* [PATCH net] net/sched: cls_route: fix fastmap use-after-free on filter
@ 2026-07-29 9:44 Jamal Hadi Salim
2026-07-29 9:47 ` Jamal Hadi Salim
0 siblings, 1 reply; 2+ messages in thread
From: Jamal Hadi Salim @ 2026-07-29 9:44 UTC (permalink / raw)
To: netdev
Cc: pabeni, kuba, davem, edumazet, horms, john.fastabend, jiri,
zdi-disclosures, santosh.kalluri129, victor, security, stable,
Jamal Hadi Salim
The route4 classifier maintains a 16-slot fastmap cache that stores raw
struct route4_filter pointers indexed by (id, iif). The reader
(route4_classify) populates this cache via route4_set_fastmap() for every
classified packet that hits a filter. The writer (route4_delete,
route4_change) clears the cache via route4_reset_fastmap() before
RCU-deferred kfree of the filter.
This creates a UAF race:
1. Reader walks the RCU-protected bucket chain, finds filter f
2. Writer unlinks f, calls route4_reset_fastmap(), then tcf_queue_work()
3. Reader calls route4_set_fastmap() and writes f into the cache
*after* the writer's reset, caching a pointer about to be freed
4. After the RCU grace period, kfree(f) executes
5. Next classified packet on the same (id, iif) tuple hits the stale
fastmap entry and reads f->res from freed memory
Reproduced with an mdelay(100) accelerator in route4_set_fastmap() and a
concurrent add/delete stress test (provided by both zdi and Santosh).
Both triggered KASAN slab-use-after-free reports in the route4 fastmap
paths.
Fix:
Introduce a per-filter boolean dying flag to suppress stale fastmap
republishing by in-flight readers.
Fixes: 1109c00547fc ("net: sched: RCU cls_route")
Reported-by: zdi-disclosures@trendmicro.com
Reported-by: Santosh Kalluri <santosh.kalluri129@gmail.com>
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Tested-by: Santosh Kalluri <santosh.kalluri129@gmail.com>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
V1->V2
1. Fix based on feedback from the Sashikos
https://sashiko.dev/#/patchset/20260723105210.817079-1-jhs%40mojatatu.com
https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260723105210.817079-1-jhs%40mojatatu.com
Restore the "if (tcf_exts_get_net(&f->exts)) route4_queue_work(f);
else __route4_delete_filter(f);"
preserving the tcf_exts_get_net() contract for netns teardown.
V2->V3
Revert earlier suggestion from Santosh's rcu sync to Paolo's suggestion to use
the dying flag approach.
---
net/sched/cls_route.c | 35 ++++++++++++++++++++++++++---------
1 file changed, 26 insertions(+), 9 deletions(-)
diff --git a/net/sched/cls_route.c b/net/sched/cls_route.c
index bd6f945bd388..eded7aacd3f7 100644
--- a/net/sched/cls_route.c
+++ b/net/sched/cls_route.c
@@ -52,6 +52,7 @@ struct route4_filter {
struct tcf_result res;
struct tcf_exts exts;
u32 handle;
+ bool dying;
struct route4_bucket *bkt;
struct tcf_proto *tp;
struct rcu_work rwork;
@@ -66,9 +67,11 @@ static inline int route4_fastmap_hash(u32 id, int iif)
static DEFINE_SPINLOCK(fastmap_lock);
static void
-route4_reset_fastmap(struct route4_head *head)
+route4_reset_fastmap(struct route4_head *head, struct route4_filter *f)
{
spin_lock_bh(&fastmap_lock);
+ if (f)
+ f->dying = true;
memset(head->fastmap, 0, sizeof(head->fastmap));
spin_unlock_bh(&fastmap_lock);
}
@@ -81,9 +84,11 @@ route4_set_fastmap(struct route4_head *head, u32 id, int iif,
/* fastmap updates must look atomic to aling id, iff, filter */
spin_lock_bh(&fastmap_lock);
- head->fastmap[h].id = id;
- head->fastmap[h].iif = iif;
- head->fastmap[h].filter = f;
+ if (f == ROUTE4_FAILURE || !f->dying) {
+ head->fastmap[h].id = id;
+ head->fastmap[h].iif = iif;
+ head->fastmap[h].filter = f;
+ }
spin_unlock_bh(&fastmap_lock);
}
@@ -297,6 +302,13 @@ static void route4_destroy(struct tcf_proto *tp, bool rtnl_held,
next = rtnl_dereference(f->next);
RCU_INIT_POINTER(b->ht[h2], next);
tcf_unbind_filter(tp, &f->res);
+ /* Mark the filter dying under fastmap_lock so
+ * any in-flight reader that still holds it
+ * will skip the republish in route4_set_fastmap().
+ */
+ spin_lock_bh(&fastmap_lock);
+ f->dying = true;
+ spin_unlock_bh(&fastmap_lock);
if (tcf_exts_get_net(&f->exts))
route4_queue_work(f);
else
@@ -307,6 +319,11 @@ static void route4_destroy(struct tcf_proto *tp, bool rtnl_held,
kfree_rcu(b, rcu);
}
}
+
+ /* All filters are unlinked and marked dying, so no in-flight
+ * reader can republish a stale entry after this reset.
+ */
+ route4_reset_fastmap(head, NULL);
kfree_rcu(head, rcu);
}
@@ -334,11 +351,11 @@ static int route4_delete(struct tcf_proto *tp, void *arg, bool *last,
/* unlink it */
RCU_INIT_POINTER(*fp, rtnl_dereference(f->next));
- /* Remove any fastmap lookups that might ref filter
- * notice we unlink'd the filter so we can't get it
- * back in the fastmap.
+ /* Clear any fastmap entries that may ref this filter and
+ * mark it dying so in-flight readers can't republish it
+ * after the reset.
*/
- route4_reset_fastmap(head);
+ route4_reset_fastmap(head, f);
/* Delete it */
tcf_unbind_filter(tp, &f->res);
@@ -558,7 +575,7 @@ static int route4_change(struct net *net, struct sk_buff *in_skb,
}
}
- route4_reset_fastmap(head);
+ route4_reset_fastmap(head, fold);
*arg = f;
if (fold) {
tcf_unbind_filter(tp, &fold->res);
--
2.34.1
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH net] net/sched: cls_route: fix fastmap use-after-free on filter
2026-07-29 9:44 [PATCH net] net/sched: cls_route: fix fastmap use-after-free on filter Jamal Hadi Salim
@ 2026-07-29 9:47 ` Jamal Hadi Salim
0 siblings, 0 replies; 2+ messages in thread
From: Jamal Hadi Salim @ 2026-07-29 9:47 UTC (permalink / raw)
To: netdev
Cc: pabeni, kuba, davem, edumazet, horms, john.fastabend, jiri,
zdi-disclosures, santosh.kalluri129, victor, security, stable
On Wed, Jul 29, 2026 at 5:44 AM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
>
> The route4 classifier maintains a 16-slot fastmap cache that stores raw
> struct route4_filter pointers indexed by (id, iif). The reader
> (route4_classify) populates this cache via route4_set_fastmap() for every
> classified packet that hits a filter. The writer (route4_delete,
> route4_change) clears the cache via route4_reset_fastmap() before
> RCU-deferred kfree of the filter.
>
Sigh. I forgot to add v3 on the subject. I could resend if needed.
cheers,
jamal
> This creates a UAF race:
> 1. Reader walks the RCU-protected bucket chain, finds filter f
> 2. Writer unlinks f, calls route4_reset_fastmap(), then tcf_queue_work()
> 3. Reader calls route4_set_fastmap() and writes f into the cache
> *after* the writer's reset, caching a pointer about to be freed
> 4. After the RCU grace period, kfree(f) executes
> 5. Next classified packet on the same (id, iif) tuple hits the stale
> fastmap entry and reads f->res from freed memory
>
> Reproduced with an mdelay(100) accelerator in route4_set_fastmap() and a
> concurrent add/delete stress test (provided by both zdi and Santosh).
> Both triggered KASAN slab-use-after-free reports in the route4 fastmap
> paths.
>
> Fix:
> Introduce a per-filter boolean dying flag to suppress stale fastmap
> republishing by in-flight readers.
>
> Fixes: 1109c00547fc ("net: sched: RCU cls_route")
> Reported-by: zdi-disclosures@trendmicro.com
> Reported-by: Santosh Kalluri <santosh.kalluri129@gmail.com>
> Suggested-by: Paolo Abeni <pabeni@redhat.com>
> Tested-by: Victor Nogueira <victor@mojatatu.com>
> Tested-by: Santosh Kalluri <santosh.kalluri129@gmail.com>
> Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
>
> ---
> V1->V2
> 1. Fix based on feedback from the Sashikos
> https://sashiko.dev/#/patchset/20260723105210.817079-1-jhs%40mojatatu.com
> https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260723105210.817079-1-jhs%40mojatatu.com
>
> Restore the "if (tcf_exts_get_net(&f->exts)) route4_queue_work(f);
> else __route4_delete_filter(f);"
> preserving the tcf_exts_get_net() contract for netns teardown.
>
> V2->V3
> Revert earlier suggestion from Santosh's rcu sync to Paolo's suggestion to use
> the dying flag approach.
> ---
> net/sched/cls_route.c | 35 ++++++++++++++++++++++++++---------
> 1 file changed, 26 insertions(+), 9 deletions(-)
>
> diff --git a/net/sched/cls_route.c b/net/sched/cls_route.c
> index bd6f945bd388..eded7aacd3f7 100644
> --- a/net/sched/cls_route.c
> +++ b/net/sched/cls_route.c
> @@ -52,6 +52,7 @@ struct route4_filter {
> struct tcf_result res;
> struct tcf_exts exts;
> u32 handle;
> + bool dying;
> struct route4_bucket *bkt;
> struct tcf_proto *tp;
> struct rcu_work rwork;
> @@ -66,9 +67,11 @@ static inline int route4_fastmap_hash(u32 id, int iif)
>
> static DEFINE_SPINLOCK(fastmap_lock);
> static void
> -route4_reset_fastmap(struct route4_head *head)
> +route4_reset_fastmap(struct route4_head *head, struct route4_filter *f)
> {
> spin_lock_bh(&fastmap_lock);
> + if (f)
> + f->dying = true;
> memset(head->fastmap, 0, sizeof(head->fastmap));
> spin_unlock_bh(&fastmap_lock);
> }
> @@ -81,9 +84,11 @@ route4_set_fastmap(struct route4_head *head, u32 id, int iif,
>
> /* fastmap updates must look atomic to aling id, iff, filter */
> spin_lock_bh(&fastmap_lock);
> - head->fastmap[h].id = id;
> - head->fastmap[h].iif = iif;
> - head->fastmap[h].filter = f;
> + if (f == ROUTE4_FAILURE || !f->dying) {
> + head->fastmap[h].id = id;
> + head->fastmap[h].iif = iif;
> + head->fastmap[h].filter = f;
> + }
> spin_unlock_bh(&fastmap_lock);
> }
>
> @@ -297,6 +302,13 @@ static void route4_destroy(struct tcf_proto *tp, bool rtnl_held,
> next = rtnl_dereference(f->next);
> RCU_INIT_POINTER(b->ht[h2], next);
> tcf_unbind_filter(tp, &f->res);
> + /* Mark the filter dying under fastmap_lock so
> + * any in-flight reader that still holds it
> + * will skip the republish in route4_set_fastmap().
> + */
> + spin_lock_bh(&fastmap_lock);
> + f->dying = true;
> + spin_unlock_bh(&fastmap_lock);
> if (tcf_exts_get_net(&f->exts))
> route4_queue_work(f);
> else
> @@ -307,6 +319,11 @@ static void route4_destroy(struct tcf_proto *tp, bool rtnl_held,
> kfree_rcu(b, rcu);
> }
> }
> +
> + /* All filters are unlinked and marked dying, so no in-flight
> + * reader can republish a stale entry after this reset.
> + */
> + route4_reset_fastmap(head, NULL);
> kfree_rcu(head, rcu);
> }
>
> @@ -334,11 +351,11 @@ static int route4_delete(struct tcf_proto *tp, void *arg, bool *last,
> /* unlink it */
> RCU_INIT_POINTER(*fp, rtnl_dereference(f->next));
>
> - /* Remove any fastmap lookups that might ref filter
> - * notice we unlink'd the filter so we can't get it
> - * back in the fastmap.
> + /* Clear any fastmap entries that may ref this filter and
> + * mark it dying so in-flight readers can't republish it
> + * after the reset.
> */
> - route4_reset_fastmap(head);
> + route4_reset_fastmap(head, f);
>
> /* Delete it */
> tcf_unbind_filter(tp, &f->res);
> @@ -558,7 +575,7 @@ static int route4_change(struct net *net, struct sk_buff *in_skb,
> }
> }
>
> - route4_reset_fastmap(head);
> + route4_reset_fastmap(head, fold);
> *arg = f;
> if (fold) {
> tcf_unbind_filter(tp, &fold->res);
> --
> 2.34.1
>
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