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Sun, 19 Jul 2026 20:45:15 -0700 (PDT) From: "Cen Zhang (Microsoft)" To: jhs@mojatatu.com, jiri@resnulli.us, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com Cc: horms@kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, AutonomousCodeSecurity@microsoft.com, tgopinath@linux.microsoft.com, kys@microsoft.com, blbllhy@gmail.com Subject: [PATCH] net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds Date: Sun, 19 Jul 2026 23:45:14 -0400 Message-ID: <20260720034514.23053-1-blbllhy@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 ยง6.5.7, 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@microsoft.com Signed-off-by: Cen Zhang (Microsoft) --- 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..e7a9a49353f9 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->flags & TC_U32_VAROFFSET && 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