This is a note to let you know that I've just added the patch titled
bpf: Fix racing between bpf_timer_cancel_and_free and bpf_timer_cancel
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
bpf-fix-racing-between-bpf_timer_cancel_and_free-and.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit e78ed942eb0efe2be97543689b8db24fa9a62b4b
Author: Martin KaFai Lau <martin.lau@xxxxxxxxxx>
Date: Thu Feb 15 13:12:17 2024 -0800
bpf: Fix racing between bpf_timer_cancel_and_free and bpf_timer_cancel
[ Upstream commit 0281b919e175bb9c3128bd3872ac2903e9436e3f ]
The following race is possible between bpf_timer_cancel_and_free
and bpf_timer_cancel. It will lead a UAF on the timer->timer.
bpf_timer_cancel();
spin_lock();
t = timer->time;
spin_unlock();
bpf_timer_cancel_and_free();
spin_lock();
t = timer->timer;
timer->timer = NULL;
spin_unlock();
hrtimer_cancel(&t->timer);
kfree(t);
/* UAF on t */
hrtimer_cancel(&t->timer);
In bpf_timer_cancel_and_free, this patch frees the timer->timer
after a rcu grace period. This requires a rcu_head addition
to the "struct bpf_hrtimer". Another kfree(t) happens in bpf_timer_init,
this does not need a kfree_rcu because it is still under the
spin_lock and timer->timer has not been visible by others yet.
In bpf_timer_cancel, rcu_read_lock() is added because this helper
can be used in a non rcu critical section context (e.g. from
a sleepable bpf prog). Other timer->timer usages in helpers.c
have been audited, bpf_timer_cancel() is the only place where
timer->timer is used outside of the spin_lock.
Another solution considered is to mark a t->flag in bpf_timer_cancel
and clear it after hrtimer_cancel() is done. In bpf_timer_cancel_and_free,
it busy waits for the flag to be cleared before kfree(t). This patch
goes with a straight forward solution and frees timer->timer after
a rcu grace period.
Fixes: b00628b1c7d5 ("bpf: Introduce bpf timers.")
Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Martin KaFai Lau <martin.lau@xxxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Acked-by: Hou Tao <houtao1@xxxxxxxxxx>
Link: https://lore.kernel.org/bpf/20240215211218.990808-1-martin.lau@xxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index e68ef39cda675..a5ce840f4fbef 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1100,6 +1100,7 @@ struct bpf_hrtimer {
struct bpf_prog *prog;
void __rcu *callback_fn;
void *value;
+ struct rcu_head rcu;
};
/* the actual struct hidden inside uapi struct bpf_timer */
@@ -1328,6 +1329,7 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_timer_kern *, timer)
if (in_nmi())
return -EOPNOTSUPP;
+ rcu_read_lock();
__bpf_spin_lock_irqsave(&timer->lock);
t = timer->timer;
if (!t) {
@@ -1349,6 +1351,7 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_timer_kern *, timer)
* if it was running.
*/
ret = ret ?: hrtimer_cancel(&t->timer);
+ rcu_read_unlock();
return ret;
}
@@ -1403,7 +1406,7 @@ void bpf_timer_cancel_and_free(void *val)
*/
if (this_cpu_read(hrtimer_running) != t)
hrtimer_cancel(&t->timer);
- kfree(t);
+ kfree_rcu(t, rcu);
}
BPF_CALL_2(bpf_kptr_xchg, void *, map_value, void *, ptr)
