[PATCH] perf: fix GPF in perf_cgroup_switch()

[Song Liu <song@kernel.org>]

GPF is observed in perf_cgroup_switch():

[ 2683.232477] general protection fault, probably for non-canonical address 0xdeacffffffffff90: 0000 [#1] SMP
[ 2683.251802] CPU: 30 PID: 0 Comm: swapper/30 Kdump: loaded Tainted: G S
[ 2683.273726] Hardware name: Quanta Twin Lakes MP/Twin Lakes Passive MP,
[ 2683.291129] RIP: 0010:perf_cgroup_switch+0xc0/0x170
[ 2683.300889] Code: 85 ff ff 48 8b 83 00 01 00 00 48 85 c0 74 04 c6 40 08 \
         00 c6 43 08 00 48 8b 83 70 01 00 00 48 8d 98 90 fe ff ff 48 39 c5 \
         74 7d <8b> 83 e4 00 00 00 85 c0 0f 84 89 00 00 00 4c 8b bb 00 01 00 00 48
[ 2683.338455] RSP: 0018:ffffc9000021fdb0 EFLAGS: 00010002
[ 2683.348904] RAX: dead000000000100 RBX: deacffffffffff90 RCX: 000000000000038f
[ 2683.363176] RDX: 0000000000000007 RSI: 0000000000000400 RDI: 0000000000000000
[ 2683.377447] RBP: ffff88903ffa77b0 R08: 0000000300000003 R09: 0000000000000004
[ 2683.391718] R10: 0000000000000003 R11: 0000000000000001 R12: 0000000000000002
[ 2683.405989] R13: 0000000000000000 R14: ffff8881013fdc00 R15: 0000000000000000
[ 2683.420261] FS:  0000000000000000(0000) GS:ffff88903ff80000(0000) knlGS:0000000000000000
[ 2683.436446] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2683.447937] CR2: 00007ffafb956020 CR3: 0000000141cd7005 CR4: 00000000007706e0
[ 2683.462209] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 2683.476481] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 2683.490752] PKRU: 55555554
[ 2683.496160] Call Trace:
[ 2683.501048]  __perf_event_task_sched_in+0xb3/0x200
[ 2683.510632]  finish_task_switch+0x186/0x270
[ 2683.518999]  __schedule+0x3b1/0x850
[ 2683.525973]  ? cpuidle_enter_state+0xa7/0x340
[ 2683.534687]  ? update_ts_time_stats+0x51/0x70
[ 2683.543399]  schedule_idle+0x1e/0x40
[ 2683.550548]  do_idle+0x148/0x200
[ 2683.557001]  cpu_startup_entry+0x19/0x20
[ 2683.564843]  start_secondary+0x104/0x140
[ 2683.572688]  secondary_startup_64_no_verify+0xb0/0xbb

which indicates list corruption on cgrp_cpuctx_list. This happens on the
following path:

  perf_cgroup_switch: list_for_each_entry(cgrp_cpuctx_list)
      cpu_ctx_sched_in
         ctx_sched_in
            ctx_pinned_sched_in
              merge_sched_in
                  perf_cgroup_event_disable: remove the event from the list

To repro this on Intel CPUs:

  /* occupy all counters with pinned events (watchdog uses another) */
  perf stat -e cycles:D,cycles:D,cycles:D,cycles:D,cycles:D -a &
  /* add a pinned cgroup event */
  perf stat -e cycles:D -G my-cgroup
  /* GPF immediately */

Fix this with list_for_each_entry_safe().

Cc: Yonghong Song <yhs@fb.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Song Liu <song@kernel.org>
---
 kernel/events/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index afbf388a5176..46babdf76d8f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -839,7 +839,7 @@ static DEFINE_PER_CPU(struct list_head, cgrp_cpuctx_list);
  */
 static void perf_cgroup_switch(struct task_struct *task, int mode)
 {
-	struct perf_cpu_context *cpuctx;
+	struct perf_cpu_context *cpuctx, *tmp;
 	struct list_head *list;
 	unsigned long flags;
 
@@ -850,7 +850,7 @@ static void perf_cgroup_switch(struct task_struct *task, int mode)
 	local_irq_save(flags);
 
 	list = this_cpu_ptr(&cgrp_cpuctx_list);
-	list_for_each_entry(cpuctx, list, cgrp_cpuctx_entry) {
+	list_for_each_entry_safe(cpuctx, tmp, list, cgrp_cpuctx_entry) {
 		WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0);
 
 		perf_ctx_lock(cpuctx, cpuctx->task_ctx);
-- 
2.30.2