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

[Masami Hiramatsu <mhiramat@kernel.org>]

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().

Thank you!


-- 
Masami Hiramatsu <mhiramat@kernel.org>