[PATCH] brd: fix overflow in __brd_direct_access

[Mikulas Patocka]

The code in __brd_direct_access multiplies the pgoff variable by page size
and divides it by 512. It can cause overflow on 32-bit architectures. The
overflow happens if we create ramdisk larger than 4G and use it as a
sparse device.

This patch replaces multiplication and division with multiplication by the
number of sectors per page.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Fixes: 1647b9b959c7 ("brd: add dax_operations support")
Cc: stable@xxxxxxxxxxxxxxx	# 4.12+

---
 drivers/block/brd.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-4.13/drivers/block/brd.c
===================================================================
--- linux-4.13.orig/drivers/block/brd.c
+++ linux-4.13/drivers/block/brd.c
@@ -339,7 +339,7 @@ static long __brd_direct_access(struct b
 
 	if (!brd)
 		return -ENODEV;
-	page = brd_insert_page(brd, PFN_PHYS(pgoff) / 512);
+	page = brd_insert_page(brd, (sector_t)pgoff << PAGE_SECTORS_SHIFT);
 	if (!page)
 		return -ENOSPC;
 	*kaddr = page_address(page);