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

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: b299cde2 ("drivers: char: mem: Check for address space wraparound with mmap()")
Cc: <stable@vger.kernel.org>
Reported-by: Nico Huber <nico.h@gmx.de>
Signed-off-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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))