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    Subject[tip:timers/nohz] context_tracking: Protect against recursion
    Commit-ID:  aed5ed47724f6a7453fa62e3c90f3cee93edbfe3
    Gitweb: http://git.kernel.org/tip/aed5ed47724f6a7453fa62e3c90f3cee93edbfe3
    Author: Frederic Weisbecker <fweisbec@gmail.com>
    AuthorDate: Wed, 6 May 2015 18:04:23 +0200
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Thu, 7 May 2015 12:02:50 +0200

    context_tracking: Protect against recursion

    Context tracking recursion can happen when an exception triggers
    in the middle of a call to a context tracking probe.

    This special case can be caused by vmalloc faults. If an access
    to a memory area allocated by vmalloc happens in the middle of
    context_tracking_enter(), we may run into an endless fault loop
    because the exception in turn calls context_tracking_enter()
    which faults on the same vmalloc'ed memory, triggering an
    exception again, etc...

    Some rare crashes have been reported so lets protect against
    this with a recursion counter.

    Reported-by: Dave Jones <davej@redhat.com>
    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    Reviewed-by: Rik van Riel <riel@redhat.com>
    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Chris Metcalf <cmetcalf@ezchip.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
    Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Rafael J . Wysocki <rafael.j.wysocki@intel.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/1430928266-24888-2-git-send-email-fweisbec@gmail.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    include/linux/context_tracking_state.h | 1 +
    kernel/context_tracking.c | 29 +++++++++++++++++++++++++++++
    2 files changed, 30 insertions(+)

    diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
    index 6b7b96a..678ecdf 100644
    --- a/include/linux/context_tracking_state.h
    +++ b/include/linux/context_tracking_state.h
    @@ -12,6 +12,7 @@ struct context_tracking {
    * may be further optimized using static keys.
    */
    bool active;
    + int recursion;
    enum ctx_state {
    CONTEXT_KERNEL = 0,
    CONTEXT_USER,
    diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
    index 72d59a1..5b11a10 100644
    --- a/kernel/context_tracking.c
    +++ b/kernel/context_tracking.c
    @@ -38,6 +38,25 @@ void context_tracking_cpu_set(int cpu)
    }
    }

    +static bool context_tracking_recursion_enter(void)
    +{
    + int recursion;
    +
    + recursion = __this_cpu_inc_return(context_tracking.recursion);
    + if (recursion == 1)
    + return true;
    +
    + WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion);
    + __this_cpu_dec(context_tracking.recursion);
    +
    + return false;
    +}
    +
    +static void context_tracking_recursion_exit(void)
    +{
    + __this_cpu_dec(context_tracking.recursion);
    +}
    +
    /**
    * context_tracking_enter - Inform the context tracking that the CPU is going
    * enter user or guest space mode.
    @@ -75,6 +94,9 @@ void context_tracking_enter(enum ctx_state state)
    WARN_ON_ONCE(!current->mm);

    local_irq_save(flags);
    + if (!context_tracking_recursion_enter())
    + goto out_irq_restore;
    +
    if ( __this_cpu_read(context_tracking.state) != state) {
    if (__this_cpu_read(context_tracking.active)) {
    /*
    @@ -105,6 +127,8 @@ void context_tracking_enter(enum ctx_state state)
    */
    __this_cpu_write(context_tracking.state, state);
    }
    + context_tracking_recursion_exit();
    +out_irq_restore:
    local_irq_restore(flags);
    }
    NOKPROBE_SYMBOL(context_tracking_enter);
    @@ -139,6 +163,9 @@ void context_tracking_exit(enum ctx_state state)
    return;

    local_irq_save(flags);
    + if (!context_tracking_recursion_enter())
    + goto out_irq_restore;
    +
    if (__this_cpu_read(context_tracking.state) == state) {
    if (__this_cpu_read(context_tracking.active)) {
    /*
    @@ -153,6 +180,8 @@ void context_tracking_exit(enum ctx_state state)
    }
    __this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
    }
    + context_tracking_recursion_exit();
    +out_irq_restore:
    local_irq_restore(flags);
    }
    NOKPROBE_SYMBOL(context_tracking_exit);

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