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    Subject[PATCH 5.15 009/167] fix race between exit_itimers() and /proc/pid/timers
    Date
    From: Oleg Nesterov <oleg@redhat.com>

    commit d5b36a4dbd06c5e8e36ca8ccc552f679069e2946 upstream.

    As Chris explains, the comment above exit_itimers() is not correct,
    we can race with proc_timers_seq_ops. Change exit_itimers() to clear
    signal->posix_timers with ->siglock held.

    Cc: <stable@vger.kernel.org>
    Reported-by: chris@accessvector.net
    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/exec.c | 2 +-
    include/linux/sched/task.h | 2 +-
    kernel/exit.c | 2 +-
    kernel/time/posix-timers.c | 19 ++++++++++++++-----
    4 files changed, 17 insertions(+), 8 deletions(-)

    --- a/fs/exec.c
    +++ b/fs/exec.c
    @@ -1298,7 +1298,7 @@ int begin_new_exec(struct linux_binprm *
    bprm->mm = NULL;

    #ifdef CONFIG_POSIX_TIMERS
    - exit_itimers(me->signal);
    + exit_itimers(me);
    flush_itimer_signals();
    #endif

    --- a/include/linux/sched/task.h
    +++ b/include/linux/sched/task.h
    @@ -81,7 +81,7 @@ static inline void exit_thread(struct ta
    extern void do_group_exit(int);

    extern void exit_files(struct task_struct *);
    -extern void exit_itimers(struct signal_struct *);
    +extern void exit_itimers(struct task_struct *);

    extern pid_t kernel_clone(struct kernel_clone_args *kargs);
    struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node);
    --- a/kernel/exit.c
    +++ b/kernel/exit.c
    @@ -796,7 +796,7 @@ void __noreturn do_exit(long code)

    #ifdef CONFIG_POSIX_TIMERS
    hrtimer_cancel(&tsk->signal->real_timer);
    - exit_itimers(tsk->signal);
    + exit_itimers(tsk);
    #endif
    if (tsk->mm)
    setmax_mm_hiwater_rss(&tsk->signal->maxrss, tsk->mm);
    --- a/kernel/time/posix-timers.c
    +++ b/kernel/time/posix-timers.c
    @@ -1051,15 +1051,24 @@ retry_delete:
    }

    /*
    - * This is called by do_exit or de_thread, only when there are no more
    - * references to the shared signal_struct.
    + * This is called by do_exit or de_thread, only when nobody else can
    + * modify the signal->posix_timers list. Yet we need sighand->siglock
    + * to prevent the race with /proc/pid/timers.
    */
    -void exit_itimers(struct signal_struct *sig)
    +void exit_itimers(struct task_struct *tsk)
    {
    + struct list_head timers;
    struct k_itimer *tmr;

    - while (!list_empty(&sig->posix_timers)) {
    - tmr = list_entry(sig->posix_timers.next, struct k_itimer, list);
    + if (list_empty(&tsk->signal->posix_timers))
    + return;
    +
    + spin_lock_irq(&tsk->sighand->siglock);
    + list_replace_init(&tsk->signal->posix_timers, &timers);
    + spin_unlock_irq(&tsk->sighand->siglock);
    +
    + while (!list_empty(&timers)) {
    + tmr = list_first_entry(&timers, struct k_itimer, list);
    itimer_delete(tmr);
    }
    }

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