[PATCH net v2] net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds

[Cen Zhang (Microsoft)]

u32_change() copies the user-provided tc_u32_sel.offshift (unsigned char,
0-255) into the kernel knode object without bounds validation. When a
packet later hits u32_classify() with TC_U32_VAROFFSET set, it evaluates
`ntohs(offmask & *data) >> offshift` where the left operand is a 16-bit
value promoted to a 32-bit int. Any offshift >= 32 is undefined behavior
per C11 6.5.7p3, triggerable by an unprivileged user via user/network
namespaces.

UBSAN: shift-out-of-bounds in net/sched/cls_u32.c:236:43
shift exponent 32 is too large for 32-bit type int

Fix this by rejecting offshift >= 16 during filter creation in
u32_change().

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: AutonomousCodeSecurity@xxxxxxxxxxxxx
Link: https://lore.kernel.org/all/20260720034514.23053-1-blbllhy@xxxxxxxxx
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@xxxxxxxxx>
Acked-by: Jamal Hadi Salim <jhs@xxxxxxxxxxxx>
Tested-by: Jamal Hadi Salim <jhs@xxxxxxxxxxxx>
Tested-by: Victor Nogueira <victor@xxxxxxxxxxxx>
---
v2:
 - Drop TC_U32_VAROFFSET condition as suggested by Jamal.

 net/sched/cls_u32.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index 8f30cc82181d..ac98b1c2144a 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -1107,6 +1107,13 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 		goto erridr;
 	}
 
+	if (s->offshift >= 16) {
+		NL_SET_ERR_MSG_MOD(extack,
+				   "offshift must be less than 16");
+		err = -EINVAL;
+		goto erridr;
+	}
+
 	n = kzalloc_flex(*n, sel.keys, s->nkeys);
 	if (n == NULL) {
 		err = -ENOBUFS;
-- 
2.53.0