[PATCH 5.15.y] X.509: Fix validation of ASN.1 certificate header

[Sasha Levin <sashal@kernel.org>]

From: Lukas Wunner <lukas@wunner.de>

[ Upstream commit 3b626ba431c4501512ad07549310685e07fe4706 ]

x509_load_certificate_list() seeks to enforce that a certificate starts
with 0x30 0x82 (ASN.1 SEQUENCE tag followed by a length of more than 256
and less than 65535 bytes).

But it only enforces that *either* of those two byte values are present,
instead of checking for the *conjunction* of the two values.  Fix it.

Fixes: 631cc66eb9ea ("MODSIGN: Provide module signing public keys to the kernel")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/r/20260508033917.B5873C2BCB0@smtp.kernel.org/
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: stable@vger.kernel.org # v3.7+
Reviewed-by: Ignat Korchagin <ignat@linux.win>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 certs/common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/certs/common.c b/certs/common.c
index 16a220887a53e8..c7bcf3f6a77e39 100644
--- a/certs/common.c
+++ b/certs/common.c
@@ -20,7 +20,7 @@ int load_certificate_list(const u8 cert_list[],
 		 */
 		if (end - p < 4)
 			goto dodgy_cert;
-		if (p[0] != 0x30 &&
+		if (p[0] != 0x30 ||
 		    p[1] != 0x82)
 			goto dodgy_cert;
 		plen = (p[2] << 8) | p[3];
-- 
2.53.0