[PATCH v2] evm: Fix a small race in init_desc()

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

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.

Imagine we have two threads and in the first thread crypto_alloc_shash()
fails and returns an error pointer.

		*tfm = crypto_alloc_shash(algo, 0, CRYPTO_NOLOAD);
		if (IS_ERR(*tfm)) {
			rc = PTR_ERR(*tfm); <--- FIRST THREAD HERE!
			pr_err("Can not allocate %s (reason: %ld)\n", algo, rc);
			*tfm = NULL;

And the second thread is here:

	if (*tfm == NULL) {  <--- SECOND THREAD HERE!
		mutex_lock(&mutex);
		if (*tfm)
			goto out;

Since "*tfm" is non-NULL, we assume that it is valid and that leads to
a crash when it dereferences "*tfm".

	desc = kmalloc(sizeof(*desc) + crypto_shash_descsize(*tfm),
                                                             ^^^^

This patch fixes the problem by introducing a temporary "tmp_tfm" and
only setting "*tfm" at the very end after everything has succeeded.  The
other change is that I reversed the initial "if (!*tfm) {" condition and
pull the code in one indent level.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: d46eb3699502b ("evm: crypto hash replaced by shash")
Reported-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
Reported-by: Krzysztof Struczynski <krzysztof.struczynski@xxxxxxxxxx>
Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
---
v2: I folded mine patch together with Roberto's

 security/integrity/evm/evm_crypto.c | 44 ++++++++++++++---------------
 1 file changed, 22 insertions(+), 22 deletions(-)

diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
index 35682852ddea9..c9f7206591b30 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -73,7 +73,7 @@ static struct shash_desc *init_desc(char type, uint8_t hash_algo)
 {
 	long rc;
 	const char *algo;
-	struct crypto_shash **tfm;
+	struct crypto_shash **tfm, *tmp_tfm;
 	struct shash_desc *desc;
 
 	if (type == EVM_XATTR_HMAC) {
@@ -91,31 +91,31 @@ static struct shash_desc *init_desc(char type, uint8_t hash_algo)
 		algo = hash_algo_name[hash_algo];
 	}
 
-	if (*tfm == NULL) {
-		mutex_lock(&mutex);
-		if (*tfm)
-			goto out;
-		*tfm = crypto_alloc_shash(algo, 0, CRYPTO_NOLOAD);
-		if (IS_ERR(*tfm)) {
-			rc = PTR_ERR(*tfm);
-			pr_err("Can not allocate %s (reason: %ld)\n", algo, rc);
-			*tfm = NULL;
+	if (*tfm)
+		goto alloc;
+	mutex_lock(&mutex);
+	if (*tfm)
+		goto unlock;
+
+	tmp_tfm = crypto_alloc_shash(algo, 0, CRYPTO_NOLOAD);
+	if (IS_ERR(tmp_tfm)) {
+		pr_err("Can not allocate %s (reason: %ld)\n", algo,
+		       PTR_ERR(tmp_tfm));
+		mutex_unlock(&mutex);
+		return ERR_CAST(tmp_tfm);
+	}
+	if (type == EVM_XATTR_HMAC) {
+		rc = crypto_shash_setkey(tmp_tfm, evmkey, evmkey_len);
+		if (rc) {
+			crypto_free_shash(tmp_tfm);
 			mutex_unlock(&mutex);
 			return ERR_PTR(rc);
 		}
-		if (type == EVM_XATTR_HMAC) {
-			rc = crypto_shash_setkey(*tfm, evmkey, evmkey_len);
-			if (rc) {
-				crypto_free_shash(*tfm);
-				*tfm = NULL;
-				mutex_unlock(&mutex);
-				return ERR_PTR(rc);
-			}
-		}
-out:
-		mutex_unlock(&mutex);
 	}
-
+	*tfm = tmp_tfm;
+unlock:
+	mutex_unlock(&mutex);
+alloc:
 	desc = kmalloc(sizeof(*desc) + crypto_shash_descsize(*tfm),
 			GFP_KERNEL);
 	if (!desc)
-- 
2.26.2