[PATCH AUTOSEL 5.6 083/606] evm: Check also if *tfm is an error pointer in init_desc()

[Sasha Levin]

From: Roberto Sassu <roberto.sassu at huawei.com>

[ Upstream commit 53de3b080d5eae31d0de219617155dcc34e7d698 ]

This patch avoids a kernel panic due to accessing an error pointer set by
crypto_alloc_shash(). It occurs especially when there are many files that
require an unsupported algorithm, as it would increase the likelihood of
the following race condition:

Task A: *tfm = crypto_alloc_shash() <= error pointer
Task B: if (*tfm == NULL) <= *tfm is not NULL, use it
Task B: rc = crypto_shash_init(desc) <= panic
Task A: *tfm = NULL

This patch uses the IS_ERR_OR_NULL macro to determine whether or not a new
crypto context must be created.

Cc: stable at vger.kernel.org
Fixes: d46eb3699502b ("evm: crypto hash replaced by shash")
Co-developed-by: Krzysztof Struczynski <krzysztof.struczynski at huawei.com>
Signed-off-by: Krzysztof Struczynski <krzysztof.struczynski at huawei.com>
Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
Signed-off-by: Mimi Zohar <zohar at linux.ibm.com>
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
 security/integrity/evm/evm_crypto.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
index d485f6fc908e..302adeb2d37b 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -93,7 +93,7 @@ static struct shash_desc *init_desc(char type, uint8_t hash_algo)
 		algo = hash_algo_name[hash_algo];
 	}
 
-	if (*tfm == NULL) {
+	if (IS_ERR_OR_NULL(*tfm)) {
 		mutex_lock(&mutex);
 		if (*tfm)
 			goto out;
-- 
2.25.1