Subject: Re: [PATCH v2] powerpc, perf: Check that current->mm is alive before getting user callchain

From: Saket Kumar Bhaskar <[email-masked]>
Date: Mon, 9 Mar 2026 21:59:48 +0530

On Mon, Mar 09, 2026 at 03:40:45PM +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
>     [...]
> 
> In addition, move storing the top-level stack entry to generic
> perf_callchain_user to make sure the top-evel entry is always captured,
> even if current->mm is NULL.
> 
> Fixes: 20002ded4d93 ("perf_counter: powerpc: Add callchain support")
> Signed-off-by: Viktor Malik <[email-masked]>
> ---
> Changes in v2:
> - Move call to perf_callchain_store() for the top-level stack entry to
>   common perf_callchain_user (Saket)
> 
>  arch/powerpc/perf/callchain.c    | 5 +++++
>  arch/powerpc/perf/callchain_32.c | 1 -
>  arch/powerpc/perf/callchain_64.c | 1 -
>  3 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/perf/callchain.c b/arch/powerpc/perf/callchain.c
> index 26aa26482c9a..992cc5c98214 100644
> --- a/arch/powerpc/perf/callchain.c
> +++ b/arch/powerpc/perf/callchain.c
> @@ -103,6 +103,11 @@ perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *re
>  void
>  perf_callchain_user(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs)
>  {
> +	perf_callchain_store(entry, perf_arch_instruction_pointer(regs));
> +
> +	if (!current->mm)
> +		return;
> +
>  	if (!is_32bit_task())
>  		perf_callchain_user_64(entry, regs);
>  	else
> diff --git a/arch/powerpc/perf/callchain_32.c b/arch/powerpc/perf/callchain_32.c
> index ddcc2d8aa64a..0de21c5d272c 100644
> --- a/arch/powerpc/perf/callchain_32.c
> +++ b/arch/powerpc/perf/callchain_32.c
> @@ -142,7 +142,6 @@ void perf_callchain_user_32(struct perf_callchain_entry_ctx *entry,
>  	next_ip = perf_arch_instruction_pointer(regs);
>  	lr = regs->link;
>  	sp = regs->gpr[1];
> -	perf_callchain_store(entry, next_ip);
>  
>  	while (entry->nr < entry->max_stack) {
>  		fp = (unsigned int __user *) (unsigned long) sp;
> diff --git a/arch/powerpc/perf/callchain_64.c b/arch/powerpc/perf/callchain_64.c
> index 115d1c105e8a..30fb61c5f0cb 100644
> --- a/arch/powerpc/perf/callchain_64.c
> +++ b/arch/powerpc/perf/callchain_64.c
> @@ -77,7 +77,6 @@ void perf_callchain_user_64(struct perf_callchain_entry_ctx *entry,
>  	next_ip = perf_arch_instruction_pointer(regs);
>  	lr = regs->link;
>  	sp = regs->gpr[1];
> -	perf_callchain_store(entry, next_ip);
>  
>  	while (entry->nr < entry->max_stack) {
>  		fp = (unsigned long __user *) sp;
> -- 
> 2.53.0
LGTM, feel free to add below tag:
Reviewed-by: Saket Kumar Bhaskar <[email-masked]>