[PATCH 4.9 076/111] evm: Dont deadlock if a crypto algorithm is unavailable

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Matthew Garrett <mjg59@xxxxxxxxxx>

[ Upstream commit e2861fa71641c6414831d628a1f4f793b6562580 ]

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@xxxxxxxxxx>
Acked-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 crypto/api.c                        |    2 +-
 include/linux/crypto.h              |    5 +++++
 security/integrity/evm/evm_crypto.c |    3 ++-
 3 files changed, 8 insertions(+), 2 deletions(-)

--- a/crypto/api.c
+++ b/crypto/api.c
@@ -215,7 +215,7 @@ struct crypto_alg *crypto_larval_lookup(
 	type &= mask;
 
 	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 &
--- a/include/linux/crypto.h
+++ b/include/linux/crypto.h
@@ -109,6 +109,11 @@
 #define CRYPTO_ALG_OPTIONAL_KEY		0x00004000
 
 /*
+ * Don't trigger module loading
+ */
+#define CRYPTO_NOLOAD			0x00008000
+
+/*
  * Transform masks and values (for crt_flags).
  */
 #define CRYPTO_TFM_NEED_KEY		0x00000001
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -94,7 +94,8 @@ static struct shash_desc *init_desc(char
 		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);