Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 15 Apr 2020 19:30:01 -0700
From:   Eric Biggers <ebiggers@kernel.org>
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     syzbot <syzbot+fc0674cde00b66844470@syzkaller.appspotmail.com>,
        davem@davemloft.net, linux-crypto@vger.kernel.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        syzkaller-bugs@googlegroups.com
Subject: Re: crypto: api - Fix use-after-free and race in crypto_spawn_alg
Message-ID: <20200416023001.GE816@sol.localdomain>
References: <0000000000002656a605a2a34356@google.com>
 <20200410060942.GA4048@gondor.apana.org.au>
 <20200416021703.GD816@sol.localdomain>
 <20200416022502.GA18386@gondor.apana.org.au>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200416022502.GA18386@gondor.apana.org.au>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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