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    SubjectRe: [PATCH] x86: check for valid irq_cfg pointer in smp_irq_move_cleanup_interrupt
    On Sat, May 26, 2012 at 12:18:21PM +0200, Thomas Gleixner wrote:
    > On Fri, 25 May 2012, Suresh Siddha wrote:
    > > On Thu, 2012-05-24 at 21:16 +0200, Thomas Gleixner wrote:
    > > There are other (not-so common) irq desc references, like in the
    > > show_interrupts() (cat /proc/interrupts path) etc, that does things like
    > > this in the process context:
    > >
    > > desc = irq_to_desc(i);
    > > if (!desc)
    > > return 0;
    > >
    > > raw_spin_lock_irqsave(&desc->lock, flags);
    > >
    > > May be we should introduce something like
    > > get_irq_desc_locked()/put_irq_desc_locked() that can safely access the
    > > irq desc with pre-emption/irq's disabled and lock it etc. And the
    > > synchronize_sched() will enable the destroy_irq()/free_desc() to free it
    > > safely etc.
    >
    > I want to avoid that and instead use proper refcounting. The reason is
    > that we want to move the irq descriptor when the affinity changes
    > nodes, and for that we need refcounting anyway.
    >

    While this proposal sounds good, in the meantime would there be any harm in
    putting the NULL cfg check into smp_irq_move_cleanup_interrupt()?

    It's a minimal change, and eliminates the panics that I've encountered thus far.

    Reposting the patch.



    A NULL pointer dereference can occur in smp_irq_move_cleanup_interrupt() if
    we haven't yet setup the irq_cfg pointer in the irq_desc.irq_data.chip_data.

    In create_irq_nr() there is a window where we have set vector_irq in
    __assign_irq_vector(), but not yet called irq_set_chip_data() to set the
    irq_cfg pointer.

    Should an IRQ_MOVE_CLEANUP_VECTOR hit the cpu in question during this time,
    smp_irq_move_cleanup_interrupt() will attempt to process the aforementioned
    irq, but panic when accessing irq_cfg.

    There is also a window in destroy_irq() where we've cleared the irq_cfg
    pointer in free_irq_cfg(), but have not yet called irq_free_desc(). Note
    that we have cleared vector_irq in __clear_irq_vector() prior to free_irq_cfg(),
    but smp_irq_move_cleanup_interrupt() might've already referenced the irq_desc.

    Only continue processing the irq if irq_cfg is non-NULL.

    Signed-off-by: Dimitri Sivanich <sivanich@sgi.com>
    ---
    arch/x86/kernel/apic/io_apic.c | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    Index: linux/arch/x86/kernel/apic/io_apic.c
    ===================================================================
    --- linux.orig/arch/x86/kernel/apic/io_apic.c
    +++ linux/arch/x86/kernel/apic/io_apic.c
    @@ -2478,9 +2478,12 @@ asmlinkage void smp_irq_move_cleanup_int
    if (!desc)
    continue;

    - cfg = irq_cfg(irq);
    raw_spin_lock(&desc->lock);

    + cfg = irq_cfg(irq);
    + if (!cfg)
    + goto unlock;
    +
    /*
    * Check if the irq migration is in progress. If so, we
    * haven't received the cleanup request yet for this irq.

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