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SubjectRe: KASAN: use-after-free Read in skcipher_recvmsg
On Sat, Nov 03, 2018 at 03:35:04PM -0700, Eric Biggers wrote:
> [+clabbe@baylibre.com]
>
> Hi Corentin, I think this is a bug in the new crypto statistics feature. In the
> skcipher_decrypt case the code is (but this applies elsewhere too!):
>
> static inline void crypto_stat_skcipher_decrypt(struct skcipher_request *req,
> int ret, struct crypto_alg *alg)
> {
> #ifdef CONFIG_CRYPTO_STATS
> if (ret && ret != -EINPROGRESS && ret != -EBUSY) {
> atomic_inc(&alg->cipher_err_cnt);
> } else {
> atomic_inc(&alg->decrypt_cnt);
> atomic64_add(req->cryptlen, &alg->decrypt_tlen);
> }
> #endif
> }
>
> static inline int crypto_skcipher_decrypt(struct skcipher_request *req)
> {
> struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
> int ret;
>
> if (crypto_skcipher_get_flags(tfm) & CRYPTO_TFM_NEED_KEY)
> ret = -ENOKEY;
> else
> ret = tfm->decrypt(req);
> crypto_stat_skcipher_decrypt(req, ret, tfm->base.__crt_alg);
> return ret;
> }
>
> The bug is the request may be issued asynchronously (as indicated by EINPROGRESS
> or EBUSY) being returned, and the stats are updated afterwards. But by that
> time, the request's completion function may have already run, and the request
> structure may have already been freed.
>
> In theory, I think the algorithm could have even been unregistered as well.
> Therefore, it's only safe to update the stats either *before* calling
> tfm->decrypt(), or afterwards if the error code was not EINPROGRESS or EBUSY.

Hello

I can store "len" and alg for later use, this will fix a part of the problem.

For the fact that algorithm could be unregistred, I think it cannot happen since at least the tfm running this crypto_skcipher_decrypt/othersamefunction still exists and that it is(should be) impossible to unregister an alg with still existing tfm which uses it.
But that needs to be verified.

Regards

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