Re: [PATCH] crypto: DRBG - guard uninstantion by lock

Dmitry Vyukov
2018-04-11T17:10:16Z

On Wed, Apr 11, 2018 at 4:26 PM, Stephan Müller <smueller@chronox.de> wrote:
> Hi Dimitry,
>
> This fix prevents the kernel from crashing when injecting the fault.

Good!

> Stack traces are yet shown but I guess that is expected every time
> a fault is injected.

Yes, nothing to fix here.

> As to why KASAN did not notice this one, I am not sure. Maybe it is
> because I use two buffer pointers to point to (almost) the same memory
> (one that is aligned and one pointing to the complete buffer)?

After looking at the fix, I figured out what happened with KASAN.
Filed https://bugzilla.kernel.org/show_bug.cgi?id=199359. In short,
tricky interplay between kzfree, ksize and double-free detection.
If KASAN worked as intended it would give a nice "double-free in this
stack for object allocated in this stack and previously freed in this
stack", which would probably make debugging much simpler.

> ---8<---
>
> During freeing of the internal buffers used by the DRBG, set the pointer
> to NULL. It is possible that the context with the freed buffers is
> reused. In case of an error during initialization where the pointers
> do not yet point to allocated memory, the NULL value prevents a double
> free.
>
> Signed-off-by: Stephan Mueller <smueller@chronox.de>
> Reported-by: syzbot+75397ee3df5c70164154@syzkaller.appspotmail.com
> ---
> crypto/drbg.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/crypto/drbg.c b/crypto/drbg.c
> index 4faa2781c964..466a112a4446 100644
> --- a/crypto/drbg.c
> +++ b/crypto/drbg.c
> @@ -1134,8 +1134,10 @@ static inline void drbg_dealloc_state(struct drbg_state *drbg)
> if (!drbg)
> return;
> kzfree(drbg->Vbuf);
> + drbg->Vbuf = NULL;
> drbg->V = NULL;
> kzfree(drbg->Cbuf);
> + drbg->Cbuf = NULL;
> drbg->C = NULL;
> kzfree(drbg->scratchpadbuf);
> drbg->scratchpadbuf = NULL;
> --
> 2.14.3
>
>
>
>
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