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From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@toke.dk>
To: Samuel Moelius <sam.moelius@trailofbits.com>
Cc: Samuel Moelius <sam.moelius@trailofbits.com>, Jamal Hadi Salim
 <jhs@mojatatu.com>, Jiri Pirko <jiri@resnulli.us>, "David S. Miller"
 <davem@davemloft.net>, Eric Dumazet <edumazet@google.com>, Jakub Kicinski
 <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>, Simon Horman
 <horms@kernel.org>, "moderated list:CAKE QDISC"
 <cake@lists.bufferbloat.net>, "open list:TC subsystem"
 <netdev@vger.kernel.org>, open list <linux-kernel@vger.kernel.org>
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References: <20260702000758.297407.e5c888d9d99d.cake-overhead-underflow@trailofbits.com>
Date: Thu, 02 Jul 2026 11:40:27 +0200
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Subject: [Cake] Re: [PATCH v3] net/sched: cake: reject overhead values that underflow length
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Samuel Moelius <sam.moelius@trailofbits.com> writes:

> 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 <sam.moelius@trailofbits.com>

Acked-by: Toke H=C3=B8iland-J=C3=B8rgensen <toke@toke.dk>