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>

1 files changed, 2 insertions, 1 deletions

diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
index b60524310855..c20e3142b541 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/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);