linux-kernel - [PATCH v2 2/6] evm: Check also if *tfm is an error pointer in init

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Date:   Mon, 27 Apr 2020 12:28:56 +0200
From:   Roberto Sassu <roberto.sassu@...wei.com>
To:     <zohar@...ux.ibm.com>, <rgoldwyn@...e.de>
CC:     <linux-integrity@...r.kernel.org>,
        <linux-security-module@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <silviu.vlasceanu@...wei.com>,
        <krzysztof.struczynski@...wei.com>,
        "Roberto Sassu" <roberto.sassu@...wei.com>,
        <stable@...r.kernel.org>
Subject: [PATCH v2 2/6] evm: Check also if *tfm is an error pointer in init_desc()

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@...r.kernel.org
Fixes: d46eb3699502b ("evm: crypto hash replaced by shash")
Co-developed-by: Krzysztof Struczynski <krzysztof.struczynski@...wei.com>
Signed-off-by: Krzysztof Struczynski <krzysztof.struczynski@...wei.com>
Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
---
 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 35682852ddea..77ad1e5a93e4 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -91,7 +91,7 @@ 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;
-- 
2.17.1