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SubjectRe: [PATCH v4] mm/oom_kill.c: futex: Don't OOM reap a process with a futex robust list
From
On 3/8/22 19:25, Nico Pache wrote:
> The pthread struct is allocated on PRIVATE|ANONYMOUS memory [1] which can
> be targeted by the oom reaper. This mapping is also used to store the futex
> robust list; the kernel does not keep a copy of the robust list and instead
> references a userspace address to maintain the robustness during a process
> death. A race can occur between exit_mm and the oom reaper that allows
> the oom reaper to clear the memory of the futex robust list before the
> exit path has handled the futex death.
>
> Prevent the OOM reaper from concurrently reaping the mappings if the dying
> process contains a robust_list. If the dying task_struct does not contain
> a pointer in tsk->robust_list, we can assume there was either never one
> setup for this task struct, or futex_cleanup has properly handled the
> futex death and we can safely reap this memory.
>
> Reproducer: https://gitlab.com/jsavitz/oom_futex_reproducer
>
> [1] https://elixir.bootlin.com/glibc/latest/source/nptl/allocatestack.c#L370
>
> Fixes: 212925802454 ("mm: oom: let oom_reap_task and exit_mmap run concurrently")
> Cc: Rafael Aquini <aquini@redhat.com>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Christoph von Recklinghausen <crecklin@redhat.com>
> Cc: Don Dutile <ddutile@redhat.com>
> Cc: Herton R. Krzesinski <herton@redhat.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: <tglx@linutronix.de>
> Cc: <mingo@redhat.com>
> Cc: <dvhart@infradead.org>
> Cc: <dave@stgolabs.net>
> Cc: <andrealmeid@collabora.com>
> Cc: <peterz@infradead.org>
> Co-developed-by: Joel Savitz <jsavitz@redhat.com>
> Signed-off-by: Joel Savitz <jsavitz@redhat.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
> mm/oom_kill.c | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 989f35a2bbb1..37af902494d8 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -587,6 +587,25 @@ static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
> goto out_unlock;
> }
>
> + /* Don't reap a process holding a robust_list as the pthread
> + * struct is allocated in userspace using PRIVATE | ANONYMOUS
> + * memory which when reaped before futex_cleanup() can leave
> + * the waiting process stuck.
> + */
> +#ifdef CONFIG_FUTEX
> + bool robust = false;
> +
> + robust = tsk->robust_list != NULL;
> +#ifdef CONFIG_COMPAT
> + robust |= tsk->compat_robust_list != NULL;
> +#endif
> + if (robust) {
> + trace_skip_task_reaping(tsk->pid);
> + pr_info("oom_reaper: skipping task as it contains a robust list");
> + goto out_finish;
> + }
> +#endif
> +
> trace_start_task_reaping(tsk->pid);
>
> /* failed to reap part of the address space. Try again later */

I believe it will be easier to read if you define a helper function like

#ifdef CONFIG_FUTEX
static inline bool has_robust_list(struct task_struct *tsk)
{
        bool robust = !!tsk->robust_list;

#ifdef CONFIG_COMPAT
        robust |= !!tsk->compat_robust_list
#endif
        return robust;
}
#else
static inline bool has_robust_list(struct task_struct *tsk)
{
        return false;
}
#endif

Then you don't need any #if/endif in oom_reap_task_mm().

Cheers,
Longman

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