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

From: Saket Kumar Bhaskar <[email-masked]>
Date: Tue, 3 Mar 2026 20:28:25 +0530

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 <[email-masked]>
> ---
>  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/[email-masked]/T/#mf901967ebe77506f1bd6e3d876c2a85824d9519d

Wondering if the above generic fix is working do we need to add this
check in powerpc specific code?

Thanks,
Saket