[PATCH v1] compiler: prevent dead store elimination

[PATCH v1] compiler: prevent dead store elimination

[Roel Kluin]

Due to optimization A call to memset() may be removed as a dead store when
the buffer is not used after its value is overwritten. The new function
secure_bzero() ensures a section of memory is padded with zeroes.

>From the GCC manual, section 5.37:
If your assembler instructions access memory in an unpredictable
fashion, add `memory' to the list of clobbered registers. This will
cause GCC to not keep memory values cached in registers across the
assembler instruction and not optimize stores or loads to that memory.

Every byte in the [p,p+n[ range must be used. If you only use the
first byte, via e.g. asm("" :: "m"(*(char*)p)), then the compiler
_will_ skip scrubbing bytes beyond the first. This works with
gcc-3.2.3 up to gcc-4.4.3.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
---
 include/linux/compiler-gcc.h   |   11 +++++++++++
 include/linux/compiler-intel.h |    2 ++
 include/linux/string.h         |    1 +
 lib/string.c                   |   13 +++++++++++++
 4 files changed, 27 insertions(+), 0 deletions(-)

Thanks all for the required information, checkpatch.pl and compile
tested. In my (non-kernel) testcase this prevents dead store elimination.
Comments?

Roel

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 73dcf80..0799938 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -12,6 +12,17 @@
 #define barrier() __asm__ __volatile__("": : :"memory")
 
 /*
+ * Dead store elimination (DSE) is an optimization that may remove a write to
+ * a buffer that is not used anymore. Use ARRAY_PREVENT_DSE after a write when
+ * the scrub is required for security reasons.
+ */
+#define ARRAY_PREVENT_DSE(p, n)				\
+	do {						\
+		struct __scrub { char c[n]; };		\
+		asm("" : : "m"(*(struct __scrub *)p));	\
+	} while (0)
+
+/*
  * This macro obfuscates arithmetic on a variable address so that gcc
  * shouldn't recognize the original var, and make assumptions about it.
  *
diff --git a/include/linux/compiler-intel.h b/include/linux/compiler-intel.h
index d8e636e..e8e11f3 100644
--- a/include/linux/compiler-intel.h
+++ b/include/linux/compiler-intel.h
@@ -14,9 +14,11 @@
  * It uses intrinsics to do the equivalent things.
  */
 #undef barrier
+#undef ARRAY_PREVENT_DSE
 #undef RELOC_HIDE
 
 #define barrier() __memory_barrier()
+#define ARRAY_PREVENT_DSE(p, n)
 
 #define RELOC_HIDE(ptr, off)					\
   ({ unsigned long __ptr;					\
diff --git a/include/linux/string.h b/include/linux/string.h
index a716ee2..36213e6 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -118,6 +118,7 @@ extern void * memchr(const void *,int,__kernel_size_t);
 extern char *kstrdup(const char *s, gfp_t gfp);
 extern char *kstrndup(const char *s, size_t len, gfp_t gfp);
 extern void *kmemdup(const void *src, size_t len, gfp_t gfp);
+extern void secure_bzero(void *, __kernel_size_t);
 
 extern char **argv_split(gfp_t gfp, const char *str, int *argcp);
 extern void argv_free(char **argv);
diff --git a/lib/string.c b/lib/string.c
index a1cdcfc..588ac31 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -559,6 +559,19 @@ void *memset(void *s, int c, size_t count)
 EXPORT_SYMBOL(memset);
 #endif
 
+/**
+ * secure_bzero - Call memset to fill a region of memory with zeroes and
+ * ensure this memset is not removed due to dead store elimination.
+ * @p: Pointer to the start of the area.
+ * @n: The size of the area.
+ */
+void secure_bzero(void *p, size_t n)
+{
+	memset(p, 0, n);
+	ARRAY_PREVENT_DSE(p, n);
+}
+EXPORT_SYMBOL(secure_bzero);
+
 #ifndef __HAVE_ARCH_MEMCPY
 /**
  * memcpy - Copy one area of memory to another

[PATCH v1] compiler: prevent dead store elimination

[Roel Kluin]

Due to optimization A call to memset() may be removed as a dead store when
the buffer is not used after its value is overwritten. The new function
secure_bzero() ensures a section of memory is padded with zeroes.

>From the GCC manual, section 5.37:
If your assembler instructions access memory in an unpredictable
fashion, add `memory' to the list of clobbered registers. This will
cause GCC to not keep memory values cached in registers across the
assembler instruction and not optimize stores or loads to that memory.

Every byte in the [p,p+n[ range must be used. If you only use the
first byte, via e.g. asm("" :: "m"(*(char*)p)), then the compiler
_will_ skip scrubbing bytes beyond the first. This works with
gcc-3.2.3 up to gcc-4.4.3.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
---
 include/linux/compiler-gcc.h   |   11 +++++++++++
 include/linux/compiler-intel.h |    2 ++
 include/linux/string.h         |    1 +
 lib/string.c                   |   13 +++++++++++++
 4 files changed, 27 insertions(+), 0 deletions(-)

Thanks all for the required information, checkpatch.pl and compile
tested. In my (non-kernel) testcase this prevents dead store elimination.
Comments?

Roel

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 73dcf80..0799938 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -12,6 +12,17 @@
 #define barrier() __asm__ __volatile__("": : :"memory")
 
 /*
+ * Dead store elimination (DSE) is an optimization that may remove a write to
+ * a buffer that is not used anymore. Use ARRAY_PREVENT_DSE after a write when
+ * the scrub is required for security reasons.
+ */
+#define ARRAY_PREVENT_DSE(p, n)				\
+	do {						\
+		struct __scrub { char c[n]; };		\
+		asm("" : : "m"(*(struct __scrub *)p));	\
+	} while (0)
+
+/*
  * This macro obfuscates arithmetic on a variable address so that gcc
  * shouldn't recognize the original var, and make assumptions about it.
  *
diff --git a/include/linux/compiler-intel.h b/include/linux/compiler-intel.h
index d8e636e..e8e11f3 100644
--- a/include/linux/compiler-intel.h
+++ b/include/linux/compiler-intel.h
@@ -14,9 +14,11 @@
  * It uses intrinsics to do the equivalent things.
  */
 #undef barrier
+#undef ARRAY_PREVENT_DSE
 #undef RELOC_HIDE
 
 #define barrier() __memory_barrier()
+#define ARRAY_PREVENT_DSE(p, n)
 
 #define RELOC_HIDE(ptr, off)					\
   ({ unsigned long __ptr;					\
diff --git a/include/linux/string.h b/include/linux/string.h
index a716ee2..36213e6 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -118,6 +118,7 @@ extern void * memchr(const void *,int,__kernel_size_t);
 extern char *kstrdup(const char *s, gfp_t gfp);
 extern char *kstrndup(const char *s, size_t len, gfp_t gfp);
 extern void *kmemdup(const void *src, size_t len, gfp_t gfp);
+extern void secure_bzero(void *, __kernel_size_t);
 
 extern char **argv_split(gfp_t gfp, const char *str, int *argcp);
 extern void argv_free(char **argv);
diff --git a/lib/string.c b/lib/string.c
index a1cdcfc..588ac31 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -559,6 +559,19 @@ void *memset(void *s, int c, size_t count)
 EXPORT_SYMBOL(memset);
 #endif
 
+/**
+ * secure_bzero - Call memset to fill a region of memory with zeroes and
+ * ensure this memset is not removed due to dead store elimination.
+ * @p: Pointer to the start of the area.
+ * @n: The size of the area.
+ */
+void secure_bzero(void *p, size_t n)
+{
+	memset(p, 0, n);
+	ARRAY_PREVENT_DSE(p, n);
+}
+EXPORT_SYMBOL(secure_bzero);
+
 #ifndef __HAVE_ARCH_MEMCPY
 /**
  * memcpy - Copy one area of memory to another

Re: [PATCH v1] compiler: prevent dead store elimination

[Andi Kleen]

> Every byte in the [p,p+n[ range must be used. If you only use the
> first byte, via e.g. asm("" :: "m"(*(char*)p)), then the compiler
> _will_ skip scrubbing bytes beyond the first. This works with
> gcc-3.2.3 up to gcc-4.4.3.

You forgot to credit Mikael who did all the hard work figuring
this out?

>  /*
> + * Dead store elimination (DSE) is an optimization that may remove a write to
> + * a buffer that is not used anymore. Use ARRAY_PREVENT_DSE after a write when
> + * the scrub is required for security reasons.
> + */
> +#define ARRAY_PREVENT_DSE(p, n)				\

Maybe it's just me, but the name is ugly.

> +	do {						\
> +		struct __scrub { char c[n]; };		\


Better typeof(*p)[n]

> +++ b/include/linux/compiler-intel.h
> @@ -14,9 +14,11 @@
>   * It uses intrinsics to do the equivalent things.
>   */
>  #undef barrier
> +#undef ARRAY_PREVENT_DSE
>  #undef RELOC_HIDE
>  
>  #define barrier() __memory_barrier()
> +#define ARRAY_PREVENT_DSE(p, n)

Who says the Intel compiler doesn't need this?

I'm sure it does dead store elimination too and it understands
gcc asm syntax.

> +/**
> + * secure_bzero - Call memset to fill a region of memory with zeroes and
> + * ensure this memset is not removed due to dead store elimination.
> + * @p: Pointer to the start of the area.
> + * @n: The size of the area.
> + */
> +void secure_bzero(void *p, size_t n)
> +{
> +	memset(p, 0, n);
> +	ARRAY_PREVENT_DSE(p, n);

I think that's a candidate for a inline

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.

[PATCH v2] compiler: prevent dead store elimination

[Roel Kluin]

Due to optimization A call to memset() may be removed as a dead store when
the buffer is not used after its value is overwritten. The new function
secure_bzero() ensures a section of memory is padded with zeroes.

>From the GCC manual, section 5.37:
If your assembler instructions access memory in an unpredictable
fashion, add `memory' to the list of clobbered registers. This will
cause GCC to not keep memory values cached in registers across the
assembler instruction and not optimize stores or loads to that memory.

Every byte in the [p,p+n[ range must be used. If you only use the
first byte, via e.g. asm("" :: "m"(*(char*)p)), then the compiler
_will_ skip scrubbing bytes beyond the first. This works with
gcc-3.2.3 up to gcc-4.4.3.

Thanks to Mikael Pettersson for this information.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
---
> You forgot to credit Mikael

Not intended, I first put it in the changelog but then decided to thank all
involved in my comments. Do credits belong in the changelog?

>> +#define ARRAY_PREVENT_DSE(p, n)				\
> 
> Maybe it's just me, but the name is ugly.

Ok, changed it to ARRAY_KEEP_STORE() or do you have a suggestion?

>> +	do {						\
>> +		struct __scrub { char c[n]; };		\
> 
> Better typeof(*p)[n]

I guess you meant `struct __scrub { typeof(*p) c[n]; };' ?

In the current implementation p is a void pointer. Is it ok to declare an array
of voids? In kernel it compiles, in my testcase it produces an error.

>> +++ b/include/linux/compiler-intel.h

>> +#define ARRAY_PREVENT_DSE(p, n)
> 
> Who says the Intel compiler doesn't need this?

There was a comment in include/linux/compiler-intel.h that it's not supported.

> I'm sure it does dead store elimination too and it understands
> gcc asm syntax.

Ok, should I put it in include/linux/compiler.h or where?

>> +void secure_bzero(void *p, size_t n)
>> +{
>> +	memset(p, 0, n);
>> +	ARRAY_PREVENT_DSE(p, n);

> I think that's a candidate for a inline

You mean the secure_bzero() function, right?

Thanks for comments, Roel

 include/linux/compiler.h |   11 +++++++++++
 include/linux/string.h   |    1 +
 lib/string.c             |   13 +++++++++++++
 3 files changed, 25 insertions(+), 0 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 188fcae..2f14199 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -302,4 +302,15 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
  */
 #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
 
+/*
+ * Dead store elimination is an optimization that may remove a write to a
+ * buffer that is not used anymore. Use ARRAY_KEEP_STORE after a write when
+ * the scrub is required for security reasons.
+ */
+#define ARRAY_KEEP_STORE(p, n)				\
+	do {						\
+		struct __scrub { typeof(*p) c[n]; };	\
+		asm("" : : "m"(*(struct __scrub *)p));	\
+	} while (0)
+
 #endif /* __LINUX_COMPILER_H */
diff --git a/include/linux/string.h b/include/linux/string.h
index a716ee2..36213e6 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -118,6 +118,7 @@ extern void * memchr(const void *,int,__kernel_size_t);
 extern char *kstrdup(const char *s, gfp_t gfp);
 extern char *kstrndup(const char *s, size_t len, gfp_t gfp);
 extern void *kmemdup(const void *src, size_t len, gfp_t gfp);
+extern void secure_bzero(void *, __kernel_size_t);
 
 extern char **argv_split(gfp_t gfp, const char *str, int *argcp);
 extern void argv_free(char **argv);
diff --git a/lib/string.c b/lib/string.c
index a1cdcfc..5576161 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -559,6 +559,19 @@ void *memset(void *s, int c, size_t count)
 EXPORT_SYMBOL(memset);
 #endif
 
+/**
+ * secure_bzero - Call memset to fill a region of memory with zeroes and
+ * ensure this memset is not removed due to dead store elimination.
+ * @p: Pointer to the start of the area.
+ * @n: The size of the area.
+ */
+inline void secure_bzero(void *p, size_t n)
+{
+	memset(p, 0, n);
+	ARRAY_KEEP_STORE(p, n);
+}
+EXPORT_SYMBOL(secure_bzero);
+
 #ifndef __HAVE_ARCH_MEMCPY
 /**
  * memcpy - Copy one area of memory to another

Re: [PATCH v1] compiler: prevent dead store elimination

[Bill Davidsen]

Andi Kleen wrote:
>> Every byte in the [p,p+n[ range must be used. If you only use the
>> first byte, via e.g. asm("" :: "m"(*(char*)p)), then the compiler
>> _will_ skip scrubbing bytes beyond the first. This works with
>> gcc-3.2.3 up to gcc-4.4.3.
> 
> You forgot to credit Mikael who did all the hard work figuring
> this out?
> 
>>  /*
>> + * Dead store elimination (DSE) is an optimization that may remove a write to
>> + * a buffer that is not used anymore. Use ARRAY_PREVENT_DSE after a write when
>> + * the scrub is required for security reasons.
>> + */
>> +#define ARRAY_PREVENT_DSE(p, n)				\
> 
> Maybe it's just me, but the name is ugly.
> 
>> +	do {						\
>> +		struct __scrub { char c[n]; };		\
> 
> 
> Better typeof(*p)[n]
> 
>> +++ b/include/linux/compiler-intel.h
>> @@ -14,9 +14,11 @@
>>   * It uses intrinsics to do the equivalent things.
>>   */
>>  #undef barrier
>> +#undef ARRAY_PREVENT_DSE
>>  #undef RELOC_HIDE
>>  
>>  #define barrier() __memory_barrier()
>> +#define ARRAY_PREVENT_DSE(p, n)
> 
> Who says the Intel compiler doesn't need this?
> 
> I'm sure it does dead store elimination too and it understands
> gcc asm syntax.
> 
According to the Intel forum, it not only doesn't, but a request for this as a 
feature was rejected, so it won't. Or am I misreading this?

http://software.intel.com/en-us/forums/showthread.php?t=46770

-- 
Bill Davidsen <davidsen@tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot

Re: [PATCH v1] compiler: prevent dead store elimination

[Arjan van de Ven]

On Sat, 27 Feb 2010 21:47:42 +0100
Roel Kluin <roel.kluin@gmail.com> wrote:
> +void secure_bzero(void *p, size_t n)
> +{
> +	memset(p, 0, n);
> +	ARRAY_PREVENT_DSE(p, n);
> +}
> +EXPORT_SYMBOL(secure_bzero);


please don't introduce bzero again to the kernel;

make it secure_memset() please.


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org

Re: [PATCH v2] compiler: prevent dead store elimination

[Mikael Pettersson]

Roel Kluin writes:
 > Due to optimization A call to memset() may be removed as a dead store when

_______________________^ lower-case a

 > the buffer is not used after its value is overwritten. The new function
 > secure_bzero() ensures a section of memory is padded with zeroes.
 > 
 > >From the GCC manual, section 5.37:
 > If your assembler instructions access memory in an unpredictable
 > fashion, add `memory' to the list of clobbered registers. This will
 > cause GCC to not keep memory values cached in registers across the
 > assembler instruction and not optimize stores or loads to that memory.

This paragraph, while accurate, is unrelated to the contents of this
patch. Note that in an asm(), a "memory" clobber (see barrier()) is
not at all the same thing as a memory operand "m"(...), which is what
we're using here. Just drop this bit.

 > Every byte in the [p,p+n[ range must be used. If you only use the
 > first byte, via e.g. asm("" :: "m"(*(char*)p)), then the compiler
 > _will_ skip scrubbing bytes beyond the first.
and then
 > This works with
 > gcc-3.2.3 up to gcc-4.4.3.

You've edited my comments and put them together in a way that doesn't
quite make sense. In particular, "This works" doesn't refer to the
previous text but the local-struct-of-variable-size trick. The text
should read something like:

To prevent a memset() from being eliminated, the compiler must belive
that the memory area is used after the memset(). Also, every byte in
the memory area must be used. If only the first byte is used, via e.g.
asm("" :: "m"(*(char*)p)), then the compiler _will_ skip scrubbing
bytes beyond the first.

The trick is to define a local type of the same size as the memory
area, and use it for the "m" operand in a dummy asm():

	static inline void fake_memory_use(void *p, size_t n)
	{
		struct __scrub { char c[n]; }
		asm("" : : "m"(*(struct __scrub *)p));
	}

This looks strange, n being a variable, but has been confirmed to
work with gcc-3.2.3 up to gcc-4.4.3.

 > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
 > index 188fcae..2f14199 100644
 > --- a/include/linux/compiler.h
 > +++ b/include/linux/compiler.h
 > @@ -302,4 +302,15 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
 >   */
 >  #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
 >  
 > +/*
 > + * Dead store elimination is an optimization that may remove a write to a
 > + * buffer that is not used anymore. Use ARRAY_KEEP_STORE after a write when
 > + * the scrub is required for security reasons.
 > + */
 > +#define ARRAY_KEEP_STORE(p, n)				\

1. This doesn't need to be a macro. Please make it a static inline function.
2. Its function is to "fake" a use of a memory area, which allows us to prevent
   unwanted DSE elsewhere. So this should be fake_memory_use() or something.

 > +	do {						\
 > +		struct __scrub { typeof(*p) c[n]; };	\

The typeof(*p) suggestion doesn't work. It would require p to always be
a pointer type with an accurate (for memset) sizeof(*p). In general however
we'll memset some array described by a void*/size_t pair, and typeof in
that case is useless. The original'struct __scrub { char c[n]; };' was Ok.

 >  extern char *kstrndup(const char *s, size_t len, gfp_t gfp);
 >  extern void *kmemdup(const void *src, size_t len, gfp_t gfp);
 > +extern void secure_bzero(void *, __kernel_size_t);

Why is this __kernel_size_t and not just plain size_t? It's not
a user-space API.

Re: [PATCH v1] compiler: prevent dead store elimination

[Mikael Pettersson]

Arjan van de Ven writes:
 > On Sat, 27 Feb 2010 21:47:42 +0100
 > Roel Kluin <roel.kluin@gmail.com> wrote:
 > > +void secure_bzero(void *p, size_t n)
 > > +{
 > > +	memset(p, 0, n);
 > > +	ARRAY_PREVENT_DSE(p, n);
 > > +}
 > > +EXPORT_SYMBOL(secure_bzero);
 > 
 > 
 > please don't introduce bzero again to the kernel;
 > 
 > make it secure_memset() please.

In principle I would agree, but bzero() avoids the unfortunately
rather common mistake of swapping the int/size_t parameters to
memset(). I.e., people writing memset(p, n, 0) not memset(p, 0, n).

Re: [PATCH v1] compiler: prevent dead store elimination

[Alexey Dobriyan]

On Mon, Mar 1, 2010 at 11:32 AM, Mikael Pettersson <mikpe@it.uu.se> wrote:
> Arjan van de Ven writes:
>  > On Sat, 27 Feb 2010 21:47:42 +0100
>  > Roel Kluin <roel.kluin@gmail.com> wrote:
>  > > +void secure_bzero(void *p, size_t n)
>  > > +{
>  > > +  memset(p, 0, n);
>  > > +  ARRAY_PREVENT_DSE(p, n);
>  > > +}
>  > > +EXPORT_SYMBOL(secure_bzero);
>  >
>  >
>  > please don't introduce bzero again to the kernel;
>  >
>  > make it secure_memset() please.

What's so secure in this function? :^)
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Re: [PATCH v1] compiler: prevent dead store elimination

[Alexey Dobriyan]

On Mon, Mar 1, 2010 at 11:32 AM, Mikael Pettersson <mikpe@it.uu.se> wrote:
> Arjan van de Ven writes:
>  > On Sat, 27 Feb 2010 21:47:42 +0100
>  > Roel Kluin <roel.kluin@gmail.com> wrote:
>  > > +void secure_bzero(void *p, size_t n)
>  > > +{
>  > > +  memset(p, 0, n);
>  > > +  ARRAY_PREVENT_DSE(p, n);
>  > > +}
>  > > +EXPORT_SYMBOL(secure_bzero);
>  >
>  >
>  > please don't introduce bzero again to the kernel;
>  >
>  > make it secure_memset() please.

What's so secure in this function? :^)

Re: [PATCH v1] compiler: prevent dead store elimination

[Andi Kleen]

On Sun, Feb 28, 2010 at 09:15:11PM -0800, Arjan van de Ven wrote:
> On Sat, 27 Feb 2010 21:47:42 +0100
> Roel Kluin <roel.kluin@gmail.com> wrote:
> > +void secure_bzero(void *p, size_t n)
> > +{
> > +	memset(p, 0, n);
> > +	ARRAY_PREVENT_DSE(p, n);
> > +}
> > +EXPORT_SYMBOL(secure_bzero);
> 
> 
> please don't introduce bzero again to the kernel;
> 
> make it secure_memset() please.

Would there ever be any reason to set the key to something
else than 0? 

IMHO bzero is less error prone. With memset there are regular 
bugs when the two end arguments get exchanged. 

You could call it differently if you have a problem with old BSD
names, but inherently there's nothing wrong with them. One possibility
would be the same name as VC++ uses.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.

Re: [PATCH v2] compiler: prevent dead store elimination

[Andi Kleen]

>  > +	do {						\
>  > +		struct __scrub { typeof(*p) c[n]; };	\
> 
> The typeof(*p) suggestion doesn't work. It would require p to always be
> a pointer type with an accurate (for memset) sizeof(*p). In general however
> we'll memset some array described by a void*/size_t pair, and typeof in
> that case is useless. The original'struct __scrub { char c[n]; };' was Ok.

I just suggested it because of the original array name of the macro
and without it it would have only worked for char arrays. With the new
naming it's ok I guess.

It could be made to work with macros and builtin_type_match I guess,
but it would be fairly ugly and not worth it.

-Andi

Re: [PATCH v2] compiler: prevent dead store elimination

[Andi Kleen]

> 
> >> +#define ARRAY_PREVENT_DSE(p, n)
> > 
> > Who says the Intel compiler doesn't need this?
> 
> There was a comment in include/linux/compiler-intel.h that it's not supported.

That's true for the ia64 version, but not for the x86 version which supports
gcc compatible inline assembler. So on x86 you can use it. On Itanium
it probably would need some other compiler built in.

Also in any case some form of memory clobber is needed because it'll surely
do dead-store-optimization.

-Andi

[PATCH v3] compiler: prevent dead store elimination

[Roel Kluin]

Due to optimization a call to memset() may be removed as a dead store when
the buffer is not used after its value is overwritten. The new function
secure_bzero() ensures a section of memory is padded with zeroes.

To prevent a memset() from being eliminated, the compiler must belive
that the memory area is used after the memset(). Also, every byte in
the memory area must be used. If only the first byte is used, via e.g.
asm("" :: "m"(*(char*)p)), then the compiler _will_ skip scrubbing
bytes beyond the first.

The trick is to define a local type of the same size as the memory
area, and use it for the "m" operand in a dummy asm():

	static inline void fake_memory_use(void *p, size_t n)
	{
		struct __scrub { char c[n]; }
		asm("" : : "m"(*(struct __scrub *)p));
	}

This looks strange, n being a variable, but has been confirmed to
work with gcc-3.2.3 up to gcc-4.4.3.

Many thanks to Mikael Pettersson and Andi Kleen.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
---
I used a trick to prevent the assembly for Itanium, defining
fake_memory_use in instrinsics.h and checking its definition
in string.c, but maybe there's a better way to do this?

To provide a memory clobber for Itanium it appears we could use
the pragma directive to set the optimization level [1]. Only when
optimization level is greater than 3, dead store removal occurs[2].
But this will be ugly:

inline void secure_bzero(void *p, size_t n)
{
#ifdef _ASM_IA64_INTRINSICS_H
#pragma OPT_LEVEL 2
	memset(p, 0, n);
#pragma OPT_LEVEL INITIAL
#else
	memset(p, 0, n);
	fake_memory_use(p, n);
#endif /* _ASM_IA64_INTRINSICS_H */
}

Do we want this?

Thanks, Roel

[1] http://docs.hp.com/en/B3901-90030/ch03s04.html
[2] http://h21007.www2.hp.com/portal/download/files/unprot/Itanium/OptimizingApps-ItaniumV9-1.pdf
(page 6)

 arch/ia64/include/asm/intrinsics.h |    3 +++
 include/linux/string.h             |    1 +
 lib/string.c                       |   27 +++++++++++++++++++++++++++
 3 files changed, 31 insertions(+), 0 deletions(-)

diff --git a/arch/ia64/include/asm/intrinsics.h b/arch/ia64/include/asm/intrinsics.h
index 111ed52..51d8035 100644
--- a/arch/ia64/include/asm/intrinsics.h
+++ b/arch/ia64/include/asm/intrinsics.h
@@ -241,6 +241,9 @@ extern long ia64_cmpxchg_called_with_bad_pointer (void);
 	IA64_INTRINSIC_API(intrin_local_irq_restore)
 #define ia64_set_rr0_to_rr4		IA64_INTRINSIC_API(set_rr0_to_rr4)
 
+/* FIXME: define fake_memory_use as a memory clobber for Itanium */
+#define fake_memory_use
+
 #endif /* !__ASSEMBLY__ */
 
 #endif /* _ASM_IA64_INTRINSICS_H */
diff --git a/include/linux/string.h b/include/linux/string.h
index a716ee2..be68efb 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -118,6 +118,7 @@ extern void * memchr(const void *,int,__kernel_size_t);
 extern char *kstrdup(const char *s, gfp_t gfp);
 extern char *kstrndup(const char *s, size_t len, gfp_t gfp);
 extern void *kmemdup(const void *src, size_t len, gfp_t gfp);
+extern void secure_bzero(void *, size_t);
 
 extern char **argv_split(gfp_t gfp, const char *str, int *argcp);
 extern void argv_free(char **argv);
diff --git a/lib/string.c b/lib/string.c
index a1cdcfc..cbc32d4 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -559,6 +559,33 @@ void *memset(void *s, int c, size_t count)
 EXPORT_SYMBOL(memset);
 #endif
 
+#ifndef fake_memory_use
+/*
+ * Dead store elimination is an optimization that may remove a write to a
+ * buffer that is not used anymore. To prevent this, e.g. for security
+ * reasons, the compiler must believe that every byte in this memory area
+ * is used after the last write.
+ */
+static inline void fake_memory_use(void *p, size_t n)
+{
+	struct __scrub { char c[n]; }
+	asm("" : : "m"(*(struct __scrub *)p));
+}
+#endif /* fake_memory_use */
+
+/**
+ * secure_bzero - Call memset to fill a region of memory with zeroes and
+ * ensure this memset is not removed due to dead store elimination.
+ * @p: Pointer to the start of the area.
+ * @n: The size of the area.
+ */
+inline void secure_bzero(void *p, size_t n)
+{
+	memset(p, 0, n);
+	fake_memory_use(p, n);
+}
+EXPORT_SYMBOL(secure_bzero);
+
 #ifndef __HAVE_ARCH_MEMCPY
 /**
  * memcpy - Copy one area of memory to another