Re: [PATCH v3] perf/core: Fix cgroup event list management

[Marco Elver]

On Tue, 14 Dec 2021 at 00:22, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> The active cgroup events are managed in the per-cpu cgrp_cpuctx_list.
> This list is only accessed from current cpu and not protected by any
> locks.  But from the commit ef54c1a476ae ("perf: Rework
> perf_event_exit_event()"), it's possible to access (actually modify)
> the list from another cpu.
>
> In the perf_remove_from_context(), it can remove an event from the
> context without an IPI when the context is not active.  This is not
> safe with cgroup events which can have some active events in the
> context even if ctx->is_active is 0 at the moment.  The target cpu
> might be in the middle of list iteration at the same time.
>
> If the event is enabled when it's about to be closed, it might call
> perf_cgroup_event_disable() and list_del() with the cgrp_cpuctx_list
> on a different cpu.
>
> This resulted in a crash due to an invalid list pointer access during
> the cgroup list traversal on the cpu which the event belongs to.
>
> Let's fallback to IPI to access the cgrp_cpuctx_list from that cpu.
> Similarly, perf_install_in_context() should use IPI for the cgroup
> events too.
>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>

The final version needs:

Fixes: ef54c1a476ae ("perf: Rework perf_event_exit_event()")

so stable kernels will see it, unless this has already been picked up
in which case we need to email stable.

Thanks,
-- Marco