linux-kernel - Re: [PATCH] fs: crypto: keyinfo: Fix a possible null-pointer dereference in derive_key

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Message-ID: <9740973d-6e59-e4df-7097-4e5d0da89235@gmail.com>
Date:   Thu, 25 Jul 2019 11:33:53 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     tytso@....edu, jaegeuk@...nel.org, ebiggers@...nel.org
Cc:     linux-fscrypt@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org
Subject: Re: [PATCH] fs: crypto: keyinfo: Fix a possible null-pointer
 dereference in derive_key_aes()

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;
>
>     alg = tfm->__crt_alg;
>
>     if (!tfm->exit && alg->cra_exit)
>         alg->cra_exit(tfm);
>     crypto_exit_ops(tfm);
>     crypto_mod_put(alg);
>     kzfree(mem);
> }
>
> The function crypto_skcipher_tfm() may return an uninitialized address 
> (&tfm->base) when tfm is NULL.
> Then crypto_destroy_tfm() uses this problematic address (tfm), which 
> may cause security problems.
>
> 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);
>
> Thus, I think it is better to check tfm before calling crypto_free_*().
>
>>
>> It appears you've sent the same patch for some of these other callers
>> (https://lore.kernel.org/lkml/?q=%22fix+a+possible+null-pointer%22), 
>> but none
>> are Cc'ed to linux-crypto or another mailing list I'm subscribed to, 
>> so I can't
>> respond to them.  But this feedback applies equally to them too.
>
> Ah, sorry.
> I just ran "get_maintainer.pl" for the kernel modules used 
> crypto_free_*(), and forgot to cc to linux-crypto...
>
>>
>> Note also that if there actually were a bug here (which again, there 
>> doesn't
>> appear to be), we'd need to fix it in crypto_free_*(), not in the 
>> callers.
>>
>
> I think a possible way is to add a check of tfm in crypto_free_*(), 
> such as:
> static inline void crypto_free_skcipher(struct crypto_skcipher *tfm)
> {
>     if (tfm)
>         crypto_destroy_tfm(tfm, crypto_skcipher_tfm(tfm));
> }
>
> If you think it is okay, I can send a patch for this.
>
>
> Best wishes,
> Jia-Ju Bai