net/sched: account classifier filter allocations to memcg

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Allocations in the tc classifier *_change() paths (filter objects,
per-CPU counters, and per-filter aux data) use plain GFP_KERNEL without
__GFP_ACCOUNT, allowing unprivileged users to pin kernel memory outside
memcg charging. The shared tcf_exts_init_ex() action array allocation in
cls_api.c was also uncharged; this patch closes it along with the
per-classifier filter-object/percpu/aux allocations that remain
unaccounted.

Add GFP_KERNEL_ACCOUNT to:
- the shared tcf_exts_init_ex() action array (cls_api.c), common to every
  filter of every classifier (32 pointers, 256 bytes);
- the filter-object, per-CPU-counter, and per-filter aux allocations in
  cls_basic, cls_bpf, cls_cgroup, cls_flow, cls_flower, cls_fw,
  cls_matchall, cls_route and cls_u32;
- the u32_init_knode() replace-path knode allocation (cls_u32.c), which
  allocates the same struct tc_u_knode + sel.keys on every replace of an
  existing knode and was missed by the create-path-only conversion.

Also fix the cls_basic error path: basic_change() inserts fnew into the
IDR before allocating the per-CPU counter. If alloc_percpu() fails the
errout path kfree'd fnew without idr_remove, leaving a dangling pointer
in the IDR. With GFP_KERNEL_ACCOUNT the percpu alloc becomes failable
on demand (memcg at memory.max), making the dead path attacker-reachable
and burning the handle permanently. Add the idr_remove on the percpu
failure path, matching the basic_set_parms failure-path pattern.

Note: vega@nebusec.ai provided a poc for basic_cls, but it was easy to
extend to the other classifiers.

Conditions to recreate the bug:
- CONFIG_NET_SCHED, CONFIG_NET_CLS_* (the classifier being used),
  CONFIG_NET_CLS_ACT, CONFIG_MEMCG, CONFIG_USER_NS, CONFIG_NET_NS.
- Unprivileged user in a fresh user+network namespace (unshare -Urn),
  or root with CAP_NET_ADMIN.
- Create a large number of tc filters (e.g. tc filter add dev lo
  ingress ... <classifier> ...) while watching a memcg-limited cgroup:
  system slab grows far faster than memory.current, pinning kernel
  memory outside memcg charging.

Fixes: 0da974f4f3 ("[NET]: Conversions from kmalloc+memset to k(z|c)alloc.")
Reported-by: vega@nebusec.ai
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260819143733.57538-1-jhs@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

...

This commit is contained in:

Jamal Hadi Salim 2026-08-19 10:37:33 -04:00 • committed by Jakub Kicinski

parent e755276c9f

commit 1beb81947e

10 changed files with 25 additions and 21 deletions

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3

net/sched/cls_api.c

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6

net/sched/cls_basic.c

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6

net/sched/cls_bpf.c

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2

net/sched/cls_cgroup.c

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net/sched/cls_flow.c

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4

net/sched/cls_flower.c

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net/sched/cls_fw.c

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4

net/sched/cls_matchall.c

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4

net/sched/cls_route.c

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11

net/sched/cls_u32.c

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