linux-kernel - Re: [RFC PATCH] tracing: Fix syscall tracepoint use-after-free

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Message-ID: <20241023145640.1499722-1-jrife@google.com>
Date: Wed, 23 Oct 2024 14:56:40 +0000
From: Jordan Rife <jrife@...gle.com>
To: jrife@...gle.com
Cc: acme@...nel.org, alexander.shishkin@...ux.intel.com, 
	andrii.nakryiko@...il.com, ast@...nel.org, bpf@...r.kernel.org, 
	joel@...lfernandes.org, linux-kernel@...r.kernel.org, mark.rutland@....com, 
	mathieu.desnoyers@...icios.com, mhiramat@...nel.org, mingo@...hat.com, 
	mjeanson@...icios.com, namhyung@...nel.org, paulmck@...nel.org, 
	peterz@...radead.org, rostedt@...dmis.org, 
	syzbot+b390c8062d8387b6272a@...kaller.appspotmail.com, yhs@...com
Subject: Re: [RFC PATCH] tracing: Fix syscall tracepoint use-after-free

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);
 	} else if (ops->dealloc)
 		ops->dealloc(link);
 }
-- 
2.47.0.105.g07ac214952-goog