ARM: 5691/1: fix cache aliasing issues between kmap() and kmap_atomic() with highmem

Let's suppose a highmem page is kmap'd with kmap().  A pkmap entry is
used, the page mapped to it, and the virtual cache is dirtied.  Then
kunmap() is used which does virtually nothing except for decrementing a
usage count.

Then, let's suppose the _same_ page gets mapped using kmap_atomic().
It is therefore mapped onto a fixmap entry instead, which has a
different virtual address unaware of the dirty cache data for that page
sitting in the pkmap mapping.

Fortunately it is easy to know if a pkmap mapping still exists for that
page and use it directly with kmap_atomic(), thanks to kmap_high_get().

And actual testing with a printk in the added code path shows that this
condition is actually met *extremely* frequently.  Seems that we've been
quite lucky that things have worked so well with highmem so far.

Cc: stable@kernel.org
Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 8 insertions, 0 deletions

diff --git a/arch/arm/mm/highmem.c b/arch/arm/mm/highmem.c
index a34954d9df7d..73cae57fa707 100644
--- a/arch/arm/mm/highmem.c
+++ b/arch/arm/mm/highmem.c
@@ -40,11 +40,16 @@ void *kmap_atomic(struct page *page, enum km_type type)
 {
         unsigned int idx;
         unsigned long vaddr;
+        void *kmap;
 
         pagefault_disable();
         if (!PageHighMem(page))
                 return page_address(page);
 
+        kmap = kmap_high_get(page);
+        if (kmap)
+                return kmap;
+
         idx = type + KM_TYPE_NR * smp_processor_id();
         vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
 #ifdef CONFIG_DEBUG_HIGHMEM
@@ -80,6 +85,9 @@ void kunmap_atomic(void *kvaddr, enum km_type type)
 #else
                 (void) idx;  /* to kill a warning */
 #endif
+        } else if (vaddr >= PKMAP_ADDR(0) && vaddr < PKMAP_ADDR(LAST_PKMAP)) {
+                /* this address was obtained through kmap_high_get() */
+                kunmap_high(pte_page(pkmap_page_table[PKMAP_NR(vaddr)]));
         }
         pagefault_enable();
 }