uprobes: Fix race in uprobe_free_utask

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] grafana/pyroscope#3673

Fixes: cfa7f3d ("perf,x86: avoid missing caller address in stack traces captured in uprobe")
Reported-by: Max Makarov <[email protected]>
Signed-off-by: Jiri Olsa <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Oleg Nesterov <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

kernel/events/uprobes.c

@@ -1915,6 +1915,7 @@ void uprobe_free_utask(struct task_struct *t)
 	if (!utask)
 		return;
 
+	t->utask = NULL;
 	WARN_ON_ONCE(utask->active_uprobe || utask->xol_vaddr);
 
 	timer_delete_sync(&utask->ri_timer);
@@ -1924,7 +1925,6 @@ void uprobe_free_utask(struct task_struct *t)
 		ri = free_ret_instance(ri, true /* cleanup_hprobe */);
 
 	kfree(utask);
-	t->utask = NULL;
 }
 
 #define RI_TIMER_PERIOD (HZ / 10) /* 100 ms */