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Message-ID: <CAADnVQKGJcS3mNc8i+dKjybMu0K7vTDVHmPQNqAtGc3X8hE6Hw@mail.gmail.com>
Date: Wed, 23 Oct 2024 18:28:10 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Jordan Rife <jrife@...gle.com>, 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 Wed, Oct 23, 2024 at 8:21 AM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> 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.
We're not going to penalize all bpf progs for that.
This patch is a non-starter.
> > 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.
I don't see how preempt_disable vs preempt_disable_notrace and
preemption in general are relevant here.
The prog dereference needs to happen under rcu_read_lock.
But since you've switched to use rcu_read_lock_trace,
so then this part:
> >> bpf_link_free()
> >> ops->release()
> >> bpf_raw_tp_link_release()
> >> bpf_probe_unregister()
> >> tracepoint_probe_unregister()
> >> tracepoint_remove_func()
> >> release_probes()
> >> call_rcu() [1]
is probably incorrect. It should be call_rcu_tasks_trace ?
and in addition all tracepoints (both sleepable and not)
should deref __data under normal rcu_read_lock() before
passing that pointer into __bpf_trace_##call.
Because that's bpf link and prog are rcu protected.
I don't have patches in front of me, so I'm guessing a bit.
So pls remind me where all these patches are?
What tree/branch are we talking about?
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