[PATCH 6.6.y] uprobes: Fix race in uprobe_free_utask

[Christian Simon <simon@xxxxxxxx>]

commit b583ef82b671c9a752fbe3e95bd4c1c51eab764d upstream.

Christian Simon verified the regression exists in v6.6.80 as per method
below and backported the mainline fix to the older version of
uprobe_free_utask. After that change I can no longer reproduce
the race with this method within 1h, while before it would show the
panic under a minute.

Max Makarov reported kernel panic [1] in perf user callchain code.

The reason for that is the race between uprobe_free_utask and bpf
profiler code doing the perf user stack unwind and is triggered
within uprobe_free_utask function:
  - after current->utask is freed and
  - before current->utask is set to NULL

 general protection fault, probably for non-canonical address 0x9e759c37ee555c76: 0000 [#1] SMP PTI
 RIP: 0010:is_uprobe_at_func_entry+0x28/0x80
 ...
  ? die_addr+0x36/0x90
  ? exc_general_protection+0x217/0x420
  ? asm_exc_general_protection+0x26/0x30
  ? is_uprobe_at_func_entry+0x28/0x80
  perf_callchain_user+0x20a/0x360
  get_perf_callchain+0x147/0x1d0
  bpf_get_stackid+0x60/0x90
  bpf_prog_9aac297fb833e2f5_do_perf_event+0x434/0x53b
  ? __smp_call_single_queue+0xad/0x120
  bpf_overflow_handler+0x75/0x110
  ...
  asm_sysvec_apic_timer_interrupt+0x1a/0x20
 RIP: 0010:__kmem_cache_free+0x1cb/0x350
 ...
  ? uprobe_free_utask+0x62/0x80
  ? acct_collect+0x4c/0x220
  uprobe_free_utask+0x62/0x80
  mm_release+0x12/0xb0
  do_exit+0x26b/0xaa0
  __x64_sys_exit+0x1b/0x20
  do_syscall_64+0x5a/0x80

It can be easily reproduced by running following commands in
separate terminals:

  # while :; do bpftrace -e 'uprobe:/bin/ls:_start  { printf("hit\n"); }' -c ls; done
  # bpftrace -e 'profile:hz:100000 { @[ustack()] = count(); }'

Fixing this by making sure current->utask pointer is set to NULL
before we start to release the utask object.

[1] https://github.com/grafana/pyroscope/issues/3673

Fixes: cfa7f3d2c526 ("perf,x86: avoid missing caller address in stack traces captured in uprobe")
Reported-by: Max Makarov <maxpain@xxxxxxxxx>
(cherry picked from commit b583ef82b671c9a752fbe3e95bd4c1c51eab764d)
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20250109141440.2692173-1-jolsa@xxxxxxxxxx
[Christian Simon: Rebased for 6.6.y, due to mainline change https://lore.kernel.org/all/20240929144239.GA9475@xxxxxxxxxx/]
Signed-off-by: Christian Simon <simon@xxxxxxxx>
---
 kernel/events/uprobes.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 6dac0b579821..70fae75c48ca 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1716,6 +1716,7 @@ void uprobe_free_utask(struct task_struct *t)
 	if (!utask)
 		return;
 
+	t->utask = NULL;
 	if (utask->active_uprobe)
 		put_uprobe(utask->active_uprobe);
 
@@ -1725,7 +1726,6 @@ void uprobe_free_utask(struct task_struct *t)
 
 	xol_free_insn_slot(t);
 	kfree(utask);
-	t->utask = NULL;
 }
 
 /*
-- 
2.47.1