bpf: Disable preemption in perf_event_output helpers code

hi,
we got report of kernel crash [1][3] within bpf_event_output helper.

The reason is the nesting protection code in bpf_event_output that expects disabled preemption, which is not guaranteed for programs executed by bpf_prog_run_array_cg.

I managed to reproduce on tracing side where we have the same problem in bpf_perf_event_output. The reproducer [2] just creates busy uprobe and call bpf_perf_event_output helper a lot.

v3 changes:
- added acks and fixed 'Fixes' tag style [Hou Tao]
- added Closes tag to patch 2

v2 changes:
- I changed 'Fixes' commits to where I saw we switched from preempt_disable to migrate_disable, but I'm not completely sure about the patch 2, because it was tricky to find, would be nice if somebody could check on that

thanks,
jirka

[1] https://github.com/cilium/cilium/issues/26756
[2] https://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git/commit/?h=bpf_output_fix_reproducer&id=8054dcc634121b884c7c331329d61d93351d03b5
[3] slack:

    [66194.378161] BUG: kernel NULL pointer dereference, address: 0000000000000001
    [66194.378324] #PF: supervisor instruction fetch in kernel mode
    [66194.378447] #PF: error_code(0x0010) - not-present page
    ...
    [66194.378692] Oops: 0010 [#1] PREEMPT SMP NOPTI
    ...
    [66194.380666]  <TASK>
    [66194.380775]  ? perf_output_sample+0x12a/0x9a0
    [66194.380902]  ? finish_task_switch.isra.0+0x81/0x280
    [66194.381024]  ? perf_event_output+0x66/0xa0
    [66194.381148]  ? bpf_event_output+0x13a/0x190
    [66194.381270]  ? bpf_event_output_data+0x22/0x40
    [66194.381391]  ? bpf_prog_dfc84bbde731b257_cil_sock4_connect+0x40a/0xacb
    [66194.381519]  ? xa_load+0x87/0xe0
    [66194.381635]  ? __cgroup_bpf_run_filter_sock_addr+0xc1/0x1a0
    [66194.381759]  ? release_sock+0x3e/0x90
    [66194.381876]  ? sk_setsockopt+0x1a1/0x12f0
    [66194.381996]  ? udp_pre_connect+0x36/0x50
    [66194.382114]  ? inet_dgram_connect+0x93/0xa0
    [66194.382233]  ? __sys_connect+0xb4/0xe0
    [66194.382353]  ? udp_setsockopt+0x27/0x40
    [66194.382470]  ? __pfx_udp_push_pending_frames+0x10/0x10
    [66194.382593]  ? __sys_setsockopt+0xdf/0x1a0
    [66194.382713]  ? __x64_sys_connect+0xf/0x20
    [66194.382832]  ? do_syscall_64+0x3a/0x90
    [66194.382949]  ? entry_SYSCALL_64_after_hwframe+0x72/0xdc
    [66194.383077]  </TASK>

---
Jiri Olsa (2):
bpf: Disable preemption in bpf_perf_event_output
bpf: Disable preemption in bpf_event_output

 kernel/trace/bpf_trace.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

The nesting protection in bpf_perf_event_output relies on disabled preemption, which is guaranteed for kprobes and tracepoints.

However bpf_perf_event_output can be also called from uprobes context through bpf_prog_run_array_sleepable function which disables migration, but keeps preemption enabled.

This can cause task to be preempted by another one inside the nesting protection and lead eventually to two tasks using same perf_sample_data buffer and cause crashes like:

kernel tried to execute NX-protected page - exploit attempt? (uid: 0)

  BUG: unable to handle page fault for address: ffffffff82be3eea
  ...
  Call Trace:
   ? __die+0x1f/0x70
   ? page_fault_oops+0x176/0x4d0
   ? exc_page_fault+0x132/0x230
   ? asm_exc_page_fault+0x22/0x30
   ? perf_output_sample+0x12b/0x910
   ? perf_event_output+0xd0/0x1d0
   ? bpf_perf_event_output+0x162/0x1d0
   ? bpf_prog_c6271286d9a4c938_krava1+0x76/0x87
   ? __uprobe_perf_func+0x12b/0x540
   ? uprobe_dispatcher+0x2c4/0x430
   ? uprobe_notify_resume+0x2da/0xce0
   ? atomic_notifier_call_chain+0x7b/0x110
   ? exit_to_user_mode_prepare+0x13e/0x290
   ? irqentry_exit_to_user_mode+0x5/0x30
   ? asm_exc_int3+0x35/0x40

Fixing this by disabling preemption in bpf_perf_event_output.

---
 kernel/trace/bpf_trace.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

index 5f2dcabad202..14c9a1a548c9 100644
@@ -661,8 +661,7 @@ static DEFINE_PER_CPU(int, bpf_trace_nest_level);
 BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,
 	   u64, flags, void *, data, u64, size)
 {
-	struct bpf_trace_sample_data *sds = this_cpu_ptr(&bpf_trace_sds);
-	int nest_level = this_cpu_inc_return(bpf_trace_nest_level);
+	struct bpf_trace_sample_data *sds;
 	struct perf_raw_record raw = {
 		.frag = {
 			.size = size,

@@ -670,7 +669,12 @@ BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,
 		},
 	};
 	struct perf_sample_data *sd;
-	int err;
+	int nest_level, err;
+
+	preempt_disable();
+
+	sds = this_cpu_ptr(&bpf_trace_sds);
+	nest_level = this_cpu_inc_return(bpf_trace_nest_level);
 
 	if (WARN_ON_ONCE(nest_level > ARRAY_SIZE(sds->sds))) {
 		err = -EBUSY;

@@ -691,6 +695,7 @@ BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,
 
 out:
 	this_cpu_dec(bpf_trace_nest_level);
+	preempt_enable();
 	return err;
 }
 

2.41.0

We received report [1] of kernel crash, which is caused by using nesting protection without disabled preemption.

The bpf_event_output can be called by programs executed by bpf_prog_run_array_cg function that disabled migration but keeps preemption enabled.

This can cause task to be preempted by another one inside the nesting protection and lead eventually to two tasks using same perf_sample_data buffer and cause crashes like:

  BUG: kernel NULL pointer dereference, address: 0000000000000001
  #PF: supervisor instruction fetch in kernel mode
  #PF: error_code(0x0010) - not-present page
  ...
  ? perf_output_sample+0x12a/0x9a0
  ? finish_task_switch.isra.0+0x81/0x280
  ? perf_event_output+0x66/0xa0
  ? bpf_event_output+0x13a/0x190
  ? bpf_event_output_data+0x22/0x40
  ? bpf_prog_dfc84bbde731b257_cil_sock4_connect+0x40a/0xacb
  ? xa_load+0x87/0xe0
  ? __cgroup_bpf_run_filter_sock_addr+0xc1/0x1a0
  ? release_sock+0x3e/0x90
  ? sk_setsockopt+0x1a1/0x12f0
  ? udp_pre_connect+0x36/0x50
  ? inet_dgram_connect+0x93/0xa0
  ? __sys_connect+0xb4/0xe0
  ? udp_setsockopt+0x27/0x40
  ? __pfx_udp_push_pending_frames+0x10/0x10
  ? __sys_setsockopt+0xdf/0x1a0
  ? __x64_sys_connect+0xf/0x20
  ? do_syscall_64+0x3a/0x90
  ? entry_SYSCALL_64_after_hwframe+0x72/0xdc

Fixing this by disabling preemption in bpf_event_output.

[1] https://github.com/cilium/cilium/issues/26756

---
 kernel/trace/bpf_trace.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

index 14c9a1a548c9..6826ebf750b0 100644
@@ -720,7 +720,6 @@ static DEFINE_PER_CPU(struct bpf_trace_sample_data, bpf_misc_sds);
 u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
 		     void *ctx, u64 ctx_size, bpf_ctx_copy_t ctx_copy)
 {
-	int nest_level = this_cpu_inc_return(bpf_event_output_nest_level);
 	struct perf_raw_frag frag = {
 		.copy		= ctx_copy,
 		.size		= ctx_size,

@@ -737,8 +736,13 @@ u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
 	};
 	struct perf_sample_data *sd;
 	struct pt_regs *regs;
+	int nest_level;
 	u64 ret;
 
+	preempt_disable();
+
+	nest_level = this_cpu_inc_return(bpf_event_output_nest_level);
+
 	if (WARN_ON_ONCE(nest_level > ARRAY_SIZE(bpf_misc_sds.sds))) {
 		ret = -EBUSY;
 		goto out;

@@ -753,6 +757,7 @@ u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
 	ret = __bpf_perf_event_output(regs, map, flags, sd);
 out:
 	this_cpu_dec(bpf_event_output_nest_level);
+	preempt_enable();
 	return ret;
 }
 

2.41.0