Re: [BUGFIX PATCH tip/master V2 3/3] kprobes/arm: Fix a possible deadlock case in kretprobe

[Masami Hiramatsu <mhiramat@kernel.org>]

On Fri, 10 Feb 2017 11:34:45 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Thu, 9 Feb 2017 16:49:00 +0000
> Russell King - ARM Linux <linux@armlinux.org.uk> wrote:
> 
> > On Fri, Feb 10, 2017 at 01:32:22AM +0900, Masami Hiramatsu wrote:
> > > Fix a possibility of deadlock case in kretprobe on arm
> > > implementation. There may be a chance that the kretprobe
> > > hash table lock can cause a dead lock.
> > > 
> > > The senario is that a user puts 2 kretprobes, one on normal
> > > function and one on a function which can be called from
> > > somewhare which can interrupt in irq disabled critical
> > > section like FIQ.
> > 
> > If we:
> > - hit a kernel tracing feature from FIQ context
> > - the tracing feature takes a lock
> > - the lock is also taken elsewhere on the same CPU with IRQs disabled
> > 
> > we will quite simply deadlock.
> 
> Correct.
> 
> > In this case, kretprobe_hash_lock() takes the hlist_lock using
> > raw_spin_lock_irqsave().
> > 
> > Now, from what I can see in the kprobes code, this lock is taken in
> > other contexts (eg, kprobe_flush_task()), which means even with this
> > fix, it's still risky if a kprobe is placed on a FIQ-called function.
> 
> Oops, right! I'll fix that too. Thanks for pointed out.
> 
> > 
> > > In this case, if the kernel hits the 1st kretprobe on a
> > > normal function return which calls trampoline_handler(),
> > > acquire a spinlock on the hash table in kretprobe_hash_lock()
> > > and disable irqs. After that, if the 2nd kretprobe is kicked
> > > from FIQ, it also calls trampoline_handler() and tries to
> > > acquire the same spinlock (since the hash is based on
> > > current task, same as the 1st kretprobe), it causes
> > > a deadlock.
> > 
> > So my deadlock scenario is:
> > 
> > - we're in the middle of kprobe_flush_task()
> > - FIQ happens, calls trampoline_handler()
> > - deadlock in kretprobe_hash_lock()
> > 
> > From what I can see, kretprobes in FIQ are just unsafe.
> 
> Yes, NMI on x86 too.
> 
> > I suspect that avoiding these deadlocks means that we have to deny
> > kprobes from FIQ context - making trampoline_handler() return
> > immediately if in_nmi() is true.
> 
> Ah, in_nmi() means FIQ on arm :)
> OK, but actually it is too late to check it in the enter of
> trampoline_handler() since we don't know where is the real
> return address at that point. So I'll check that in setup site
> - kretprobe_pre_handler().

Hmm, pre_handler_kretprobe() already checked in_nmi().
So, I think this will no problem on FIQ too.

Thanks,

-- 
Masami Hiramatsu <mhiramat@kernel.org>