[PATCH 5.10/5.15.y] crypto: rsa-pkcs1pad: Don't WARN on an empty digest

[Doruk Tan Ozturk <doruk@0sec.ai>]

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(!req->dst_len).  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.

This is the 5.10.y/5.15.y form of the fix, where the length is read
directly from req->dst_len rather than cached in a digest_size local.
Mainline does not contain this code path; commit 1e562deacecc
("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.  The 6.1.y/6.6.y/6.12.y form
(WARN_ON(!digest_size)) is sent as a separate patch.

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

Fixes: c7381b012872 ("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>
---
 crypto/rsa-pkcs1pad.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/crypto/rsa-pkcs1pad.c b/crypto/rsa-pkcs1pad.c
index e2f4ccbd71dd..d10076bb6ec2 100644
--- a/crypto/rsa-pkcs1pad.c
+++ b/crypto/rsa-pkcs1pad.c
@@ -535,7 +535,7 @@ static int pkcs1pad_verify(struct akcipher_request *req)
 	int err;
 
 	if (WARN_ON(req->dst) ||
-	    WARN_ON(!req->dst_len) ||
+	    !req->dst_len ||
 	    !ctx->key_size || req->src_len != ctx->key_size)
 		return -EINVAL;
 
-- 
2.43.0