* Re: [PATCH] net/sched: em_meta: clamp shift to value width
2026-11-08 22:31 [PATCH] net/sched: em_meta: clamp shift to value width Noah Weinberger
@ 2026-11-09 23:27 ` netdev-bot+sashiko
0 siblings, 0 replies; 2+ messages in thread
From: netdev-bot+sashiko @ 2026-11-09 23:27 UTC (permalink / raw)
To: NoahWeinberger2003
Cc: jhs, jiri, davem, edumazet, kuba, pabeni, netdev, linux-kernel
Thank you for your contribution! Sashiko AI review found 3 potential
issue(s) to consider.
Critical: 0 · High: 0 · Medium: 1 · Low: 2
If you believe none of the issues are legitimate, please reply to each
issue of Medium severity or higher and explain why the AI is wrong.
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(one such reply per series is enough).
- [Medium] In net/sched/em_meta.c, em_meta_change() (lines 942-945)
silently rewrites a user-supplied hdr.shift above BITS_PER_LONG-1 down…
- [Low] Commit-message and DCO problems.
- [Low] The patch has no Fixes: tag even though it fixes user-reachable
undefined behaviour.
--
net/sched: em_meta: clamp shift to value width
This patch clamps the user-supplied hdr.shift of both em_meta operands to
BITS_PER_LONG - 1 in em_meta_change(). The aim is to avoid an undefined
right shift in meta_int_apply_extras() when the u8 shift is BITS_PER_LONG
or larger.
> Hello,
>
[ ... ]
> Affected version/commit: mainline fa4ad798f6f681031775266044ba1b0cf3c670bd (v7.4-rc1, 2026-11-07), net/sched/em_meta.c:762-770 (meta_int_apply_extras), 906-941 (em_meta_change).
> Longstanding code; the sibling fix pattern exists in act_pedit.
>
> This was found during an assisted (AI-supported) static source review of
> mainline fa4ad798f6f681031775266044ba1b0cf3c670bd (v7.4-rc1, 2026-11-07); every code path cited below was re-verified by the
> reporter against that tree. The patch below has NOT been compiled and has
> NOT been runtime tested, and no reproducer was produced - the triggering
> conditions are described above instead. Please treat this as a report for
> verification rather than a battle-tested fix.
>
> Reported-by: Noah Weinberger <noahweinberger2003@proton.me>
[Severity: Low]
The commit has no Signed-off-by: line, so it can't be applied under the
DCO. Could it be resent with one?
The only trailer is this Reported-by:, and it names the patch author.
Is it needed?
The greeting, the "Affected version/commit" line with its tree hash and
line numbers, and the paragraph on compile and test status read like
cover letter material. Should they go below the --- line, or be dropped?
The paragraph also says "every code path cited below was re-verified",
but nothing is cited after that sentence.
[Severity: Low]
The changelog only says "Longstanding code". The shift in
meta_int_apply_extras() goes back to the initial git import. Should this
carry a tag so stable trees pick up the fix?
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> diff --git a/net/sched/em_meta.c b/net/sched/em_meta.c
> index 48bdd77df0a3e..6681c6793cb80 100644
> --- a/net/sched/em_meta.c
> +++ b/net/sched/em_meta.c
> @@ -936,6 +936,14 @@ static int em_meta_change(struct net *net, void *data, int len,
> memcpy(&meta->lvalue.hdr, &hdr->left, sizeof(hdr->left));
> memcpy(&meta->rvalue.hdr, &hdr->right, sizeof(hdr->right));
>
> + /* Keep the shift within the value width to avoid undefined
> + * behaviour in meta_int_apply_extras(), like act_pedit does.
> + */
> + if (meta->lvalue.hdr.shift > BITS_PER_LONG - 1)
> + meta->lvalue.hdr.shift = BITS_PER_LONG - 1;
> + if (meta->rvalue.hdr.shift > BITS_PER_LONG - 1)
> + meta->rvalue.hdr.shift = BITS_PER_LONG - 1;
[Severity: Medium]
Can this silent clamp change the match result for an out-of-range shift?
meta_int_apply_extras() uses the clamped value directly:
net/sched/em_meta.c:meta_int_apply_extras() {
if (v->hdr.shift)
dst->value >>= v->hdr.shift;
...
}
When shift is BITS_PER_LONG - 1, the top bit survives. The result is 0 or
1, not the 0 you would get if every bit were shifted out.
For example, on a 32-bit kernel, meta(random shift 40 eq 0) would fail
about half the time. On 64-bit, the same request always gives 0 for a
u32-sourced value. The same __u8 uAPI value now gives a verdict that
depends on BITS_PER_LONG.
em_meta_dump() also reports the rewritten shift back to userspace:
net/sched/em_meta.c:em_meta_dump() {
...
memcpy(&hdr.left, &meta->lvalue.hdr, sizeof(hdr.left));
memcpy(&hdr.right, &meta->rvalue.hdr, sizeof(hdr.right));
if (nla_put(skb, TCA_EM_META_HDR, sizeof(hdr), &hdr))
...
}
So tc filter show would print 63 or 31 instead of the configured shift.
Is that intended?
The new comment ties the clamp to the value width and to
meta_int_apply_extras(). However, the clamp also rewrites
TCF_META_TYPE_VAR operands. For those, shift is a byte trim count, and
a large value is not undefined:
net/sched/em_meta.c:meta_var_apply_extras() {
int shift = v->hdr.shift;
if (shift && shift < dst->len)
dst->len -= shift;
}
var_dev and var_sk_bound_if are at most 15 bytes long, so VAR matching
does not change. The dump does change, though, and the comment doesn't
describe that path.
In act_pedit the shift is used to compute an offset. Here, the shifted
value is compared directly with the user operand.
Would one of these be a better fit?
- Reject shift >= BITS_PER_LONG for TCF_META_TYPE_INT with -EINVAL.
- Leave hdr untouched, and set dst->value to 0 in meta_int_apply_extras()
when shift >= BITS_PER_LONG.
Either way, could the changelog describe the behaviour change?
> +
> if (!meta_is_supported(&meta->lvalue) ||
> !meta_is_supported(&meta->rvalue)) {
> err = -EOPNOTSUPP;
--
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/PonpZoq156a2aiUCGrHWYeg_kl3yFtp5oE2JaeCRJosmPfqWvGCrSlM-dn8lG7kSQpaAJuSaDN1zwr8X9UTC_DxfkFYZQVEQpV9kb-dMWxc%3D%40proton.me
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