Re: [PATCH] powerpc, perf: Check that current->mm is alive before getting user callchain

Viktor Malik
2026-03-09T14:43:36Z

On 3/9/26 13:17, Viktor Malik wrote:
> On 3/9/26 12:05, Saket Kumar Bhaskar wrote:
>> On Thu, Mar 05, 2026 at 01:45:44PM +0100, Viktor Malik wrote:
>>> On 3/3/26 15:58, Saket Kumar Bhaskar wrote:
>>>> On Fri, Feb 27, 2026 at 09:25:02AM +0100, Viktor Malik wrote:
>>>>> It may happen that mm is already released, which leads to kernel panic.
>>>>> This adds the NULL check for current->mm, similarly to 20afc60f892d
>>>>> ("x86, perf: Check that current->mm is alive before getting user
>>>>> callchain").
>>>>>
>>>>> I was getting this panic when running a profiling BPF program
>>>>> (profile.py from bcc-tools):
>>>>>
>>>>> [26215.051935] Kernel attempted to read user page (588) - exploit attempt? (uid: 0)
>>>>> [26215.051950] BUG: Kernel NULL pointer dereference on read at 0x00000588
>>>>> [26215.051952] Faulting instruction address: 0xc00000000020fac0
>>>>> [26215.051957] Oops: Kernel access of bad area, sig: 11 [#1]
>>>>> [...]
>>>>> [26215.052049] Call Trace:
>>>>> [26215.052050] [c000000061da6d30] [c00000000020fc10] perf_callchain_user_64+0x2d0/0x490 (unreliable)
>>>>> [26215.052054] [c000000061da6dc0] [c00000000020f92c] perf_callchain_user+0x1c/0x30
>>>>> [26215.052057] [c000000061da6de0] [c0000000005ab2a0] get_perf_callchain+0x100/0x360
>>>>> [26215.052063] [c000000061da6e70] [c000000000573bc8] bpf_get_stackid+0x88/0xf0
>>>>> [26215.052067] [c000000061da6ea0] [c008000000042258] bpf_prog_16d4ab9ab662f669_do_perf_event+0xf8/0x274
>>>>> [...]
>>>>>
>>>>> Fixes: 20002ded4d93 ("perf_counter: powerpc: Add callchain support")
>>>>> Signed-off-by: Viktor Malik <vmalik@redhat.com>
>>>>> ---
>>>>> arch/powerpc/perf/callchain_32.c | 3 +++
>>>>> arch/powerpc/perf/callchain_64.c | 3 +++
>>>>> 2 files changed, 6 insertions(+)
>>>>>
>>>>> diff --git a/arch/powerpc/perf/callchain_32.c b/arch/powerpc/perf/callchain_32.c
>>>>> index ddcc2d8aa64a..b46e21679566 100644
>>>>> --- a/arch/powerpc/perf/callchain_32.c
>>>>> +++ b/arch/powerpc/perf/callchain_32.c
>>>>> @@ -144,6 +144,9 @@ void perf_callchain_user_32(struct perf_callchain_entry_ctx *entry,
>>>>> sp = regs->gpr[1];
>>>>> perf_callchain_store(entry, next_ip);
>>>>>
>>>>> + if (!current->mm)
>>>>> + return;
>>>>> +
>>>>> while (entry->nr < entry->max_stack) {
>>>>> fp = (unsigned int __user *) (unsigned long) sp;
>>>>> if (invalid_user_sp(sp) || read_user_stack_32(fp, &next_sp))
>>>>> diff --git a/arch/powerpc/perf/callchain_64.c b/arch/powerpc/perf/callchain_64.c
>>>>> index 115d1c105e8a..eaaadd6fa81b 100644
>>>>> --- a/arch/powerpc/perf/callchain_64.c
>>>>> +++ b/arch/powerpc/perf/callchain_64.c
>>>>> @@ -79,6 +79,9 @@ void perf_callchain_user_64(struct perf_callchain_entry_ctx *entry,
>>>>> sp = regs->gpr[1];
>>>>> perf_callchain_store(entry, next_ip);
>>>>>
>>>>> + if (!current->mm)
>>>>> + return;
>>>>> +
>>>>> while (entry->nr < entry->max_stack) {
>>>>> fp = (unsigned long __user *) sp;
>>>>> if (invalid_user_sp(sp) || read_user_stack_64(fp, &next_sp))
>>>>> --
>>>>> 2.53.0
>>>>>
>>>> Sorry, I missed adding cc list for the last conversation so adding this for reference:
>>>>
>>>>> Wouldn't be good if we check this in perf_callchain_user() as it will
>>>>> cover both cases.
>>>>
>>>> to which Viktor replied:
>>>> I considered it but in that case, we'd also miss the top-level stack
>>>> frame (the perf_callchain_store call above). Other arches include it so
>>>> I followed the behavior for powerpc.
>>>>
>>>> Viktor, agreed with your first point. I have another concern:
>>>>
>>>> I was hitting this issue with stacktrace_build_id_nmi in bpf and
>>>> applied this patch https://lore.kernel.org/bpf/20260126074331.815684-1-chen.dylane@linux.dev/T/#mf901967ebe77506f1bd6e3d876c2a85824d9519d
>>>>
>>>> Wondering if the above generic fix is working do we need to add this
>>>> check in powerpc specific code?
>>>
>>> I tried to apply that patch series but, unfortunately, keep getting the
>>> panic when running the BCC profile tool.
>>>
>>> Also, looking at the patch, it seems that it would only solve the issue
>>> when perf_callchain_user is called from a BPF context, however, I assume
>>> that it may be called from other contexts, too.
>>>
>>> Since perf_callchain_user_{32,64} are dereferencing current->mm while
>>> walking the stack, I think that an explicit protection against
>>> current->mm being NULL makes sense here, even in the presence of the
>>> above patch. Especially since other arches have it, too.
>>>
>>> Viktor
>>>
>> Ok that looks convincing then, another thing is that, how about moving perf_callchain_store
>> to perf_callchain_user and checking current->mm == NULL there for both perf_callchain_user_32/64.
>> next_ip, lr and sp can be passed to perf_callchain_user_32/64.
>
> Yeah, that should be possible. I'll send v2.

v2 sent:

https://lore.kernel.org/linuxppc-dev/20260309144045.169427-1-vmalik@redhat.com/T/#u

Viktor

>
> Viktor
>
>>
>> Thanks,
>> Saket
>>
>