linux-kernel - Re: [PATCH] arm: Remove early stack deallocation from restore_user

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Message-ID: <20150109191647.GA1933@kroah.com>
Date:	Fri, 9 Jan 2015 11:16:47 -0800
From:	Greg KH <greg@...ah.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Daniel Thompson <daniel.thompson@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>
Subject: Re: [PATCH] arm: Remove early stack deallocation from
 restore_user_regs

On Fri, Jan 09, 2015 at 05:20:35PM +0000, Russell King - ARM Linux wrote:
> On Fri, Jan 09, 2015 at 05:06:54PM +0000, Daniel Thompson wrote:
> > On 09/01/15 16:46, Russell King - ARM Linux wrote:
> > > On Mon, Jan 05, 2015 at 03:12:38PM +0000, Daniel Thompson wrote:
> > >> Currently restore_user_regs deallocates the SVC stack early in
> > >> its execution and relies on no exception being taken between
> > >> the deallocation and the registers being restored. The introduction
> > >> of a default FIQ handler that also uses the SVC stack breaks this
> > >> assumption and can result in corrupted register state.
> > >>
> > >> This patch works around the problem by removing the early
> > >> stack deallocation and using r2 as a temporary instead. I have
> > >> not found a way to do this without introducing an extra mov
> > >> instruction to the macro.
> > >>
> > >> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> > >> ---
> > > 
> > > Please put it in the patch system, thanks.
> > 
> > Will do.
> > 
> > 
> > > I think we should queue
> > > this one for stable too, as I think we need this for v3.18
> > > (as a result of c0e7f7ee717e2b4c5791e7422424c96b5008c39e,
> > > ARM: 8150/3: fiq: Replace default FIQ handler)?
> > 
> > It's a close call.
> 
> I agree.
> 
> > Before 8150/3 the system would probably crash if the default FIQ handler
> > ran. After 8150/3 the system is also likely to crash since there's no
> > code hooked into the handler in v3.18 that can clear the source of FIQ
> > leaving us stuck re-entering the FIQ handler.
> > 
> > Nevertheless, this is a nasty gremlin to leave for backporters (it
> > wasn't easy to find) so I'd be very happy to Cc: stable and see what
> > they think.
> 
> Well, we could ask Greg now.  It's not a regression (as nothing makes
> use of the previously merged changes yet), but it is a nasty latent bug
> which we could easily solve.
> 
> Having thought about it some more, I'm tempted to say... leave the
> stable tag off it, and we can make the decision later - after it's had
> a chance to really prove itself to a much wider audience.  That'd be
> the lowest risk for the 3.18 stable tree.

That sounds like a good idea, you can always email stable@ to let us
know of a patch we should add at a later time.

thanks,

greg k-h
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