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authorRussell King <rmk+kernel@arm.linux.org.uk>2009-10-05 15:34:22 +0100
committerRussell King <rmk+kernel@arm.linux.org.uk>2009-10-05 15:42:16 +0100
commit2725898fc9bb2121ac0fb1b5e4faf4fc09014729 (patch)
treef2397446518cdfd8dc42a1d3808ed77005f427cc /arch/arm/Kconfig
parentf00a75c094c340c4e7435665816c3273c870e849 (diff)
downloadlinux-2.6.34-ux500-2725898fc9bb2121ac0fb1b5e4faf4fc09014729.tar.gz
ARM: Flush user mapping on VIVT processors when copying a page
Steven Walter <stevenrwalter@gmail.com> writes: > 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: > > Thread A: flush_cache_mm() (dup_mmap) > Thread B: writes to a page in the above mm > Thread A: pte_wrprotect() the above page (copy_one_pte) > 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. Now there are two conflicting copies of the same > cache-line in dcache. If the userspace cache-line flushes before > the kernel cache-line, we lose the changes made during the first > write. do_wp_page does call flush_dcache_page on the newly-copied > page, but there's still a window where the CPU could flush the > userspace cache-line before then. Resolve this by flushing the user mapping before copying the page on processors with a writeback VIVT cache. Note: this does have a performance impact, and so needs further consideration before being merged - can we optimize out some of the cache flushes if, eg, we know that the page isn't yet mapped? Thread: <e06498070903061426o5875ad13hc6328aa0d3f08ed7@mail.gmail.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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