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>

arch/arm/mm/copypage-feroceon.c

@@ -74,6 +74,7 @@ void feroceon_copy_user_highpage(struct page *to, struct page *from,
 
 	kto = kmap_atomic(to, KM_USER0);
 	kfrom = kmap_atomic(from, KM_USER1);
+	flush_cache_page(vma, vaddr, page_to_pfn(from));
 	feroceon_copy_user_page(kto, kfrom);
 	kunmap_atomic(kfrom, KM_USER1);
 	kunmap_atomic(kto, KM_USER0);

arch/arm/mm/copypage-v4wb.c

@@ -54,6 +54,7 @@ void v4wb_copy_user_highpage(struct page *to, struct page *from,
 
 	kto = kmap_atomic(to, KM_USER0);
 	kfrom = kmap_atomic(from, KM_USER1);
+	flush_cache_page(vma, vaddr, page_to_pfn(from));
 	v4wb_copy_user_page(kto, kfrom);
 	kunmap_atomic(kfrom, KM_USER1);
 	kunmap_atomic(kto, KM_USER0);

arch/arm/mm/copypage-xsc3.c

@@ -77,6 +77,7 @@ void xsc3_mc_copy_user_highpage(struct page *to, struct page *from,
 
 	kto = kmap_atomic(to, KM_USER0);
 	kfrom = kmap_atomic(from, KM_USER1);
+	flush_cache_page(vma, vaddr, page_to_pfn(from));
 	xsc3_mc_copy_user_page(kto, kfrom);
 	kunmap_atomic(kfrom, KM_USER1);
 	kunmap_atomic(kto, KM_USER0);