Re: [PATCH] fs: crypto: keyinfo: Fix a possible null-pointer dereference in derive_key_aes()

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Jul 25, 2019 at 11:33:53AM +0800, Jia-Ju Bai wrote:
> Sorry, I forgot to send to Eric, so send it again.
> 
> On 2019/7/25 11:30, Jia-Ju Bai wrote:
> > 
> > 
> > On 2019/7/25 0:07, Eric Biggers wrote:
> > > [+Cc linux-crypto]
> > > 
> > > On Wed, Jul 24, 2019 at 06:02:04PM +0800, Jia-Ju Bai wrote:
> > > > In derive_key_aes(), tfm is assigned to NULL on line 46, and then
> > > > crypto_free_skcipher(tfm) is executed.
> > > > 
> > > > crypto_free_skcipher(tfm)
> > > >      crypto_skcipher_tfm(tfm)
> > > >          return &tfm->base;
> > > > 
> > > > Thus, a possible null-pointer dereference may occur.
> > > This analysis is incorrect because only the address &tfm->base is taken.
> > > There's no pointer dereference.
> > > 
> > > In fact all the crypto_free_*() functions are no-ops on NULL
> > > pointers, and many
> > > other callers rely on it.  So there's no bug here.
> > 
> > Thanks for the reply :)
> > I admit that "&tfm->base" is not a null-pointer dereference when tfm is
> > NULL.
> > But I still think crypto_free_skcipher(tfm) can cause security problems
> > when tfm is NULL.
> > 
> > Looking at the code:
> > 
> > static inline void crypto_free_skcipher(struct crypto_skcipher *tfm)
> > {
> >     crypto_destroy_tfm(tfm, crypto_skcipher_tfm(tfm));
> > }
> > 
> > static inline struct crypto_tfm *crypto_skcipher_tfm(
> >     struct crypto_skcipher *tfm)
> > {
> >     return &tfm->base;
> > }
> > 
> > void crypto_destroy_tfm(void *mem, struct crypto_tfm *tfm)
> > {
> >     struct crypto_alg *alg;
> > 
> >     if (unlikely(!mem))
> >         return;

When the original pointer is NULL, mem == NULL here so crypto_destroy_tfm() is a
no-op.

> > Besides, I also find that some kernel modules check tfm before calling
> > crypto_free_*(), such as:
> > 
> > drivers/crypto/vmx/aes_xts.c:
> >     if (ctx->fallback) {
> >         crypto_free_skcipher(ctx->fallback);
> >         ctx->fallback = NULL;
> >     }
> > 
> > net/rxrpc/rxkad.c:
> >     if (conn->cipher)
> >         crypto_free_skcipher(conn->cipher);
> > 
> > drivers/crypto/chelsio/chcr_algo.c:
> >     if (ablkctx->aes_generic)
> >         crypto_free_cipher(ablkctx->aes_generic);
> > 
> > net/mac80211/wep.c:
> >     if (!IS_ERR(local->wep_tx_tfm))
> >         crypto_free_cipher(local->wep_tx_tfm);
> > 

Well, people sometimes do that for kfree() too.  But that doesn't mean it's
needed, or that it's the preferred style (it's not).

- Eric