[PATCH v5] powerpc: Do not make the entire heap executable

[PATCH v5] powerpc: Do not make the entire heap executable

[Denys Vlasenko]

On 32-bit powerpc the ELF PLT sections of binaries (built with --bss-plt,
or with a toolchain which defaults to it) look like this:

  [17] .sbss             NOBITS          0002aff8 01aff8 000014 00  WA  0   0  4
  [18] .plt              NOBITS          0002b00c 01aff8 000084 00 WAX  0   0  4
  [19] .bss              NOBITS          0002b090 01aff8 0000a4 00  WA  0   0  4

Which results in an ELF load header:

  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  LOAD           0x019c70 0x00029c70 0x00029c70 0x01388 0x014c4 RWE 0x10000

This is all correct, the load region containing the PLT is marked as
executable. Note that the PLT starts at 0002b00c but the file mapping ends at
0002aff8, so the PLT falls in the 0 fill section described by the load header,
and after a page boundary.

Unfortunately the generic ELF loader ignores the X bit in the load headers
when it creates the 0 filled non-file backed mappings. It assumes all of these
mappings are RW BSS sections, which is not the case for PPC.

gcc/ld has an option (--secure-plt) to not do this, this is said to incur
a small performance penalty.

Currently, to support 32-bit binaries with PLT in BSS kernel maps *entire
brk area* with executable rights for all binaries, even --secure-plt ones.

Stop doing that.

Teach the ELF loader to check the X bit in the relevant load header
and create 0 filled anonymous mappings that are executable
if the load header requests that.

The patch was originally posted in 2012 by Jason Gunthorpe
and apparently ignored:

https://lkml.org/lkml/2012/9/30/138

Lightly run-tested.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <paulus@samba.org>
CC: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
CC: Kees Cook <keescook@chromium.org>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Michael Ellerman <mpe@ellerman.id.au>
CC: Florian Weimer <fweimer@redhat.com>
CC: linux-mm@kvack.org
CC: linuxppc-dev@lists.ozlabs.org
CC: linux-kernel@vger.kernel.org
---
Changes since v4:
* if (current->personality & READ_IMPLIES_EXEC), still use VM_EXEC
  for 32-bit executables.

Changes since v3:
* typo fix in commit message
* rebased to current Linus tree

Changes since v2:
* moved capability to map with VM_EXEC into vm_brk_flags()

Changes since v1:
* wrapped lines to not exceed 79 chars
* improved comment
* expanded CC list

 arch/powerpc/include/asm/page.h |  3 ++-
 fs/binfmt_elf.c                 | 30 ++++++++++++++++++++++--------
 include/linux/mm.h              |  1 +
 mm/mmap.c                       | 21 ++++++++++++++++-----
 4 files changed, 41 insertions(+), 14 deletions(-)

diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h
index 56398e7..d188f51 100644
--- a/arch/powerpc/include/asm/page.h
+++ b/arch/powerpc/include/asm/page.h
@@ -230,7 +230,9 @@ extern long long virt_phys_offset;
  * and needs to be executable.  This means the whole heap ends
  * up being executable.
  */
-#define VM_DATA_DEFAULT_FLAGS32	(VM_READ | VM_WRITE | VM_EXEC | \
+#define VM_DATA_DEFAULT_FLAGS32 \
+	(((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0) | \
+				 VM_READ | VM_WRITE | \
 				 VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)
 
 #define VM_DATA_DEFAULT_FLAGS64	(VM_READ | VM_WRITE | \
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 7f6aff3f..2b57b5a 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -91,12 +91,18 @@ static struct linux_binfmt elf_format = {
 
 #define BAD_ADDR(x) ((unsigned long)(x) >= TASK_SIZE)
 
-static int set_brk(unsigned long start, unsigned long end)
+static int set_brk(unsigned long start, unsigned long end, int prot)
 {
 	start = ELF_PAGEALIGN(start);
 	end = ELF_PAGEALIGN(end);
 	if (end > start) {
-		int error = vm_brk(start, end - start);
+		/*
+		 * Map the last of the bss segment.
+		 * If the header is requesting these pages to be
+		 * executable, honour that (ppc32 needs this).
+		 */
+		int error = vm_brk_flags(start, end - start,
+				prot & PROT_EXEC ? VM_EXEC : 0);
 		if (error)
 			return error;
 	}
@@ -524,6 +530,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
 	unsigned long load_addr = 0;
 	int load_addr_set = 0;
 	unsigned long last_bss = 0, elf_bss = 0;
+	int bss_prot = 0;
 	unsigned long error = ~0UL;
 	unsigned long total_size;
 	int i;
@@ -606,8 +613,10 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
 			 * elf_bss and last_bss is the bss section.
 			 */
 			k = load_addr + eppnt->p_vaddr + eppnt->p_memsz;
-			if (k > last_bss)
+			if (k > last_bss) {
 				last_bss = k;
+				bss_prot = elf_prot;
+			}
 		}
 	}
 
@@ -623,13 +632,14 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
 	/*
 	 * Next, align both the file and mem bss up to the page size,
 	 * since this is where elf_bss was just zeroed up to, and where
-	 * last_bss will end after the vm_brk() below.
+	 * last_bss will end after the vm_brk_flags() below.
 	 */
 	elf_bss = ELF_PAGEALIGN(elf_bss);
 	last_bss = ELF_PAGEALIGN(last_bss);
 	/* Finally, if there is still more bss to allocate, do it. */
 	if (last_bss > elf_bss) {
-		error = vm_brk(elf_bss, last_bss - elf_bss);
+		error = vm_brk_flags(elf_bss, last_bss - elf_bss,
+				bss_prot & PROT_EXEC ? VM_EXEC : 0);
 		if (error)
 			goto out;
 	}
@@ -674,6 +684,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	unsigned long error;
 	struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
 	unsigned long elf_bss, elf_brk;
+	int bss_prot = 0;
 	int retval, i;
 	unsigned long elf_entry;
 	unsigned long interp_load_addr = 0;
@@ -881,7 +892,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
 			   before this one. Map anonymous pages, if needed,
 			   and clear the area.  */
 			retval = set_brk(elf_bss + load_bias,
-					 elf_brk + load_bias);
+					 elf_brk + load_bias,
+					 bss_prot);
 			if (retval)
 				goto out_free_dentry;
 			nbyte = ELF_PAGEOFFSET(elf_bss);
@@ -975,8 +987,10 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		if (end_data < k)
 			end_data = k;
 		k = elf_ppnt->p_vaddr + elf_ppnt->p_memsz;
-		if (k > elf_brk)
+		if (k > elf_brk) {
+			bss_prot = elf_prot;
 			elf_brk = k;
+		}
 	}
 
 	loc->elf_ex.e_entry += load_bias;
@@ -992,7 +1006,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	 * mapping in the interpreter, to make sure it doesn't wind
 	 * up getting placed where the bss needs to go.
 	 */
-	retval = set_brk(elf_bss, elf_brk);
+	retval = set_brk(elf_bss, elf_brk, bss_prot);
 	if (retval)
 		goto out_free_dentry;
 	if (likely(elf_bss != elf_brk) && unlikely(padzero(elf_bss))) {
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 08ed53e..3ffa76c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2058,6 +2058,7 @@ static inline void mm_populate(unsigned long addr, unsigned long len) {}
 
 /* These take the mm semaphore themselves */
 extern int __must_check vm_brk(unsigned long, unsigned long);
+extern int __must_check vm_brk_flags(unsigned long, unsigned long, unsigned long);
 extern int vm_munmap(unsigned long, size_t);
 extern unsigned long __must_check vm_mmap(struct file *, unsigned long,
         unsigned long, unsigned long,
diff --git a/mm/mmap.c b/mm/mmap.c
index ca9d91b..4d5b3f3 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2653,11 +2653,11 @@ static inline void verify_mm_writelocked(struct mm_struct *mm)
  *  anonymous maps.  eventually we may be able to do some
  *  brk-specific accounting here.
  */
-static int do_brk(unsigned long addr, unsigned long request)
+static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma, *prev;
-	unsigned long flags, len;
+	unsigned long len;
 	struct rb_node **rb_link, *rb_parent;
 	pgoff_t pgoff = addr >> PAGE_SHIFT;
 	int error;
@@ -2668,7 +2668,7 @@ static int do_brk(unsigned long addr, unsigned long request)
 	if (!len)
 		return 0;
 
-	flags = VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
+	flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
 
 	error = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
 	if (offset_in_page(error))
@@ -2736,7 +2736,12 @@ out:
 	return 0;
 }
 
-int vm_brk(unsigned long addr, unsigned long len)
+static int do_brk(unsigned long addr, unsigned long len)
+{
+	return do_brk_flags(addr, len, 0);
+}
+
+int vm_brk_flags(unsigned long addr, unsigned long len, unsigned long flags)
 {
 	struct mm_struct *mm = current->mm;
 	int ret;
@@ -2745,13 +2750,19 @@ int vm_brk(unsigned long addr, unsigned long len)
 	if (down_write_killable(&mm->mmap_sem))
 		return -EINTR;
 
-	ret = do_brk(addr, len);
+	ret = do_brk_flags(addr, len, flags);
 	populate = ((mm->def_flags & VM_LOCKED) != 0);
 	up_write(&mm->mmap_sem);
 	if (populate && !ret)
 		mm_populate(addr, len);
 	return ret;
 }
+EXPORT_SYMBOL(vm_brk_flags);
+
+int vm_brk(unsigned long addr, unsigned long len)
+{
+	return vm_brk_flags(addr, len, 0);
+}
 EXPORT_SYMBOL(vm_brk);
 
 /* Release all mmaps. */
-- 
2.9.2

Re: [PATCH v5] powerpc: Do not make the entire heap executable

[Michael Ellerman]

Denys Vlasenko <dvlasenk@redhat.com> writes:

> On 32-bit powerpc the ELF PLT sections of binaries (built with --bss-plt,
> or with a toolchain which defaults to it) look like this:

Or (it seems), for all programs built with -pg (profiling).

>   [17] .sbss             NOBITS          0002aff8 01aff8 000014 00  WA  0   0  4
>   [18] .plt              NOBITS          0002b00c 01aff8 000084 00 WAX  0   0  4
>   [19] .bss              NOBITS          0002b090 01aff8 0000a4 00  WA  0   0  4
>
> Which results in an ELF load header:
>
>   Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
>   LOAD           0x019c70 0x00029c70 0x00029c70 0x01388 0x014c4 RWE 0x10000
>
> This is all correct, the load region containing the PLT is marked as
> executable. Note that the PLT starts at 0002b00c but the file mapping ends at
> 0002aff8, so the PLT falls in the 0 fill section described by the load header,
> and after a page boundary.
>
> Unfortunately the generic ELF loader ignores the X bit in the load headers
> when it creates the 0 filled non-file backed mappings. It assumes all of these
> mappings are RW BSS sections, which is not the case for PPC.
>
> gcc/ld has an option (--secure-plt) to not do this, this is said to incur
> a small performance penalty.
>
> Currently, to support 32-bit binaries with PLT in BSS kernel maps *entire
> brk area* with executable rights for all binaries, even --secure-plt ones.
>
> Stop doing that.
>
> Teach the ELF loader to check the X bit in the relevant load header
> and create 0 filled anonymous mappings that are executable
> if the load header requests that.
...
>
> Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> CC: Michael Ellerman <mpe@ellerman.id.au>

This looks OK to me, I've tested it with --bss-plt and --secure-plt and
also -pg.

So for the powerpc part I'll give you an:

Acked-by: Michael Ellerman <mpe@ellerman.id.au>

But this is not really a powerpc patch, and I'm not an ELF expert. So
I'm not comfortable merging it via the powerpc tree. It doesn't look
like we really have a maintainer for binfmt_elf.c, so I'm not sure who
should be acking that part.

I've added Al Viro to Cc, he maintains fs/ and might be interested.

I've also added Andrew Morton who might be happy to put this in his
tree, and see if anyone complains?

Also here: https://patchwork.ozlabs.org/patch/661595/

cheers

> ---
> Changes since v4:
> * if (current->personality & READ_IMPLIES_EXEC), still use VM_EXEC
>   for 32-bit executables.
>
> Changes since v3:
> * typo fix in commit message
> * rebased to current Linus tree
>
> Changes since v2:
> * moved capability to map with VM_EXEC into vm_brk_flags()
>
> Changes since v1:
> * wrapped lines to not exceed 79 chars
> * improved comment
> * expanded CC list
>
>  arch/powerpc/include/asm/page.h |  3 ++-
>  fs/binfmt_elf.c                 | 30 ++++++++++++++++++++++--------
>  include/linux/mm.h              |  1 +
>  mm/mmap.c                       | 21 ++++++++++++++++-----
>  4 files changed, 41 insertions(+), 14 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h
> index 56398e7..d188f51 100644
> --- a/arch/powerpc/include/asm/page.h
> +++ b/arch/powerpc/include/asm/page.h
> @@ -230,7 +230,9 @@ extern long long virt_phys_offset;
>   * and needs to be executable.  This means the whole heap ends
>   * up being executable.
>   */
> -#define VM_DATA_DEFAULT_FLAGS32	(VM_READ | VM_WRITE | VM_EXEC | \
> +#define VM_DATA_DEFAULT_FLAGS32 \
> +	(((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0) | \
> +				 VM_READ | VM_WRITE | \
>  				 VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)
>  
>  #define VM_DATA_DEFAULT_FLAGS64	(VM_READ | VM_WRITE | \
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 7f6aff3f..2b57b5a 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -91,12 +91,18 @@ static struct linux_binfmt elf_format = {
>  
>  #define BAD_ADDR(x) ((unsigned long)(x) >= TASK_SIZE)
>  
> -static int set_brk(unsigned long start, unsigned long end)
> +static int set_brk(unsigned long start, unsigned long end, int prot)
>  {
>  	start = ELF_PAGEALIGN(start);
>  	end = ELF_PAGEALIGN(end);
>  	if (end > start) {
> -		int error = vm_brk(start, end - start);
> +		/*
> +		 * Map the last of the bss segment.
> +		 * If the header is requesting these pages to be
> +		 * executable, honour that (ppc32 needs this).
> +		 */
> +		int error = vm_brk_flags(start, end - start,
> +				prot & PROT_EXEC ? VM_EXEC : 0);
>  		if (error)
>  			return error;
>  	}
> @@ -524,6 +530,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>  	unsigned long load_addr = 0;
>  	int load_addr_set = 0;
>  	unsigned long last_bss = 0, elf_bss = 0;
> +	int bss_prot = 0;
>  	unsigned long error = ~0UL;
>  	unsigned long total_size;
>  	int i;
> @@ -606,8 +613,10 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>  			 * elf_bss and last_bss is the bss section.
>  			 */
>  			k = load_addr + eppnt->p_vaddr + eppnt->p_memsz;
> -			if (k > last_bss)
> +			if (k > last_bss) {
>  				last_bss = k;
> +				bss_prot = elf_prot;
> +			}
>  		}
>  	}
>  
> @@ -623,13 +632,14 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>  	/*
>  	 * Next, align both the file and mem bss up to the page size,
>  	 * since this is where elf_bss was just zeroed up to, and where
> -	 * last_bss will end after the vm_brk() below.
> +	 * last_bss will end after the vm_brk_flags() below.
>  	 */
>  	elf_bss = ELF_PAGEALIGN(elf_bss);
>  	last_bss = ELF_PAGEALIGN(last_bss);
>  	/* Finally, if there is still more bss to allocate, do it. */
>  	if (last_bss > elf_bss) {
> -		error = vm_brk(elf_bss, last_bss - elf_bss);
> +		error = vm_brk_flags(elf_bss, last_bss - elf_bss,
> +				bss_prot & PROT_EXEC ? VM_EXEC : 0);
>  		if (error)
>  			goto out;
>  	}
> @@ -674,6 +684,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  	unsigned long error;
>  	struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
>  	unsigned long elf_bss, elf_brk;
> +	int bss_prot = 0;
>  	int retval, i;
>  	unsigned long elf_entry;
>  	unsigned long interp_load_addr = 0;
> @@ -881,7 +892,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  			   before this one. Map anonymous pages, if needed,
>  			   and clear the area.  */
>  			retval = set_brk(elf_bss + load_bias,
> -					 elf_brk + load_bias);
> +					 elf_brk + load_bias,
> +					 bss_prot);
>  			if (retval)
>  				goto out_free_dentry;
>  			nbyte = ELF_PAGEOFFSET(elf_bss);
> @@ -975,8 +987,10 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  		if (end_data < k)
>  			end_data = k;
>  		k = elf_ppnt->p_vaddr + elf_ppnt->p_memsz;
> -		if (k > elf_brk)
> +		if (k > elf_brk) {
> +			bss_prot = elf_prot;
>  			elf_brk = k;
> +		}
>  	}
>  
>  	loc->elf_ex.e_entry += load_bias;
> @@ -992,7 +1006,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  	 * mapping in the interpreter, to make sure it doesn't wind
>  	 * up getting placed where the bss needs to go.
>  	 */
> -	retval = set_brk(elf_bss, elf_brk);
> +	retval = set_brk(elf_bss, elf_brk, bss_prot);
>  	if (retval)
>  		goto out_free_dentry;
>  	if (likely(elf_bss != elf_brk) && unlikely(padzero(elf_bss))) {
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 08ed53e..3ffa76c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2058,6 +2058,7 @@ static inline void mm_populate(unsigned long addr, unsigned long len) {}
>  
>  /* These take the mm semaphore themselves */
>  extern int __must_check vm_brk(unsigned long, unsigned long);
> +extern int __must_check vm_brk_flags(unsigned long, unsigned long, unsigned long);
>  extern int vm_munmap(unsigned long, size_t);
>  extern unsigned long __must_check vm_mmap(struct file *, unsigned long,
>          unsigned long, unsigned long,
> diff --git a/mm/mmap.c b/mm/mmap.c
> index ca9d91b..4d5b3f3 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2653,11 +2653,11 @@ static inline void verify_mm_writelocked(struct mm_struct *mm)
>   *  anonymous maps.  eventually we may be able to do some
>   *  brk-specific accounting here.
>   */
> -static int do_brk(unsigned long addr, unsigned long request)
> +static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
>  {
>  	struct mm_struct *mm = current->mm;
>  	struct vm_area_struct *vma, *prev;
> -	unsigned long flags, len;
> +	unsigned long len;
>  	struct rb_node **rb_link, *rb_parent;
>  	pgoff_t pgoff = addr >> PAGE_SHIFT;
>  	int error;
> @@ -2668,7 +2668,7 @@ static int do_brk(unsigned long addr, unsigned long request)
>  	if (!len)
>  		return 0;
>  
> -	flags = VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
> +	flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
>  
>  	error = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
>  	if (offset_in_page(error))
> @@ -2736,7 +2736,12 @@ out:
>  	return 0;
>  }
>  
> -int vm_brk(unsigned long addr, unsigned long len)
> +static int do_brk(unsigned long addr, unsigned long len)
> +{
> +	return do_brk_flags(addr, len, 0);
> +}
> +
> +int vm_brk_flags(unsigned long addr, unsigned long len, unsigned long flags)
>  {
>  	struct mm_struct *mm = current->mm;
>  	int ret;
> @@ -2745,13 +2750,19 @@ int vm_brk(unsigned long addr, unsigned long len)
>  	if (down_write_killable(&mm->mmap_sem))
>  		return -EINTR;
>  
> -	ret = do_brk(addr, len);
> +	ret = do_brk_flags(addr, len, flags);
>  	populate = ((mm->def_flags & VM_LOCKED) != 0);
>  	up_write(&mm->mmap_sem);
>  	if (populate && !ret)
>  		mm_populate(addr, len);
>  	return ret;
>  }
> +EXPORT_SYMBOL(vm_brk_flags);
> +
> +int vm_brk(unsigned long addr, unsigned long len)
> +{
> +	return vm_brk_flags(addr, len, 0);
> +}
>  EXPORT_SYMBOL(vm_brk);
>  
>  /* Release all mmaps. */
> -- 
> 2.9.2

Re: [PATCH v5] powerpc: Do not make the entire heap executable

[Jason Gunthorpe]

On Wed, Sep 28, 2016 at 11:42:11AM +1000, Michael Ellerman wrote:

> But this is not really a powerpc patch, and I'm not an ELF expert. So
> I'm not comfortable merging it via the powerpc tree. It doesn't look
> like we really have a maintainer for binfmt_elf.c, so I'm not sure who
> should be acking that part.

Thanks a bunch for looking at this Michael.

> I've added Al Viro to Cc, he maintains fs/ and might be interested.

> I've also added Andrew Morton who might be happy to put this in his
> tree, and see if anyone complains?

For those added to the CC, I would re-state my original commit message
more clearly.

My research showed that the ELF loader bug fixed in this patch is the
root cause bug fix required to implement this hunk:

> > -#define VM_DATA_DEFAULT_FLAGS32	(VM_READ | VM_WRITE | VM_EXEC | \
> > +#define VM_DATA_DEFAULT_FLAGS32 \
> > +	(((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0) | \
> > +				 VM_READ | VM_WRITE | \
> >  				 VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)

Eg that 32 bit powerpc currently unconditionally injects writable,
executable pages into a user space process.

This critically undermines all the W^X security work that has been
done in the tool chain and user space by the PPC community.

I would encourage people to view this as an important security patch
for 32 bit powerpc environments.

Regards,
Jason

Re: [PATCH v5] powerpc: Do not make the entire heap executable

[Florian Weimer]

* Jason Gunthorpe:

> Eg that 32 bit powerpc currently unconditionally injects writable,
> executable pages into a user space process.
>
> This critically undermines all the W^X security work that has been
> done in the tool chain and user space by the PPC community.

Exactly, this is how we found it.  I have pretty extensive execmod
tests, and I'm going to put them into glibc eventually.  It would be
nice to cut down the number of architectures where it will fail.
(Even if you don't believe in security hardening.)

Re: [PATCH v5] powerpc: Do not make the entire heap executable

[Kees Cook]

On Tue, Sep 27, 2016 at 6:42 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
> Denys Vlasenko <dvlasenk@redhat.com> writes:
>
>> On 32-bit powerpc the ELF PLT sections of binaries (built with --bss-plt,
>> or with a toolchain which defaults to it) look like this:
>
> Or (it seems), for all programs built with -pg (profiling).
>
>>   [17] .sbss             NOBITS          0002aff8 01aff8 000014 00  WA  0   0  4
>>   [18] .plt              NOBITS          0002b00c 01aff8 000084 00 WAX  0   0  4
>>   [19] .bss              NOBITS          0002b090 01aff8 0000a4 00  WA  0   0  4
>>
>> Which results in an ELF load header:
>>
>>   Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
>>   LOAD           0x019c70 0x00029c70 0x00029c70 0x01388 0x014c4 RWE 0x10000
>>
>> This is all correct, the load region containing the PLT is marked as
>> executable. Note that the PLT starts at 0002b00c but the file mapping ends at
>> 0002aff8, so the PLT falls in the 0 fill section described by the load header,
>> and after a page boundary.
>>
>> Unfortunately the generic ELF loader ignores the X bit in the load headers
>> when it creates the 0 filled non-file backed mappings. It assumes all of these
>> mappings are RW BSS sections, which is not the case for PPC.
>>
>> gcc/ld has an option (--secure-plt) to not do this, this is said to incur
>> a small performance penalty.
>>
>> Currently, to support 32-bit binaries with PLT in BSS kernel maps *entire
>> brk area* with executable rights for all binaries, even --secure-plt ones.
>>
>> Stop doing that.
>>
>> Teach the ELF loader to check the X bit in the relevant load header
>> and create 0 filled anonymous mappings that are executable
>> if the load header requests that.
> ...
>>
>> Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
>> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
>> Reviewed-by: Kees Cook <keescook@chromium.org>
>> CC: Michael Ellerman <mpe@ellerman.id.au>
>
> This looks OK to me, I've tested it with --bss-plt and --secure-plt and
> also -pg.
>
> So for the powerpc part I'll give you an:
>
> Acked-by: Michael Ellerman <mpe@ellerman.id.au>
>
> But this is not really a powerpc patch, and I'm not an ELF expert. So
> I'm not comfortable merging it via the powerpc tree. It doesn't look
> like we really have a maintainer for binfmt_elf.c, so I'm not sure who
> should be acking that part.

I've done a bunch of recent work in and around binfmt_elf.c, so I'd be
happy to change my "Reviewed-by" to an "Acked-by" a change that I
suggested earlier. (Details below.)

> I've added Al Viro to Cc, he maintains fs/ and might be interested.
>
> I've also added Andrew Morton who might be happy to put this in his
> tree, and see if anyone complains?
>
> Also here: https://patchwork.ozlabs.org/patch/661595/
>
> cheers
>
>> ---
>> Changes since v4:
>> * if (current->personality & READ_IMPLIES_EXEC), still use VM_EXEC
>>   for 32-bit executables.
>>
>> Changes since v3:
>> * typo fix in commit message
>> * rebased to current Linus tree
>>
>> Changes since v2:
>> * moved capability to map with VM_EXEC into vm_brk_flags()
>>
>> Changes since v1:
>> * wrapped lines to not exceed 79 chars
>> * improved comment
>> * expanded CC list
>>
>>  arch/powerpc/include/asm/page.h |  3 ++-
>>  fs/binfmt_elf.c                 | 30 ++++++++++++++++++++++--------
>>  include/linux/mm.h              |  1 +
>>  mm/mmap.c                       | 21 ++++++++++++++++-----
>>  4 files changed, 41 insertions(+), 14 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h
>> index 56398e7..d188f51 100644
>> --- a/arch/powerpc/include/asm/page.h
>> +++ b/arch/powerpc/include/asm/page.h
>> @@ -230,7 +230,9 @@ extern long long virt_phys_offset;
>>   * and needs to be executable.  This means the whole heap ends
>>   * up being executable.
>>   */
>> -#define VM_DATA_DEFAULT_FLAGS32      (VM_READ | VM_WRITE | VM_EXEC | \
>> +#define VM_DATA_DEFAULT_FLAGS32 \
>> +     (((current->personality & READ_IMPLIES_EXEC) ? VM_EXEC : 0) | \
>> +                              VM_READ | VM_WRITE | \
>>                                VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC)
>>
>>  #define VM_DATA_DEFAULT_FLAGS64      (VM_READ | VM_WRITE | \
>> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
>> index 7f6aff3f..2b57b5a 100644
>> --- a/fs/binfmt_elf.c
>> +++ b/fs/binfmt_elf.c
>> @@ -91,12 +91,18 @@ static struct linux_binfmt elf_format = {
>>
>>  #define BAD_ADDR(x) ((unsigned long)(x) >= TASK_SIZE)
>>
>> -static int set_brk(unsigned long start, unsigned long end)
>> +static int set_brk(unsigned long start, unsigned long end, int prot)
>>  {
>>       start = ELF_PAGEALIGN(start);
>>       end = ELF_PAGEALIGN(end);
>>       if (end > start) {
>> -             int error = vm_brk(start, end - start);
>> +             /*
>> +              * Map the last of the bss segment.
>> +              * If the header is requesting these pages to be
>> +              * executable, honour that (ppc32 needs this).
>> +              */
>> +             int error = vm_brk_flags(start, end - start,
>> +                             prot & PROT_EXEC ? VM_EXEC : 0);
>>               if (error)
>>                       return error;
>>       }
>> @@ -524,6 +530,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>>       unsigned long load_addr = 0;
>>       int load_addr_set = 0;
>>       unsigned long last_bss = 0, elf_bss = 0;
>> +     int bss_prot = 0;
>>       unsigned long error = ~0UL;
>>       unsigned long total_size;
>>       int i;
>> @@ -606,8 +613,10 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>>                        * elf_bss and last_bss is the bss section.
>>                        */
>>                       k = load_addr + eppnt->p_vaddr + eppnt->p_memsz;
>> -                     if (k > last_bss)
>> +                     if (k > last_bss) {
>>                               last_bss = k;
>> +                             bss_prot = elf_prot;
>> +                     }
>>               }
>>       }
>>
>> @@ -623,13 +632,14 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>>       /*
>>        * Next, align both the file and mem bss up to the page size,
>>        * since this is where elf_bss was just zeroed up to, and where
>> -      * last_bss will end after the vm_brk() below.
>> +      * last_bss will end after the vm_brk_flags() below.
>>        */
>>       elf_bss = ELF_PAGEALIGN(elf_bss);
>>       last_bss = ELF_PAGEALIGN(last_bss);
>>       /* Finally, if there is still more bss to allocate, do it. */
>>       if (last_bss > elf_bss) {
>> -             error = vm_brk(elf_bss, last_bss - elf_bss);
>> +             error = vm_brk_flags(elf_bss, last_bss - elf_bss,
>> +                             bss_prot & PROT_EXEC ? VM_EXEC : 0);
>>               if (error)
>>                       goto out;
>>       }
>> @@ -674,6 +684,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>>       unsigned long error;
>>       struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
>>       unsigned long elf_bss, elf_brk;
>> +     int bss_prot = 0;
>>       int retval, i;
>>       unsigned long elf_entry;
>>       unsigned long interp_load_addr = 0;
>> @@ -881,7 +892,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
>>                          before this one. Map anonymous pages, if needed,
>>                          and clear the area.  */
>>                       retval = set_brk(elf_bss + load_bias,
>> -                                      elf_brk + load_bias);
>> +                                      elf_brk + load_bias,
>> +                                      bss_prot);
>>                       if (retval)
>>                               goto out_free_dentry;
>>                       nbyte = ELF_PAGEOFFSET(elf_bss);
>> @@ -975,8 +987,10 @@ static int load_elf_binary(struct linux_binprm *bprm)
>>               if (end_data < k)
>>                       end_data = k;
>>               k = elf_ppnt->p_vaddr + elf_ppnt->p_memsz;
>> -             if (k > elf_brk)
>> +             if (k > elf_brk) {
>> +                     bss_prot = elf_prot;
>>                       elf_brk = k;
>> +             }
>>       }
>>
>>       loc->elf_ex.e_entry += load_bias;
>> @@ -992,7 +1006,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>>        * mapping in the interpreter, to make sure it doesn't wind
>>        * up getting placed where the bss needs to go.
>>        */
>> -     retval = set_brk(elf_bss, elf_brk);
>> +     retval = set_brk(elf_bss, elf_brk, bss_prot);
>>       if (retval)
>>               goto out_free_dentry;
>>       if (likely(elf_bss != elf_brk) && unlikely(padzero(elf_bss))) {
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 08ed53e..3ffa76c 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2058,6 +2058,7 @@ static inline void mm_populate(unsigned long addr, unsigned long len) {}
>>
>>  /* These take the mm semaphore themselves */
>>  extern int __must_check vm_brk(unsigned long, unsigned long);
>> +extern int __must_check vm_brk_flags(unsigned long, unsigned long, unsigned long);
>>  extern int vm_munmap(unsigned long, size_t);
>>  extern unsigned long __must_check vm_mmap(struct file *, unsigned long,
>>          unsigned long, unsigned long,
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index ca9d91b..4d5b3f3 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -2653,11 +2653,11 @@ static inline void verify_mm_writelocked(struct mm_struct *mm)
>>   *  anonymous maps.  eventually we may be able to do some
>>   *  brk-specific accounting here.
>>   */
>> -static int do_brk(unsigned long addr, unsigned long request)
>> +static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
>>  {
>>       struct mm_struct *mm = current->mm;
>>       struct vm_area_struct *vma, *prev;
>> -     unsigned long flags, len;
>> +     unsigned long len;
>>       struct rb_node **rb_link, *rb_parent;
>>       pgoff_t pgoff = addr >> PAGE_SHIFT;
>>       int error;
>> @@ -2668,7 +2668,7 @@ static int do_brk(unsigned long addr, unsigned long request)
>>       if (!len)
>>               return 0;
>>
>> -     flags = VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
>> +     flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;

This is where the flags are actually built from what's coming in
through the newly created exported function vm_brk_flags() below. The
only flag we're acting on is VM_EXEC (passed in from set_brk() above).
I think do_brk_flags() should mask the valid flags, or we'll regret it
in the future. I'd like to see something like:

    /* Until we need other flags, refuse anything except VM_EXEC. */
    if ((flags & (~VM_EXEC)) != 0)
        return -EINVAL;
    flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;

With this, I'd be happy to Ack. :)

-Kees

>>
>>       error = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
>>       if (offset_in_page(error))
>> @@ -2736,7 +2736,12 @@ out:
>>       return 0;
>>  }
>>
>> -int vm_brk(unsigned long addr, unsigned long len)
>> +static int do_brk(unsigned long addr, unsigned long len)
>> +{
>> +     return do_brk_flags(addr, len, 0);
>> +}
>> +
>> +int vm_brk_flags(unsigned long addr, unsigned long len, unsigned long flags)
>>  {
>>       struct mm_struct *mm = current->mm;
>>       int ret;
>> @@ -2745,13 +2750,19 @@ int vm_brk(unsigned long addr, unsigned long len)
>>       if (down_write_killable(&mm->mmap_sem))
>>               return -EINTR;
>>
>> -     ret = do_brk(addr, len);
>> +     ret = do_brk_flags(addr, len, flags);
>>       populate = ((mm->def_flags & VM_LOCKED) != 0);
>>       up_write(&mm->mmap_sem);
>>       if (populate && !ret)
>>               mm_populate(addr, len);
>>       return ret;
>>  }
>> +EXPORT_SYMBOL(vm_brk_flags);
>> +
>> +int vm_brk(unsigned long addr, unsigned long len)
>> +{
>> +     return vm_brk_flags(addr, len, 0);
>> +}
>>  EXPORT_SYMBOL(vm_brk);
>>
>>  /* Release all mmaps. */
>> --
>> 2.9.2



-- 
Kees Cook
Nexus Security

Re: [PATCH v5] powerpc: Do not make the entire heap executable

[Oleg Nesterov]

On 09/28, Kees Cook wrote:
>
> This is where the flags are actually built from what's coming in
> through the newly created exported function vm_brk_flags() below. The
> only flag we're acting on is VM_EXEC (passed in from set_brk() above).
> I think do_brk_flags() should mask the valid flags, or we'll regret it
> in the future. I'd like to see something like:
>
>     /* Until we need other flags, refuse anything except VM_EXEC. */
>     if ((flags & (~VM_EXEC)) != 0)
>         return -EINVAL;
>     flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;

I tried to suggest this too. In particular it would be simply wrong
to accept VM_LOCKED in flags.

Oleg.