BUG/MEDIUM: jwt: Clear SSL error queue on error when checking the signature

When the signature included in a JWT is verified, if an error occurred, one
or more SSL errors are queued and never cleared. These errors may be then
caught by the SSL stack and a fatal SSL error may be erroneously reported
during a SSL received or send.

So we must take care to clear the SSL error queue when the signature
verification failed.

This patch should fix issue #2643. It must be backported as far as 2.6.

(cherry picked from commit 46b1fec0e9a6afe2c12fd4dff7c8a0d788aa6dd4)
Signed-off-by: Willy Tarreau <w@1wt.eu>

src/jwt.c

@@ -364,6 +364,13 @@ jwt_jwsverify_rsa_ecdsa(const struct jwt_ctx *ctx, struct buffer *decoded_signat
 
 end:
 	EVP_MD_CTX_free(evp_md_ctx);
+	if (retval != JWT_VRFY_OK) {
+		/* Don't forget to remove SSL errors to be sure they cannot be
+		 * caught elsewhere. The error queue is cleared because it seems
+		 * at least 2 errors are produced.
+		 */
+		ERR_clear_error();
+	}
 	return retval;
 }