crypto: rsa-pkcs1pad: Don't WARN on an empty digest

KEYCTL_PKEY_VERIFY lets an unprivileged caller supply a zero-length
digest (in_len == 0).  keyctl_pkey_params_get_2() accepts the zero
length and the request reaches pkcs1pad_verify(), where the empty
digest is rejected but only after being passed through
WARN_ON(!digest_size).  The warning is therefore directly
user-triggerable, and on kernels built with panic_on_warn=1 an
unprivileged process can panic the machine -- a local denial of
service.  Reproduced as UID 65534 in a setuid sandbox.

Keep rejecting the invalid request with -EINVAL, but do not emit a
warning for the user-controlled length.

Mainline does not contain this code path; commit 1e562deace
("crypto: rsassa-pkcs1 - Migrate to sig_alg backend") removed
pkcs1pad_verify() in v6.13-rc1.  This is a minimal fix for the
affected stable branches.  It applies as-is to 6.1.y, 6.6.y and
6.12.y (identical pkcs1pad_verify); the 5.10.y/5.15.y form is sent
as a separate patch due to the older req->dst_len spelling.

Found by 0sec automated security-research tooling (https://0sec.ai).

Fixes: c7381b0128 ("crypto: akcipher - new verify API for public key algorithms")
Cc: stable@vger.kernel.org
Assisted-by: 0sec:multi-model
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>

crypto/rsa-pkcs1pad.c

@@ -534,7 +534,7 @@ static int pkcs1pad_verify(struct akcipher_request *req)
 	const unsigned int digest_size = req->dst_len;
 	int err;
 
-	if (WARN_ON(req->dst) || WARN_ON(!digest_size) ||
+	if (WARN_ON(req->dst) || !digest_size ||
 	    !ctx->key_size || sig_size != ctx->key_size)
 		return -EINVAL;