linux-kernel - Re: cache aliasing in dup

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Message-ID: <20090306225525.GB3212@n2100.arm.linux.org.uk>
Date:	Fri, 6 Mar 2009 22:55:25 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Steven Walter <stevenrwalter@...il.com>
Cc:	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: cache aliasing in dup_mmap

On Fri, Mar 06, 2009 at 05:26:24PM -0500, Steven Walter wrote:
> I've been tracking down an instance of userspace data corruption, and I
> believe I have found a window during fork where data can be lost.  The
> corruption is occurring on an ARMv5 system with VIVT caches.  Here's the
> scenario in question.  Thread A is forking, Thread B is running in
> userspace:

With VIVT caches, you're missing a few things here:

> Thread A: flush_cache_mm (dup_mmap)

-- cache written back and invalidated

> Thread B: writes to a page in the above mm

-- cache written back and invalidated

> Thread A: pte_wrprotect the above page (copy_one_pte)

-- cache written back and invalidated

> Thread B: writes to the same page again
> 
> During thread B's second write, he'll take a fault and enter the do_wp_page
> case.  We'll end up calling copy_page, which notably uses the kernel virtual
> addresses for the old and new pages.  This means that the new page does not
> necessarily have the data from the first write.

Given the additional flushing I've mentioned above, where could the
problem be?
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