Re: [PATCH net] net/sched: account classifier filter allocations to memcg

[Breno Leitao <leitao@debian.org>]

On Wed, Aug 19, 2026 at 10:37:33AM -0400, Jamal Hadi Salim wrote:
> 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: 0da974f4f303 ("[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>