linux-kernel - Re: crypto: api - Fix use-after-free and race in crypto_spawn

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Message-ID: <20200416023001.GE816@sol.localdomain>
Date:   Wed, 15 Apr 2020 19:30:01 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     syzbot <syzbot+fc0674cde00b66844470@...kaller.appspotmail.com>,
        davem@...emloft.net, linux-crypto@...r.kernel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: crypto: api - Fix use-after-free and race in crypto_spawn_alg

On Thu, Apr 16, 2020 at 12:25:02PM +1000, Herbert Xu wrote:
> On Wed, Apr 15, 2020 at 07:17:03PM -0700, Eric Biggers wrote:
> > 
> > Wouldn't it be a bit simpler to set 'target = NULL', remove 'shoot',
> > and use 'if (target)' instead of 'if (shoot)'?
> 
> Yes it is simpler but it's actually semantically different because
> the compiler doesn't know that spawn->alg cannot be NULL in this
> case.
> 

I'm not sure what you mean here.  crypto_alg_get() is:

static inline struct crypto_alg *crypto_alg_get(struct crypto_alg *alg)
{
        refcount_inc(&alg->cra_refcnt);
        return alg;
}

So given:

	target = crypto_alg_get(alg);

Both alg and target have to be non-NULL.

- Eric