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Message-ID: <7bcea009-b58c-4a00-b7cd-f2fc06b90a02@efficios.com>
Date: Wed, 23 Oct 2024 11:19:40 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
Jordan Rife <jrife@...gle.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Andrii Nakryiko <andrii.nakryiko@...il.com>,
Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
Joel Fernandes <joel@...lfernandes.org>, LKML
<linux-kernel@...r.kernel.org>, Mark Rutland <mark.rutland@....com>,
Masami Hiramatsu <mhiramat@...nel.org>, Ingo Molnar <mingo@...hat.com>,
Michael Jeanson <mjeanson@...icios.com>, Namhyung Kim <namhyung@...nel.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>,
syzbot+b390c8062d8387b6272a@...kaller.appspotmail.com,
Yonghong Song <yhs@...com>
Subject: Re: [RFC PATCH] tracing: Fix syscall tracepoint use-after-free
On 2024-10-23 11:14, Alexei Starovoitov wrote:
> On Wed, Oct 23, 2024 at 7:56 AM Jordan Rife <jrife@...gle.com> wrote:
>>
>> Mathieu's patch alone does not seem to be enough to prevent the
>> use-after-free issue reported by syzbot.
>>
>> Link: https://lore.kernel.org/bpf/67121037.050a0220.10f4f4.000f.GAE@google.com/T/#u
>>
>> I reran the repro script with his patch applied to my tree and was
>> still able to get the same KASAN crash to occur.
>>
>> In this case, when bpf_link_free is invoked it kicks off three instances
>> of call_rcu*.
>>
>> bpf_link_free()
>> ops->release()
>> bpf_raw_tp_link_release()
>> bpf_probe_unregister()
>> tracepoint_probe_unregister()
>> tracepoint_remove_func()
>> release_probes()
>> call_rcu() [1]
>> bpf_prog_put()
>> __bpf_prog_put()
>> bpf_prog_put_deferred()
>> __bpf_prog_put_noref()
>> call_rcu() [2]
>> call_rcu() [3]
>>
>> With Mathieu's patch, [1] is chained with call_rcu_tasks_trace()
>> making the grace period suffiently long to safely free the probe itself.
>> The callback for [2] and [3] may be invoked before the
>> call_rcu_tasks_trace() grace period has elapsed leading to the link or
>> program itself being freed while still in use. I was able to prevent
>> any crashes with the patch below which also chains
>> call_rcu_tasks_trace() and call_rcu() at [2] and [3].
>>
>> ---
>> kernel/bpf/syscall.c | 24 ++++++++++--------------
>> 1 file changed, 10 insertions(+), 14 deletions(-)
>>
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index 59de664e580d..5290eccb465e 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -2200,6 +2200,14 @@ static void __bpf_prog_put_rcu(struct rcu_head *rcu)
>> bpf_prog_free(aux->prog);
>> }
>>
>> +static void __bpf_prog_put_tasks_trace_rcu(struct rcu_head *rcu)
>> +{
>> + if (rcu_trace_implies_rcu_gp())
>> + __bpf_prog_put_rcu(rcu);
>> + else
>> + call_rcu(rcu, __bpf_prog_put_rcu);
>> +}
>> +
>> static void __bpf_prog_put_noref(struct bpf_prog *prog, bool deferred)
>> {
>> bpf_prog_kallsyms_del_all(prog);
>> @@ -2212,10 +2220,7 @@ static void __bpf_prog_put_noref(struct bpf_prog *prog, bool deferred)
>> btf_put(prog->aux->attach_btf);
>>
>> if (deferred) {
>> - if (prog->sleepable)
>> - call_rcu_tasks_trace(&prog->aux->rcu, __bpf_prog_put_rcu);
>> - else
>> - call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu);
>> + call_rcu_tasks_trace(&prog->aux->rcu, __bpf_prog_put_tasks_trace_rcu);
>> } else {
>> __bpf_prog_put_rcu(&prog->aux->rcu);
>> }
>> @@ -2996,24 +3001,15 @@ static void bpf_link_defer_dealloc_mult_rcu_gp(struct rcu_head *rcu)
>> static void bpf_link_free(struct bpf_link *link)
>> {
>> const struct bpf_link_ops *ops = link->ops;
>> - bool sleepable = false;
>>
>> bpf_link_free_id(link->id);
>> if (link->prog) {
>> - sleepable = link->prog->sleepable;
>> /* detach BPF program, clean up used resources */
>> ops->release(link);
>> bpf_prog_put(link->prog);
>> }
>> if (ops->dealloc_deferred) {
>> - /* schedule BPF link deallocation; if underlying BPF program
>> - * is sleepable, we need to first wait for RCU tasks trace
>> - * sync, then go through "classic" RCU grace period
>> - */
>> - if (sleepable)
>> - call_rcu_tasks_trace(&link->rcu, bpf_link_defer_dealloc_mult_rcu_gp);
>> - else
>> - call_rcu(&link->rcu, bpf_link_defer_dealloc_rcu_gp);
>> + call_rcu_tasks_trace(&link->rcu, bpf_link_defer_dealloc_mult_rcu_gp);
>
> This patch is completely wrong.
Actually I suspect Jordan's patch works.
> Looks like Mathieu patch broke bpf program contract somewhere.
My patch series introduced this in the probe:
#define __BPF_DECLARE_TRACE_SYSCALL(call, proto, args) \
static notrace void \
__bpf_trace_##call(void *__data, proto) \
{ \
might_fault(); \
preempt_disable_notrace(); \
CONCATENATE(bpf_trace_run, COUNT_ARGS(args))(__data, CAST_TO_U64(args)); \
preempt_enable_notrace(); \
}
To ensure we'd call the bpf program from preempt-off context.
> The tracepoint type bpf programs must be called with rcu_read_lock held.
Calling the bpf program with preempt off is equivalent. __DO_TRACE() calls
the probes with preempt_disable_notrace() in the normal case.
> Looks like it's not happening anymore.
The issue here is not about the context in which the bpf program runs, that's
still preempt off. The problem is about expectations that a call_rcu grace period
is enough to delay reclaim after unregistration of the tracepoint. Now that
__DO_TRACE() uses rcu_read_lock_trace() to protect RCU dereference, it's not
sufficient anymore, at least for syscall tracepoints.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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