Re: [PATCH] crypto: Prevent kernel-infoleak in rng_recvmsg

[Kees Cook <kees@kernel.org>]

On Tue, Aug 26, 2025 at 03:51:04PM +0200, Ard Biesheuvel wrote:
> (cc Kees)
> 
> On Sat, 16 Aug 2025 at 11:17, Herbert Xu <herbert@gondor.apana.org.au> wrote:
> >
> > On Sat, Aug 09, 2025 at 05:59:43PM +0800, Edward Adam Davis wrote:
> > > Initialize the intermediary array member to 0 to prevent the kernel from
> > > leaking uninitialized data to user space.
> > >
> > > Reported-by: syzbot+e8bcd7ee3db6cb5cb875@syzkaller.appspotmail.com
> > > Closes: https://syzkaller.appspot.com/bug?extid=e8bcd7ee3db6cb5cb875
> > > Tested-by: syzbot+e8bcd7ee3db6cb5cb875@syzkaller.appspotmail.com
> > > Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> > > ---
> > >  crypto/jitterentropy-kcapi.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/crypto/jitterentropy-kcapi.c b/crypto/jitterentropy-kcapi.c
> > > index c24d4ff2b4a8..9e9e069f55af 100644
> > > --- a/crypto/jitterentropy-kcapi.c
> > > +++ b/crypto/jitterentropy-kcapi.c
> > > @@ -107,7 +107,7 @@ int jent_hash_time(void *hash_state, __u64 time, u8 *addtl,
> > >  {
> > >       struct shash_desc *hash_state_desc = (struct shash_desc *)hash_state;
> > >       SHASH_DESC_ON_STACK(desc, hash_state_desc->tfm);
> > > -     u8 intermediary[SHA3_256_DIGEST_SIZE];
> > > +     u8 intermediary[SHA3_256_DIGEST_SIZE] = { 0 };
> >
> > This is not a leak! The stack memroy is hashed and fed into the
> > entropy pool.
> 
> Is there still a point to doing this now that the compiler
> zero-initializes automatic variables? Or does that not apply to u8
> arrays? (asking Kees)

Wait, the jitterentropy _depends_ on "uninitialized" stack contents?
That is not a safe way to get entropy (especially since the stack
contents are likely predictable). But regardless, yes, arrays would be
fully wiped by zero-initialized automatic variables.

To force it to be uninit, use __uninitialized.

-- 
Kees Cook