executes do_nothing() at
> > > its original location allthough using a copied function descriptor.
> > > 
> > > So, fix that by really copying do_nothing() text and build a new
> > > function descriptor by copying do_nothing() function descriptor and
> > > updating the target address with the new location.
> > > 
> > > Also fix the displayed addresses by dereferencing do_nothing()
> > > function descriptor.
> > > 
> > > Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> > 
> > Do you have any comment to this patch and to patch 12 ?
> > 
> > If not, is it ok to get your acked-by ?
> 
> Any feedback please, even if it's to say no feedback ?

Hi! Thanks for the ping; I haven't had time yet to look at this, but
with -rc1 coming, I should be able to task-switch back to LKDTM for the
dev cycle and I can give some feedback.

-Kees

> 
> Many thanks,
> Christophe
> 
> > 
> > Thanks
> > Christophe
> > 
> > > ---
> > >   drivers/misc/lkdtm/perms.c | 37 ++++++++++++++++++++++++++++---------
> > >   1 file changed, 28 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> > > index 035fcca441f0..1cf24c4a79e9 100644
> > > --- a/drivers/misc/lkdtm/perms.c
> > > +++ b/drivers/misc/lkdtm/perms.c
> > > @@ -44,19 +44,34 @@ static noinline void do_overwritten(void)
> > >       return;
> > >   }
> > > +static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
> > > +{
> > > +    if (!have_function_descriptors())
> > > +        return dst;
> > > +
> > > +    memcpy(fdesc, do_nothing, sizeof(*fdesc));
> > > +    fdesc->addr = (unsigned long)dst;
> > > +    barrier();
> > > +
> > > +    return fdesc;
> > > +}
> > > +
> > >   static noinline void execute_location(void *dst, bool write)
> > >   {
> > > -    void (*func)(void) = dst;
> > > +    void (*func)(void);
> > > +    func_desc_t fdesc;
> > > +    void *do_nothing_text = dereference_function_descriptor(do_nothing);
> > > -    pr_info("attempting ok execution at %px\n", do_nothing);
> > > +    pr_info("attempting ok execution at %px\n", do_nothing_text);
> > >       do_nothing();
> > >       if (write == CODE_WRITE) {
> > > -        memcpy(dst, do_nothing, EXEC_SIZE);
> > > +        memcpy(dst, do_nothing_text, EXEC_SIZE);
> > >           flush_icache_range((unsigned long)dst,
> > >                      (unsigned long)dst + EXEC_SIZE);
> > >       }
> > > -    pr_info("attempting bad execution at %px\n", func);
> > > +    pr_info("attempting bad execution at %px\n", dst);
> > > +    func = setup_function_descriptor(&fdesc, dst);
> > >       func();
> > >       pr_err("FAIL: func returned\n");
> > >   }
> > > @@ -66,16 +81,19 @@ static void execute_user_location(void *dst)
> > >       int copied;
> > >       /* Intentionally crossing kernel/user memory boundary. */
> > > -    void (*func)(void) = dst;
> > > +    void (*func)(void);
> > > +    func_desc_t fdesc;
> > > +    void *do_nothing_text = dereference_function_descriptor(do_nothing);
> > > -    pr_info("attempting ok execution at %px\n", do_nothing);
> > > +    pr_info("attempting ok execution at %px\n", do_nothing_text);
> > >       do_nothing();
> > > -    copied = access_process_vm(current, (unsigned long)dst, do_nothing,
> > > +    copied = access_process_vm(current, (unsigned long)dst,
> > > do_nothing_text,
> > >                      EXEC_SIZE, FOLL_WRITE);
> > >       if (copied < EXEC_SIZE)
> > >           return;
> > > -    pr_info("attempting bad execution at %px\n", func);
> > > +    pr_info("attempting bad execution at %px\n", dst);
> > > +    func = setup_function_descriptor(&fdesc, dst);
> > >       func();
> > >       pr_err("FAIL: func returned\n");
> > >   }
> > > @@ -153,7 +171,8 @@ void lkdtm_EXEC_VMALLOC(void)
> > >   void lkdtm_EXEC_RODATA(void)
> > >   {
> > > -    execute_location(lkdtm_rodata_do_nothing, CODE_AS_IS);
> > > +    execute_location(dereference_function_descriptor(lkdtm_rodata_do_nothing),
> > > 
> > > +             CODE_AS_IS);
> > >   }
> > >   void lkdtm_EXEC_USERSPACE(void)
> > > 

-- 
Kees Cook

Re: [PATCH v3 11/12] lkdtm: Fix execute_[user]_location()

[Kees Cook]

On Sun, Oct 17, 2021 at 02:38:24PM +0200, Christophe Leroy wrote:
> execute_location() and execute_user_location() intent
> to copy do_nothing() text and execute it at a new location.
> However, at the time being it doesn't copy do_nothing() function
> but do_nothing() function descriptor which still points to the
> original text. So at the end it still executes do_nothing() at
> its original location allthough using a copied function descriptor.
> 
> So, fix that by really copying do_nothing() text and build a new
> function descriptor by copying do_nothing() function descriptor and
> updating the target address with the new location.
> 
> Also fix the displayed addresses by dereferencing do_nothing()
> function descriptor.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

This looks good. I might rename variables in the future (e.g. to avoid
the churn from adding _text) but also, that does help keep it clear. :)

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  drivers/misc/lkdtm/perms.c | 37 ++++++++++++++++++++++++++++---------
>  1 file changed, 28 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index 035fcca441f0..1cf24c4a79e9 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -44,19 +44,34 @@ static noinline void do_overwritten(void)
>  	return;
>  }
>  
> +static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
> +{
> +	if (!have_function_descriptors())
> +		return dst;
> +
> +	memcpy(fdesc, do_nothing, sizeof(*fdesc));
> +	fdesc->addr = (unsigned long)dst;
> +	barrier();
> +
> +	return fdesc;
> +}
> +
>  static noinline void execute_location(void *dst, bool write)
>  {
> -	void (*func)(void) = dst;
> +	void (*func)(void);
> +	func_desc_t fdesc;
> +	void *do_nothing_text = dereference_function_descriptor(do_nothing);
>  
> -	pr_info("attempting ok execution at %px\n", do_nothing);
> +	pr_info("attempting ok execution at %px\n", do_nothing_text);
>  	do_nothing();
>  
>  	if (write == CODE_WRITE) {
> -		memcpy(dst, do_nothing, EXEC_SIZE);
> +		memcpy(dst, do_nothing_text, EXEC_SIZE);
>  		flush_icache_range((unsigned long)dst,
>  				   (unsigned long)dst + EXEC_SIZE);
>  	}
> -	pr_info("attempting bad execution at %px\n", func);
> +	pr_info("attempting bad execution at %px\n", dst);
> +	func = setup_function_descriptor(&fdesc, dst);
>  	func();
>  	pr_err("FAIL: func returned\n");
>  }
> @@ -66,16 +81,19 @@ static void execute_user_location(void *dst)
>  	int copied;
>  
>  	/* Intentionally crossing kernel/user memory boundary. */
> -	void (*func)(void) = dst;
> +	void (*func)(void);
> +	func_desc_t fdesc;
> +	void *do_nothing_text = dereference_function_descriptor(do_nothing);
>  
> -	pr_info("attempting ok execution at %px\n", do_nothing);
> +	pr_info("attempting ok execution at %px\n", do_nothing_text);
>  	do_nothing();
>  
> -	copied = access_process_vm(current, (unsigned long)dst, do_nothing,
> +	copied = access_process_vm(current, (unsigned long)dst, do_nothing_text,
>  				   EXEC_SIZE, FOLL_WRITE);
>  	if (copied < EXEC_SIZE)
>  		return;
> -	pr_info("attempting bad execution at %px\n", func);
> +	pr_info("attempting bad execution at %px\n", dst);
> +	func = setup_function_descriptor(&fdesc, dst);
>  	func();
>  	pr_err("FAIL: func returned\n");
>  }
> @@ -153,7 +171,8 @@ void lkdtm_EXEC_VMALLOC(void)
>  
>  void lkdtm_EXEC_RODATA(void)
>  {
> -	execute_location(lkdtm_rodata_do_nothing, CODE_AS_IS);
> +	execute_location(dereference_function_descriptor(lkdtm_rodata_do_nothing),
> +			 CODE_AS_IS);
>  }
>  
>  void lkdtm_EXEC_USERSPACE(void)
> -- 
> 2.31.1
> 

-- 
Kees Cook

[PATCH v3 12/12] lkdtm: Add a test for function descriptors protection

[Christophe Leroy]

Add WRITE_OPD to check that you can't modify function
descriptors.

Gives the following result when function descriptors are
not protected:

	lkdtm: Performing direct entry WRITE_OPD
	lkdtm: attempting bad 16 bytes write at c00000000269b358
	lkdtm: FAIL: survived bad write
	lkdtm: do_nothing was hijacked!

Looks like a standard compiler barrier() is not enough to force
GCC to use the modified function descriptor. Had to add a fake empty
inline assembly to force GCC to reload the function descriptor.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 drivers/misc/lkdtm/core.c  |  1 +
 drivers/misc/lkdtm/lkdtm.h |  1 +
 drivers/misc/lkdtm/perms.c | 22 ++++++++++++++++++++++
 3 files changed, 24 insertions(+)

diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index fe6fd34b8caf..de092aa03b5d 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -148,6 +148,7 @@ static const struct crashtype crashtypes[] = {
 	CRASHTYPE(WRITE_RO),
 	CRASHTYPE(WRITE_RO_AFTER_INIT),
 	CRASHTYPE(WRITE_KERN),
+	CRASHTYPE(WRITE_OPD),
 	CRASHTYPE(REFCOUNT_INC_OVERFLOW),
 	CRASHTYPE(REFCOUNT_ADD_OVERFLOW),
 	CRASHTYPE(REFCOUNT_INC_NOT_ZERO_OVERFLOW),
diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index c212a253edde..188bd0fd6575 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -105,6 +105,7 @@ void __init lkdtm_perms_init(void);
 void lkdtm_WRITE_RO(void);
 void lkdtm_WRITE_RO_AFTER_INIT(void);
 void lkdtm_WRITE_KERN(void);
+void lkdtm_WRITE_OPD(void);
 void lkdtm_EXEC_DATA(void);
 void lkdtm_EXEC_STACK(void);
 void lkdtm_EXEC_KMALLOC(void);
diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 1cf24c4a79e9..2c6aba3ff32b 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -44,6 +44,11 @@ static noinline void do_overwritten(void)
 	return;
 }
 
+static noinline void do_almost_nothing(void)
+{
+	pr_info("do_nothing was hijacked!\n");
+}
+
 static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
 {
 	if (!have_function_descriptors())
@@ -144,6 +149,23 @@ void lkdtm_WRITE_KERN(void)
 	do_overwritten();
 }
 
+void lkdtm_WRITE_OPD(void)
+{
+	size_t size = sizeof(func_desc_t);
+	void (*func)(void) = do_nothing;
+
+	if (!have_function_descriptors()) {
+		pr_info("XFAIL: Platform doesn't use function descriptors.\n");
+		return;
+	}
+	pr_info("attempting bad %zu bytes write at %px\n", size, do_nothing);
+	memcpy(do_nothing, do_almost_nothing, size);
+	pr_err("FAIL: survived bad write\n");
+
+	asm("" : "=m"(func));
+	func();
+}
+
 void lkdtm_EXEC_DATA(void)
 {
 	execute_location(data_area, CODE_WRITE);
-- 
2.31.1

Re: [PATCH v3 12/12] lkdtm: Add a test for function descriptors protection

[Kees Cook]

On Sun, Oct 17, 2021 at 02:38:25PM +0200, Christophe Leroy wrote:
> Add WRITE_OPD to check that you can't modify function
> descriptors.
> 
> Gives the following result when function descriptors are
> not protected:
> 
> 	lkdtm: Performing direct entry WRITE_OPD
> 	lkdtm: attempting bad 16 bytes write at c00000000269b358
> 	lkdtm: FAIL: survived bad write
> 	lkdtm: do_nothing was hijacked!
> 
> Looks like a standard compiler barrier() is not enough to force
> GCC to use the modified function descriptor. Had to add a fake empty
> inline assembly to force GCC to reload the function descriptor.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
>  drivers/misc/lkdtm/core.c  |  1 +
>  drivers/misc/lkdtm/lkdtm.h |  1 +
>  drivers/misc/lkdtm/perms.c | 22 ++++++++++++++++++++++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index fe6fd34b8caf..de092aa03b5d 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -148,6 +148,7 @@ static const struct crashtype crashtypes[] = {
>  	CRASHTYPE(WRITE_RO),
>  	CRASHTYPE(WRITE_RO_AFTER_INIT),
>  	CRASHTYPE(WRITE_KERN),
> +	CRASHTYPE(WRITE_OPD),
>  	CRASHTYPE(REFCOUNT_INC_OVERFLOW),
>  	CRASHTYPE(REFCOUNT_ADD_OVERFLOW),
>  	CRASHTYPE(REFCOUNT_INC_NOT_ZERO_OVERFLOW),
> diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> index c212a253edde..188bd0fd6575 100644
> --- a/drivers/misc/lkdtm/lkdtm.h
> +++ b/drivers/misc/lkdtm/lkdtm.h
> @@ -105,6 +105,7 @@ void __init lkdtm_perms_init(void);
>  void lkdtm_WRITE_RO(void);
>  void lkdtm_WRITE_RO_AFTER_INIT(void);
>  void lkdtm_WRITE_KERN(void);
> +void lkdtm_WRITE_OPD(void);
>  void lkdtm_EXEC_DATA(void);
>  void lkdtm_EXEC_STACK(void);
>  void lkdtm_EXEC_KMALLOC(void);
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index 1cf24c4a79e9..2c6aba3ff32b 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -44,6 +44,11 @@ static noinline void do_overwritten(void)
>  	return;
>  }
>  
> +static noinline void do_almost_nothing(void)
> +{
> +	pr_info("do_nothing was hijacked!\n");
> +}
> +
>  static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
>  {
>  	if (!have_function_descriptors())
> @@ -144,6 +149,23 @@ void lkdtm_WRITE_KERN(void)
>  	do_overwritten();
>  }
>  
> +void lkdtm_WRITE_OPD(void)
> +{
> +	size_t size = sizeof(func_desc_t);
> +	void (*func)(void) = do_nothing;
> +
> +	if (!have_function_descriptors()) {
> +		pr_info("XFAIL: Platform doesn't use function descriptors.\n");
> +		return;
> +	}
> +	pr_info("attempting bad %zu bytes write at %px\n", size, do_nothing);
> +	memcpy(do_nothing, do_almost_nothing, size);
> +	pr_err("FAIL: survived bad write\n");

Non-function-descriptor architectures would successfully crash at the
memcpy too, right? (i.e. for them this is just repeating WRITE_KERN)

I'm pondering the utility of the XFAIL vs just letting is succeed, but I
think it more accurate to say "hey, no OPD" as you have it.

> +
> +	asm("" : "=m"(func));
> +	func();
> +}
> +
>  void lkdtm_EXEC_DATA(void)
>  {
>  	execute_location(data_area, CODE_WRITE);
> -- 
> 2.31.1
> 

One tiny suggestion, since I think you need to respin for the
EXPORT_SYMBOL_GPL() anyway. Please update the selftests too:

diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt
index 6b36b7f5dcf9..243c781f0780 100644
--- a/tools/testing/selftests/lkdtm/tests.txt
+++ b/tools/testing/selftests/lkdtm/tests.txt
@@ -44,6 +44,7 @@ ACCESS_NULL
 WRITE_RO
 WRITE_RO_AFTER_INIT
 WRITE_KERN
+WRITE_OPD
 REFCOUNT_INC_OVERFLOW
 REFCOUNT_ADD_OVERFLOW
 REFCOUNT_INC_NOT_ZERO_OVERFLOW

(Though for the future I've been considering making the selftests an
opt-out list so the "normal" stuff doesn't need to keep getting added
there.)

Thanks!

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

-- 
Kees Cook

Re: [PATCH v3 12/12] lkdtm: Add a test for function descriptors protection

[Christophe Leroy]

Le 11/02/2022 à 02:09, Kees Cook a écrit :
> On Sun, Oct 17, 2021 at 02:38:25PM +0200, Christophe Leroy wrote:
>> Add WRITE_OPD to check that you can't modify function
>> descriptors.
>>
>> Gives the following result when function descriptors are
>> not protected:
>>
>> 	lkdtm: Performing direct entry WRITE_OPD
>> 	lkdtm: attempting bad 16 bytes write at c00000000269b358
>> 	lkdtm: FAIL: survived bad write
>> 	lkdtm: do_nothing was hijacked!
>>
>> Looks like a standard compiler barrier() is not enough to force
>> GCC to use the modified function descriptor. Had to add a fake empty
>> inline assembly to force GCC to reload the function descriptor.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>> ---
>>   drivers/misc/lkdtm/core.c  |  1 +
>>   drivers/misc/lkdtm/lkdtm.h |  1 +
>>   drivers/misc/lkdtm/perms.c | 22 ++++++++++++++++++++++
>>   3 files changed, 24 insertions(+)
>>
>> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
>> index fe6fd34b8caf..de092aa03b5d 100644
>> --- a/drivers/misc/lkdtm/core.c
>> +++ b/drivers/misc/lkdtm/core.c
>> @@ -148,6 +148,7 @@ static const struct crashtype crashtypes[] = {
>>   	CRASHTYPE(WRITE_RO),
>>   	CRASHTYPE(WRITE_RO_AFTER_INIT),
>>   	CRASHTYPE(WRITE_KERN),
>> +	CRASHTYPE(WRITE_OPD),
>>   	CRASHTYPE(REFCOUNT_INC_OVERFLOW),
>>   	CRASHTYPE(REFCOUNT_ADD_OVERFLOW),
>>   	CRASHTYPE(REFCOUNT_INC_NOT_ZERO_OVERFLOW),
>> diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
>> index c212a253edde..188bd0fd6575 100644
>> --- a/drivers/misc/lkdtm/lkdtm.h
>> +++ b/drivers/misc/lkdtm/lkdtm.h
>> @@ -105,6 +105,7 @@ void __init lkdtm_perms_init(void);
>>   void lkdtm_WRITE_RO(void);
>>   void lkdtm_WRITE_RO_AFTER_INIT(void);
>>   void lkdtm_WRITE_KERN(void);
>> +void lkdtm_WRITE_OPD(void);
>>   void lkdtm_EXEC_DATA(void);
>>   void lkdtm_EXEC_STACK(void);
>>   void lkdtm_EXEC_KMALLOC(void);
>> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
>> index 1cf24c4a79e9..2c6aba3ff32b 100644
>> --- a/drivers/misc/lkdtm/perms.c
>> +++ b/drivers/misc/lkdtm/perms.c
>> @@ -44,6 +44,11 @@ static noinline void do_overwritten(void)
>>   	return;
>>   }
>>   
>> +static noinline void do_almost_nothing(void)
>> +{
>> +	pr_info("do_nothing was hijacked!\n");
>> +}
>> +
>>   static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
>>   {
>>   	if (!have_function_descriptors())
>> @@ -144,6 +149,23 @@ void lkdtm_WRITE_KERN(void)
>>   	do_overwritten();
>>   }
>>   
>> +void lkdtm_WRITE_OPD(void)
>> +{
>> +	size_t size = sizeof(func_desc_t);
>> +	void (*func)(void) = do_nothing;
>> +
>> +	if (!have_function_descriptors()) {
>> +		pr_info("XFAIL: Platform doesn't use function descriptors.\n");
>> +		return;
>> +	}
>> +	pr_info("attempting bad %zu bytes write at %px\n", size, do_nothing);
>> +	memcpy(do_nothing, do_almost_nothing, size);
>> +	pr_err("FAIL: survived bad write\n");
> 
> Non-function-descriptor architectures would successfully crash at the
> memcpy too, right? (i.e. for them this is just repeating WRITE_KERN)

Yes it should. But not for the good reason.

> 
> I'm pondering the utility of the XFAIL vs just letting is succeed, but I
> think it more accurate to say "hey, no OPD" as you have it.
> 
>> +
>> +	asm("" : "=m"(func));
>> +	func();
>> +}
>> +
>>   void lkdtm_EXEC_DATA(void)
>>   {
>>   	execute_location(data_area, CODE_WRITE);
>> -- 
>> 2.31.1
>>
> 
> One tiny suggestion, since I think you need to respin for the
> EXPORT_SYMBOL_GPL() anyway. Please update the selftests too:
> 
> diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt
> index 6b36b7f5dcf9..243c781f0780 100644
> --- a/tools/testing/selftests/lkdtm/tests.txt
> +++ b/tools/testing/selftests/lkdtm/tests.txt
> @@ -44,6 +44,7 @@ ACCESS_NULL
>   WRITE_RO
>   WRITE_RO_AFTER_INIT
>   WRITE_KERN
> +WRITE_OPD
>   REFCOUNT_INC_OVERFLOW
>   REFCOUNT_ADD_OVERFLOW
>   REFCOUNT_INC_NOT_ZERO_OVERFLOW
> 
> (Though for the future I've been considering making the selftests an
> opt-out list so the "normal" stuff doesn't need to keep getting added
> there.)
> 
> Thanks!
> 
> Acked-by: Kees Cook <keescook@chromium.org>
> 

Done.

Thanks
Christophe