Re: [PATCH] crypto: algapi - fix NULL dereference in crypto_remove_spawns()

[Herbert Xu]

On Fri, Dec 29, 2017 at 02:30:19PM -0600, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
> 
> syzkaller triggered a NULL pointer dereference in crypto_remove_spawns()
> via a program that repeatedly and concurrently requests AEADs
> "authenc(cmac(des3_ede-asm),pcbc-aes-aesni)" and hashes "cmac(des3_ede)"
> through AF_ALG, where the hashes are requested as "untested"
> (CRYPTO_ALG_TESTED is set in ->salg_mask but clear in ->salg_feat; this
> causes the template to be instantiated for every request).
> 
> Although AF_ALG users really shouldn't be able to request an "untested"
> algorithm, the NULL pointer dereference is actually caused by a
> longstanding race condition where crypto_remove_spawns() can encounter
> an instance which has had spawn(s) "grabbed" but hasn't yet been
> registered, resulting in ->cra_users still being NULL.
> 
> We probably should properly initialize ->cra_users earlier, but that
> would require updating many templates individually.  For now just fix
> the bug in a simple way that can easily be backported: make
> crypto_remove_spawns() treat a NULL ->cra_users list as empty.
> 
> Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
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