Re: [PATCH] uprobes: Fix race in uprobe_free_utask

[Andrii Nakryiko]

On Thu, Jan 9, 2025 at 6:14 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> 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
> Reported-by: Max Makarov <maxpain@xxxxxxxxx>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>  kernel/events/uprobes.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>

ah, it's interrupt/NMI checking current->utask, makes total sense,
thanks for the fix!

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index fa04b14a7d72..5d71ef85420c 100644
> --- a/kernel/events/uprobes.c
> +++ b/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 */
> --
> 2.47.0
>