Re: [PATCH] uprobes: Fix race in uprobe_free_utask

[Daniel Borkmann]

On 1/9/25 3:14 PM, Jiri Olsa 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.

Lets add Fixes tag for stable team:

Fixes: cfa7f3d2c526 ("perf,x86: avoid missing caller address in stack traces captured in uprobe")

> [1] https://github.com/grafana/pyroscope/issues/3673
> Reported-by: Max Makarov <maxpain@xxxxxxxxx>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>

fwiw, the other version we were potentially thinking of was below, but
just moving the t->utask NULL assignment seemed better.

Thanks,
Daniel

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index c75c482d4c52..05f9cedf2691 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2835,6 +2835,8 @@ static bool is_uprobe_at_func_entry(struct pt_regs *regs)

        if (!current->utask)
                return false;
+       if (!current->utask->active_uprobe)
+               return false;

        auprobe = current->utask->auprobe;
        if (!auprobe)