evm: Don't deadlock if a crypto algorithm is unavailable

When EVM attempts to appraise a file signed with a crypto algorithm the
kernel doesn't have support for, it will cause the kernel to trigger a
module load. If the EVM policy includes appraisal of kernel modules this
will in turn call back into EVM - since EVM is holding a lock until the
crypto initialisation is complete, this triggers a deadlock. Add a
CRYPTO_NOLOAD flag and skip module loading if it's set, and add that flag
in the EVM case in order to fail gracefully with an error message
instead of deadlocking.
Signed-off-by: Matthew Garrett <mjg59@google.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

crypto/api.c

@@ -229,7 +229,7 @@ static struct crypto_alg *crypto_larval_lookup(const char *name, u32 type,
 	mask &= ~(CRYPTO_ALG_LARVAL | CRYPTO_ALG_DEAD);
 
 	alg = crypto_alg_lookup(name, type, mask);
-	if (!alg) {
+	if (!alg && !(mask & CRYPTO_NOLOAD)) {
 		request_module("crypto-%s", name);
 
 		if (!((type ^ CRYPTO_ALG_NEED_FALLBACK) & mask &

include/linux/crypto.h

@@ -112,6 +112,11 @@
  */
 #define CRYPTO_ALG_OPTIONAL_KEY		0x00004000
 
+/*
+ * Don't trigger module loading
+ */
+#define CRYPTO_NOLOAD			0x00008000
+
 /*
  * Transform masks and values (for crt_flags).
  */

security/integrity/evm/evm_crypto.c

@@ -97,7 +97,8 @@ static struct shash_desc *init_desc(char type)
 		mutex_lock(&mutex);
 		if (*tfm)
 			goto out;
-		*tfm = crypto_alloc_shash(algo, 0, CRYPTO_ALG_ASYNC);
+		*tfm = crypto_alloc_shash(algo, 0,
+					  CRYPTO_ALG_ASYNC | CRYPTO_NOLOAD);
 		if (IS_ERR(*tfm)) {
 			rc = PTR_ERR(*tfm);
 			pr_err("Can not allocate %s (reason: %ld)\n", algo, rc);