[PATCH] X.509: Fix crash caused by NULL pointer

[Tianjia Zhang]

On the following call path, `sig->pkey_algo` is not assigned
in asymmetric_key_verify_signature(), which causes runtime
crash in public_key_verify_signature().

  keyctl_pkey_verify
    asymmetric_key_verify_signature
      verify_signature
        public_key_verify_signature

This patch simply check this situation and fixes the crash
caused by NULL pointer.

Fixes: 215525639631 ("X.509: support OSCCA SM2-with-SM3 certificate 
verification")
Cc: sta...@vger.kernel.org # v5.10+
Reported-by: Tobias Markus <tob...@markus-regensburg.de>
Signed-off-by: Tianjia Zhang <tianjia.zh...@linux.alibaba.com>
---
 crypto/asymmetric_keys/public_key.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/crypto/asymmetric_keys/public_key.c 
b/crypto/asymmetric_keys/public_key.c
index 8892908ad58c..788a4ba1e2e7 100644
--- a/crypto/asymmetric_keys/public_key.c
+++ b/crypto/asymmetric_keys/public_key.c
@@ -356,7 +356,8 @@ int public_key_verify_signature(const struct public_key 
*pkey,
        if (ret)
                goto error_free_key;
 
-       if (strcmp(sig->pkey_algo, "sm2") == 0 && sig->data_size) {
+       if (sig->pkey_algo && strcmp(sig->pkey_algo, "sm2") == 0 &&
+           sig->data_size) {
                ret = cert_sig_digest_update(sig, tfm);
                if (ret)
                        goto error_free_key;
-- 
2.19.1.3.ge56e4f7