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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , cake@lists.bufferbloat.net (moderated list:CAKE QDISC), netdev@vger.kernel.org (open list:TC subsystem), linux-kernel@vger.kernel.org (open list) Subject: [PATCH] net/sched: cake: reject overhead values that underflow length Date: Wed, 1 Jul 2026 23:56:57 +0000 Message-ID: <20260701235656.296208.68e320e0746d.cake-overhead-underflow@trailofbits.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit CAKE accepts signed overhead values and stores them in an s16, but the adjusted packet length calculation uses unsigned arithmetic. A negative effective length can therefore wrap to a large value. Such configurations make rate accounting depend on integer wraparound rather than on the packet size userspace intended to model. A static netlink lower bound is not enough because packets reaching CAKE can be smaller than any reasonable manual-overhead allowance. Fold the signed overhead adjustment into the existing datapath MPU clamp so negative adjusted lengths are clamped before link-layer framing adjustments. Fixes: a729b7f0bd5b ("sch_cake: Add overhead compensation support to the rate shaper") Assisted-by: Codex:gpt-5.5-cyber-preview Signed-off-by: Samuel Moelius --- Changes in v3: - Adjust how check is performed Changes in v2: - Add fixes tag net/sched/sch_cake.c | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-) diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c index a3c185505afc..f78f8e950776 100644 --- a/net/sched/sch_cake.c +++ b/net/sched/sch_cake.c @@ -1389,10 +1389,7 @@ static u32 cake_calc_overhead(struct cake_sched_data *qd, u32 len, u32 off) if (qd->min_netlen > len) WRITE_ONCE(qd->min_netlen, len); - len += q->rate_overhead; - - if (len < q->rate_mpu) - len = q->rate_mpu; + len = max((s32)len + q->rate_overhead, (s32)q->rate_mpu); if (q->atm_mode == CAKE_ATM_ATM) { len += 47; -- 2.43.0