[PATCH] drivers: char: mem: Fix wraparound check to allow mappings up to the end

[Julius Werner]

A recent fix to /dev/mem prevents mappings from wrapping around the end
of physical address space. However, the check was written in a way that
also prevents a mapping reaching just up to the end of physical address
space, which may be a valid use case (especially on 32-bit systems).
This patch fixes it by checking the last mapped address (instead of the
first address behind that) for overflow.

Fixes: b299cde245 ("drivers: char: mem: Check for address space wraparound with mmap()")
Cc: <stable@xxxxxxxxxxxxxxx>
Reported-by: Nico Huber <nico.h@xxxxxx>
Signed-off-by: Julius Werner <jwerner@xxxxxxxxxxxx>
---
 drivers/char/mem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 6e0cbe092220..593a8818aca9 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -343,7 +343,7 @@ static int mmap_mem(struct file *file, struct vm_area_struct *vma)
 	phys_addr_t offset = (phys_addr_t)vma->vm_pgoff << PAGE_SHIFT;
 
 	/* It's illegal to wrap around the end of the physical address space. */
-	if (offset + (phys_addr_t)size < offset)
+	if (offset + (phys_addr_t)size - 1 < offset)
 		return -EINVAL;
 
 	if (!valid_mmap_phys_addr_range(vma->vm_pgoff, size))
-- 
2.12.2