On 2024-10-23 10:56, Jordan Rife 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@xxxxxxxxxx/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].
Right, so removal of the tracepoint probe is done by
tracepoint_probe_unregister by effectively removing the
probe function from the array. The read-side counterpart
of that is in __DO_TRACE(), where the rcu dereference is
protected by rcu_read_lock_trace for syscall tracepoints
now.
We cannot expect that surrounding the ebpf probe execution
with preempt disable like so:
#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(); \
}
Is sufficient to delay reclaim with call_rcu() after a tracepoint
unregister, because the preempt disable does not include the rcu
dereference done by the tracepoint in its critical section.
So relying on a call_rcu() to delay reclaim of the bpf objects
after unregistering their associated tracepoint is indeed not
enough. Chaining call_rcu with call_rcu_tasks_trace works though.
That question is relevant for ftrace and perf too: are there data
structures that are reclaimed with call_rcu() after being unregistered
from syscall tracepoints ?
Thanks Jordan for your thorough analysis,
Mathieu
---
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);
} else if (ops->dealloc)
ops->dealloc(link);
}
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
