[Cake] [PATCH] net/sched: cake: reject overhead values that underflow length

[Cake] [PATCH] net/sched: cake: reject overhead values that underflow length

[Samuel Moelius]

CAKE accepts overhead values that can make adjusted packet length
arithmetic underflow.  A negative effective length can wrap through
unsigned arithmetic and become a large value.

Such configurations make rate accounting depend on integer wraparound
rather than on the packet size userspace intended to model.

Validate overhead settings before using them in adjusted length
calculations.

Assisted-by: Codex:gpt-5.5-cyber-preview
Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
---
 net/sched/sch_cake.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index 5862933be8d7..03972e5525b5 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -2308,12 +2308,18 @@ static void cake_reset(struct Qdisc *sch)
 		cake_clear_tin(sch, c);
 }
 
+static const struct netlink_range_validation_signed cake_overhead_range = {
+	.min = -64,
+	.max = 256,
+};
+
 static const struct nla_policy cake_policy[TCA_CAKE_MAX + 1] = {
 	[TCA_CAKE_BASE_RATE64]   = { .type = NLA_U64 },
 	[TCA_CAKE_DIFFSERV_MODE] = { .type = NLA_U32 },
 	[TCA_CAKE_ATM]		 = { .type = NLA_U32 },
 	[TCA_CAKE_FLOW_MODE]     = { .type = NLA_U32 },
-	[TCA_CAKE_OVERHEAD]      = { .type = NLA_S32 },
+	[TCA_CAKE_OVERHEAD]      =
+		NLA_POLICY_FULL_RANGE_SIGNED(NLA_S32, &cake_overhead_range),
 	[TCA_CAKE_RTT]		 = { .type = NLA_U32 },
 	[TCA_CAKE_TARGET]	 = { .type = NLA_U32 },
 	[TCA_CAKE_AUTORATE]      = { .type = NLA_U32 },
-- 
2.43.0

[Cake] Re: [PATCH] net/sched: cake: reject overhead values that underflow length

[Eric Dumazet]

On Mon, Jun 8, 2026 at 5:06 PM Samuel Moelius
<sam.moelius@trailofbits.com> wrote:
>
> CAKE accepts overhead values that can make adjusted packet length
> arithmetic underflow.  A negative effective length can wrap through
> unsigned arithmetic and become a large value.
>
> Such configurations make rate accounting depend on integer wraparound
> rather than on the packet size userspace intended to model.
>
> Validate overhead settings before using them in adjusted length
> calculations.
>
> Assisted-by: Codex:gpt-5.5-cyber-preview
> Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
> ---

We need a Fixes: tag

pw-bot: cr