bpf: Check size of the signature buffer

Accept only a SHA256 sized buffer.

Fixes: 3492715 ("bpf: Implement signature verification for BPF programs")
Reported-by: Chris Mason <[email protected]>
Signed-off-by: KP Singh <[email protected]>

Author: sinkap

kernel/bpf/syscall.c

@@ -2826,6 +2826,9 @@ static int bpf_prog_verify_signature(struct bpf_prog *prog, union bpf_attr *attr
 	void *sig;
 	int err = 0;
 
+	if (attr->signature_size != SHA256_DIGEST_SIZE)
+		return -EINVAL;
+
 	if (system_keyring_id_check(attr->keyring_id) == 0)
 		key = bpf_lookup_system_key(attr->keyring_id);
 	else