Re: [PATCH] sha: prevent removal of memset as dead store in sha1_update()

[Brian Gerst]

On Thu, Feb 25, 2010 at 10:56 AM, Mikael Pettersson <mikpe@xxxxxxxx> wrote:
> Roel Kluin writes:
> Â> Due to optimization A call to memset() may be removed as a dead store when
> Â> the buffer is not used after its value is overwritten.
> Â>
> Â> Signed-off-by: Roel Kluin <roel.kluin@xxxxxxxxx>
> Â> ---
> Â> see http://cwe.mitre.org/data/slices/2000.html#14
> Â>
> Â> checkpatch.pl, compile and sparse tested. Comments?
> Â>
> Â> diff --git a/crypto/sha1_generic.c b/crypto/sha1_generic.c
> Â> index 0416091..86de0da 100644
> Â> --- a/crypto/sha1_generic.c
> Â> +++ b/crypto/sha1_generic.c
> Â> @@ -49,8 +49,8 @@ static int sha1_update(struct shash_desc *desc, const u8 *data,
> Â> Â Â Âsrc = data;
> Â>
> Â> Â Â Âif ((partial + len) > 63) {
> Â> - Â Â Â Â Â Âu32 temp[SHA_WORKSPACE_WORDS];
> Â> -
> Â> + Â Â Â Â Â Âu32 *temp = kzalloc(SHA_WORKSPACE_WORDS * sizeof(u32),
> Â> + Â Â Â Â Â Â Â Â Â Â Â Â Â ÂGFP_KERNEL);
> Â> Â Â Â Â Â Â Âif (partial) {
> Â> Â Â Â Â Â Â Â Â Â Â Âdone = -partial;
> Â> Â Â Â Â Â Â Â Â Â Â Âmemcpy(sctx->buffer + partial, data, done + 64);
> Â> @@ -64,6 +64,7 @@ static int sha1_update(struct shash_desc *desc, const u8 *data,
> Â> Â Â Â Â Â Â Â} while (done + 63 < len);
> Â>
> Â> Â Â Â Â Â Â Âmemset(temp, 0, sizeof(temp));
> Â> + Â Â Â Â Â Âkfree(temp);
> Â> Â Â Â Â Â Â Âpartial = 0;
> Â> Â Â Â}
> Â> Â Â Âmemcpy(sctx->buffer + partial, src, len - done);
>
> At best this might solve the issue right now, but it's not
> future-proof by any margin.
>
> One problem is that just like the lifetimes of auto variables are
> known to the compiler, allowing dead store elimination (DSE) on them,
> there is development going on to make malloc() and free() known to
> the compiler. I don't think it's complete yet, but once free() is
> known, the sequence "memset(p, 0, n); free(p);" will obviously be
> DSE:d just like in the current case with the auto variable.
>
> And as soon as gcc can optimize malloc() and free(), you can be sure that
> some eager kernel hacker will mark the kernel's allocators accordingly,
> and then we're back to square one.
>
> I fear that the only portable (across compiler versions) and safe
> solution is to invoke an assembly-coded dummy function with prototype
>
> Â Â Â Âvoid use(void *p);
>
> and rewrite the code above as
>
> Â Â Â Â{
> Â Â Â Â Â Â Â Âu32 temp[...];
> Â Â Â Â Â Â Â Â...
> Â Â Â Â Â Â Â Âmemset(temp, 0, sizeof temp);
> Â Â Â Â Â Â Â Âuse(temp);
> Â Â Â Â}
>
> This forces the compiler to consider the buffer live after the
> memset, so the memset cannot be eliminated.
>
> The reason the use() function needs to be in assembly code is that
> with link-time optimizations soon commonplace (LTO in gcc-4.5),
> a compiler can possibly discover that even an out-of-line function
>
> Â Â Â Âvoid use(void *p) { }
>
> doesn't in fact use *p, which then enables (in theory) the
> preceeding memset() to be DSE:d.


Would barrier() (which is a simple memory clobber) after the memset work?

--
Brian Gerst
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