Problem with commit ccf11dbaa07b ("evm: Fix memleak in init_desc")

[Guozihua (Scott)]

Hi Mimi,

I found an issue with commit ccf11dbaa07b ("evm: Fix memleak in init_desc").

This commit tries to free variable "tmp_tfm" if something went wrong 
after the "alloc" label in function init_desc, which would potentially 
cause a user-after-free issue

The codes are as follows:

   1 static struct shash_desc *init_desc(char type, uint8_t hash_algo)
   2 {
   3 	long rc;
   4 	const char *algo;
   5 	struct crypto_shash **tfm, *tmp_tfm = NULL;
   6 	struct shash_desc *desc;
   7
   8 	if (type == EVM_XATTR_HMAC) {
   9 		if (!(evm_initialized & EVM_INIT_HMAC)) {
  10 			pr_err_once("HMAC key is not set\n");
  11 			return ERR_PTR(-ENOKEY);
  12 		}
  13 		tfm = &hmac_tfm;
  14 		algo = evm_hmac;
  15 	} else {
  16 		if (hash_algo >= HASH_ALGO__LAST)
  17 			return ERR_PTR(-EINVAL);
  18
  19 		tfm = &evm_tfm[hash_algo];
  20 		algo = hash_algo_name[hash_algo];
  21 	}
  22
  23 	if (*tfm)
  24 		goto alloc;
  25 	mutex_lock(&mutex);
  26 	if (*tfm)
  27 		goto unlock;
  28
  29 	tmp_tfm = crypto_alloc_shash(algo, 0, CRYPTO_NOLOAD);
  30 	if (IS_ERR(tmp_tfm)) {
  31 		pr_err("Can not allocate %s (reason: %ld)\n", algo,
  32 		       PTR_ERR(tmp_tfm));
  33 		mutex_unlock(&mutex);
  34 		return ERR_CAST(tmp_tfm);
  35 	}
  36 	if (type == EVM_XATTR_HMAC) {
  37 		rc = crypto_shash_setkey(tmp_tfm, evmkey, evmkey_len);
  38 		if (rc) {
  39 			crypto_free_shash(tmp_tfm);
  40 			⋅mutex_unlock(&mutex);
  41 			return ERR_PTR(rc);
  42 		}
  43 	}
  44 	*tfm = tmp_tfm;
  45 unlock:
  46 	mutex_unlock(&mutex);
  47 alloc:
  48 	desc = kmalloc(sizeof(*desc) + crypto_shash_descsize(*tfm),
  49 			GFP_KERNEL);
  50 	if (!desc) {
  51 		crypto_free_shash(tmp_tfm);
  52 		return ERR_PTR(-ENOMEM);
  53 	}
  54
  55 	desc->tfm = *tfm;
  56
  57 	rc = crypto_shash_init(desc);
  58 	if (rc) {
  59 		crypto_free_shash(tmp_tfm);
  60 		kfree(desc);
  61 		return ERR_PTR(rc);
  62 	}
  63 	return desc;
  64 }

As we can see, variable *tfm points to one of the two global variable 
hmac_tfm or evm_tfm[hash_algo]. tmp_tfm is used as an intermediate 
variable for initializing these global variables. Freeing tmp_tfm after 
line 44 would invalidate these global variables and potentially cause a 
user-after-free issue.

I think this commit should be reverted.

Reference: commit 843385694721 ("evm: Fix a small race in init_desc()")

-- 
Best
GUO Zihua