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SubjectRe: [PATCH] perf/core: Avoid freeing static PMU contexts when PMU is unregistered
On Tue, Oct 03, 2017 at 03:20:50PM +0100, Will Deacon wrote:
> Since commit 1fd7e4169954 ("perf/core: Remove perf_cpu_context::unique_pmu"),
> when a PMU is unregistered then its associated ->pmu_cpu_context is
> unconditionally freed. Whilst this is fine for dynamically allocated
> context types (i.e. those registered using perf_invalid_context), this
> causes a problem for sharing of static contexts such as
> perf_{sw,hw}_context, which are used by multiple built-in PMUs and
> effectively have a global lifetime.
>
> Whilst testing the ARM SPE driver, which must use perf_sw_context to
> support per-task AUX tracing, unregistering the driver as a result of a
> module unload resulted in:
>
> Unable to handle kernel NULL pointer dereference at virtual address 00000038
> Internal error: Oops: 96000004 [#1] PREEMPT SMP
> Modules linked in: [last unloaded: arm_spe_pmu]
> PC is at ctx_resched+0x38/0xe8
> LR is at perf_event_exec+0x20c/0x278
> [...]
> ctx_resched+0x38/0xe8
> perf_event_exec+0x20c/0x278
> setup_new_exec+0x88/0x118
> load_elf_binary+0x26c/0x109c
> search_binary_handler+0x90/0x298
> do_execveat_common.isra.14+0x540/0x618
> SyS_execve+0x38/0x48
>
> since the software context has been freed and the ctx.pmu->pmu_disable_count
> field has been set to NULL.
>
> This patch fixes the problem by avoiding the freeing of static PMU contexts
> altogether. Whilst the sharing of dynamic contexts is questionable, this
> actually requires the caller to share their context pointer explicitly
> and so the burden is on them to manage the object lifetime.
>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Fixes: 1fd7e4169954 ("perf/core: Remove perf_cpu_context::unique_pmu")
> Reported-by: Kim Phillips <kim.phillips@arm.com>
> Signed-off-by: Will Deacon <will.deacon@arm.com>

FWIW:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
> kernel/events/core.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 6bc21e202ae4..243bfc68d0fe 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -8955,6 +8955,14 @@ static struct perf_cpu_context __percpu *find_pmu_context(int ctxn)
>
> static void free_pmu_context(struct pmu *pmu)
> {
> + /*
> + * Static contexts such as perf_sw_context have a global lifetime
> + * and may be shared between different PMUs. Avoid freeing them
> + * when a single PMU is going away.
> + */
> + if (pmu->task_ctx_nr > perf_invalid_context)
> + return;
> +
> mutex_lock(&pmus_lock);
> free_percpu(pmu->pmu_cpu_context);
> mutex_unlock(&pmus_lock);
> --
> 2.1.4
>

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