6.9-stable review patch. If anyone has any objections, please let me know.
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From: Kumar Kartikeya Dwivedi memxor@gmail.com
[ Upstream commit d4523831f07a267a943f0dde844bf8ead7495f13 ]
Given a schedule:
timer1 cb timer2 cb
bpf_timer_cancel(timer2); bpf_timer_cancel(timer1);
Both bpf_timer_cancel calls would wait for the other callback to finish executing, introducing a lockup.
Add an atomic_t count named 'cancelling' in bpf_hrtimer. This keeps track of all in-flight cancellation requests for a given BPF timer. Whenever cancelling a BPF timer, we must check if we have outstanding cancellation requests, and if so, we must fail the operation with an error (-EDEADLK) since cancellation is synchronous and waits for the callback to finish executing. This implies that we can enter a deadlock situation involving two or more timer callbacks executing in parallel and attempting to cancel one another.
Note that we avoid incrementing the cancelling counter for the target timer (the one being cancelled) if bpf_timer_cancel is not invoked from a callback, to avoid spurious errors. The whole point of detecting cur->cancelling and returning -EDEADLK is to not enter a busy wait loop (which may or may not lead to a lockup). This does not apply in case the caller is in a non-callback context, the other side can continue to cancel as it sees fit without running into errors.
Background on prior attempts:
Earlier versions of this patch used a bool 'cancelling' bit and used the following pattern under timer->lock to publish cancellation status.
lock(t->lock); t->cancelling = true; mb(); if (cur->cancelling) return -EDEADLK; unlock(t->lock); hrtimer_cancel(t->timer); t->cancelling = false;
The store outside the critical section could overwrite a parallel requests t->cancelling assignment to true, to ensure the parallely executing callback observes its cancellation status.
It would be necessary to clear this cancelling bit once hrtimer_cancel is done, but lack of serialization introduced races. Another option was explored where bpf_timer_start would clear the bit when (re)starting the timer under timer->lock. This would ensure serialized access to the cancelling bit, but may allow it to be cleared before in-flight hrtimer_cancel has finished executing, such that lockups can occur again.
Thus, we choose an atomic counter to keep track of all outstanding cancellation requests and use it to prevent lockups in case callbacks attempt to cancel each other while executing in parallel.
Reported-by: Dohyun Kim dohyunkim@google.com Reported-by: Neel Natu neelnatu@google.com Fixes: b00628b1c7d5 ("bpf: Introduce bpf timers.") Signed-off-by: Kumar Kartikeya Dwivedi memxor@gmail.com Link: https://lore.kernel.org/r/20240709185440.1104957-2-memxor@gmail.com Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/helpers.c | 38 +++++++++++++++++++++++++++++++++++--- 1 file changed, 35 insertions(+), 3 deletions(-)
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index ff18b467d7d75..79cb5681cf136 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -1107,6 +1107,7 @@ struct bpf_async_cb { struct bpf_hrtimer { struct bpf_async_cb cb; struct hrtimer timer; + atomic_t cancelling; };
/* the actual struct hidden inside uapi struct bpf_timer */ @@ -1205,6 +1206,7 @@ static int __bpf_async_init(struct bpf_async_kern *async, struct bpf_map *map, u clockid = flags & (MAX_CLOCKS - 1); t = (struct bpf_hrtimer *)cb;
+ atomic_set(&t->cancelling, 0); hrtimer_init(&t->timer, clockid, HRTIMER_MODE_REL_SOFT); t->timer.function = bpf_timer_cb; cb->value = (void *)async - map->record->timer_off; @@ -1368,7 +1370,8 @@ static void drop_prog_refcnt(struct bpf_async_cb *async)
BPF_CALL_1(bpf_timer_cancel, struct bpf_async_kern *, timer) { - struct bpf_hrtimer *t; + struct bpf_hrtimer *t, *cur_t; + bool inc = false; int ret = 0;
if (in_nmi()) @@ -1380,14 +1383,41 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_async_kern *, timer) ret = -EINVAL; goto out; } - if (this_cpu_read(hrtimer_running) == t) { + + cur_t = this_cpu_read(hrtimer_running); + if (cur_t == t) { /* If bpf callback_fn is trying to bpf_timer_cancel() * its own timer the hrtimer_cancel() will deadlock - * since it waits for callback_fn to finish + * since it waits for callback_fn to finish. + */ + ret = -EDEADLK; + goto out; + } + + /* Only account in-flight cancellations when invoked from a timer + * callback, since we want to avoid waiting only if other _callbacks_ + * are waiting on us, to avoid introducing lockups. Non-callback paths + * are ok, since nobody would synchronously wait for their completion. + */ + if (!cur_t) + goto drop; + atomic_inc(&t->cancelling); + /* Need full barrier after relaxed atomic_inc */ + smp_mb__after_atomic(); + inc = true; + if (atomic_read(&cur_t->cancelling)) { + /* We're cancelling timer t, while some other timer callback is + * attempting to cancel us. In such a case, it might be possible + * that timer t belongs to the other callback, or some other + * callback waiting upon it (creating transitive dependencies + * upon us), and we will enter a deadlock if we continue + * cancelling and waiting for it synchronously, since it might + * do the same. Bail! */ ret = -EDEADLK; goto out; } +drop: drop_prog_refcnt(&t->cb); out: __bpf_spin_unlock_irqrestore(&timer->lock); @@ -1395,6 +1425,8 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_async_kern *, timer) * if it was running. */ ret = ret ?: hrtimer_cancel(&t->timer); + if (inc) + atomic_dec(&t->cancelling); rcu_read_unlock(); return ret; }