linux-kernel - Re: [patch] Crash on evdev disconnect.

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Date:	Mon, 7 Aug 2006 15:41:44 -0400
From:	"Zephaniah E. Hull" <warp@...allh.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	linux-input@...ey.karlin.mff.cuni.cz, linux-kernel@...r.kernel.org
Subject: Re: [patch] Crash on evdev disconnect.

On Mon, Aug 07, 2006 at 03:04:36PM -0400, Dmitry Torokhov wrote:
> On 8/7/06, Zephaniah E. Hull <warp@...allh.com> wrote:
> >On Mon, Aug 07, 2006 at 01:35:50PM -0400, Dmitry Torokhov wrote:
> >> Hi,
> >>
> >> On 8/7/06, Zephaniah E. Hull <warp@...allh.com> wrote:
> >> >       if (evdev->open) {
> >> >               input_close_device(handle);
> >> >               wake_up_interruptible(&evdev->wait);
> >> >-               list_for_each_entry(list, &evdev->list, node)
> >> >+               list_for_each_entry_safe(list, next, &evdev->list, node)
> >> >                       kill_fasync(&list->fasync, SIGIO, POLL_HUP);
> >>
> >> NAK. kill_fasync does not affect the list state so using _safe does
> >> not buy us anything.
> >
> >Sorry, but you're wrong.
> >
> >Immediately before the kill_fasync call list->node.next is a valid
> >pointer, immediately afterwords it is 0x100100, which happens to be
> >list_poison.  kill_fasync is triggering a close somehow, evdev_close
> >deletes that element of the list, which poisons the next value, which
> >can make us crash and burn.
> >
> >I have a 100% reproducible crash case, which is fixed by the change.
> >
> >If kill_fasync shouldn't be making it close that's another issue, but at
> >the moment it is and this is a fairly non-invasive change which fixes
> >it.
> >
> 
> Unfortunately it does not really fix the problem, it just papers over
> the issue. The crash will still happen if for some reason
> evdev_release runs at a bad moment.

Almost agreed, it papers over the lack of locking, however most forms of
locking may need this to avoid a deadlock anyhow.  (I don't know the
semantics of kill_fasync, and thus I don't know if attempting to take a
lock in the close that is held here would deadlock, or if execution
would move back to here, but I have a nasty feeling that it's the
former.)

That said, at the moment, we have a 100% guaranteed oops on rmmod usbhid
if something has the device open, which is bad.

Zephaniah E. Hull.

-- 
	  1024D/E65A7801 Zephaniah E. Hull <warp@...allh.com>
	   92ED 94E4 B1E6 3624 226D  5727 4453 008B E65A 7801
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