xen: Fix page <-> pfn conversion on 32 bit systems

Commit 1084b1988d22dc165c9dbbc2b0e057f9248ac4db (xen: Add Xen specific
page definition) caused a regression in 4.4.

The xen functions to convert between pages and pfns fail due to an
overflow on systems where a physical address may not fit in an
unsigned long (e.g. x86 32 bit PAE systems). Rework the conversion to
avoid overflow. This should also result in simpler object code.

This bug manifested itself as disk corruption with Linux 4.4 when
using blkfront in a Xen HVM x86 32 bit guest with more than 4 GiB of
memory.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Cc: <stable@vger.kernel.org> # 4.4+
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

diff --git a/include/xen/page.h b/include/xen/page.h
index 96294ac937552178531266cb16ce04185e47c99a..9dc46cb8a0fd79be7f4bdae7ae8f5f4dbdcdb7f4 100644 (file)
--- a/include/xen/page.h
+++ b/include/xen/page.h
@@ -15,9 +15,9 @@
  */
 
 #define xen_pfn_to_page(xen_pfn)       \
-       ((pfn_to_page(((unsigned long)(xen_pfn) << XEN_PAGE_SHIFT) >> PAGE_SHIFT)))
+       (pfn_to_page((unsigned long)(xen_pfn) >> (PAGE_SHIFT - XEN_PAGE_SHIFT)))
 #define page_to_xen_pfn(page)          \
-       (((page_to_pfn(page)) << PAGE_SHIFT) >> XEN_PAGE_SHIFT)
+       ((page_to_pfn(page)) << (PAGE_SHIFT - XEN_PAGE_SHIFT))
 
 #define XEN_PFN_PER_PAGE       (PAGE_SIZE / XEN_PAGE_SIZE)