[PATCH 4.9 06/64] evm: Check also if *tfm is an error pointer in init_desc()

From: Greg Kroah-Hartman
Date: Tue May 26 2020 - 15:31:36 EST

From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

[ 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@xxxxxxxxxxxxxxx
Fixes: d46eb3699502b ("evm: crypto hash replaced by shash")
Co-developed-by: Krzysztof Struczynski <krzysztof.struczynski@xxxxxxxxxx>
Signed-off-by: Krzysztof Struczynski <krzysztof.struczynski@xxxxxxxxxx>
Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
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 c783fefa558a..e034dc21421e 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -90,7 +90,7 @@ static struct shash_desc *init_desc(char type)
algo = evm_hash;

- if (*tfm == NULL) {
+ if (IS_ERR_OR_NULL(*tfm)) {
if (*tfm)
goto out;