crypto: rsa-pkcs1pad - fix buffer overread in pkcs1pad_verify_complete()

Before checking whether the expected digest_info is present, we need to
check that there are enough bytes remaining.

Fixes: a49de377e051 ("crypto: Add hash param to pkcs1pad")
Cc: <[e-mail]> # v4.6+
Cc: Tadeusz Struk <[e-mail]>
Signed-off-by: Eric Biggers <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

crypto/rsa-pkcs1pad.c

···
 	pos++;
 
 	if (digest_info) {
+		if (digest_info->size > dst_len - pos)
+			goto done;
 		if (crypto_memneq(out_buf + pos, digest_info->data,
 				  digest_info->size))
 			goto done;