[bug report] evm: Check also if *tfm is an error pointer in init_desc()

[Roberto Sassu]

> From: Dan Carpenter [mailto:dan.carpenter at oracle.com]
> Sent: Tuesday, May 12, 2020 12:48 PM
> 
> Hello Roberto Sassu,
> 
> The patch 53de3b080d5e: "evm: Check also if *tfm is an error pointer
> in init_desc()" from Apr 27, 2020, leads to the following static
> checker warning:
> 
> 	security/integrity/evm/evm_crypto.c:119 init_desc()
> 	error: '*tfm' dereferencing possible ERR_PTR()
> 
> security/integrity/evm/evm_crypto.c
>     89
>     90                  tfm = &evm_tfm[hash_algo];
>     91                  algo = hash_algo_name[hash_algo];
>     92          }
>     93
>     94          if (IS_ERR_OR_NULL(*tfm)) {
> 
> This used to be a "if (!*tfm)" check.
> 
>     95                  mutex_lock(&mutex);
>     96                  if (*tfm)
>     97                          goto out;
> 
> Then we test again with the lock held.  But in the new code if "*tfm"
> is an error pointer then we jump directly to the unlock and crash on the
> next line.  I can't see how the commit would fix anything.

Hello Dan

you are right. The fix should be applied in both places.

if (!IS_ERR_OR_NULL(*tfm))
	goto out;

>     98                  *tfm = crypto_alloc_shash(algo, 0, CRYPTO_NOLOAD);
>     99                  if (IS_ERR(*tfm)) {
>    100                          rc = PTR_ERR(*tfm);
>    101                          pr_err("Can not allocate %s (reason: %ld)\n", algo, rc);
>    102                          *tfm = NULL;
>    103                          mutex_unlock(&mutex);
>    104                          return ERR_PTR(rc);
>    105                  }
>    106                  if (type == EVM_XATTR_HMAC) {
>    107                          rc = crypto_shash_setkey(*tfm, evmkey, evmkey_len);
>    108                          if (rc) {
>    109                                  crypto_free_shash(*tfm);
>    110                                  *tfm = NULL;
>    111                                  mutex_unlock(&mutex);
>    112                                  return ERR_PTR(rc);
>    113                          }
>    114                  }
>    115  out:
>    116                  mutex_unlock(&mutex);
>    117          }
>    118
>    119          desc = kmalloc(sizeof(*desc) + crypto_shash_descsize(*tfm),
>                                                                      ^^^^
> I don't understand how using *tfm outside of a lock is safe at all
> anyway.

I think the purpose of the mutex is just to  prevent two concurrent
allocations. Later, it should not be a problem, as *tfm is never freed.

Thanks

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli


>    120                          GFP_KERNEL);
>    121          if (!desc)
>    122                  return ERR_PTR(-ENOMEM);
>    123
> 
> regards,
> dan carpenter