[PATCH 3.16 101/129] mm/vmalloc: fix size check for remap_vmalloc_range_partial()

[Ben Hutchings]

3.16.70-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Roman Penyaev <rpenyaev@xxxxxxx>

commit 401592d2e095947344e10ec0623adbcd58934dd4 upstream.

When VM_NO_GUARD is not set area->size includes adjacent guard page,
thus for correct size checking get_vm_area_size() should be used, but
not area->size.

This fixes possible kernel oops when userspace tries to mmap an area on
1 page bigger than was allocated by vmalloc_user() call: the size check
inside remap_vmalloc_range_partial() accounts non-existing guard page
also, so check successfully passes but vmalloc_to_page() returns NULL
(guard page does not physically exist).

The following code pattern example should trigger an oops:

  static int oops_mmap(struct file *file, struct vm_area_struct *vma)
  {
        void *mem;

        mem = vmalloc_user(4096);
        BUG_ON(!mem);
        /* Do not care about mem leak */

        return remap_vmalloc_range(vma, mem, 0);
  }

And userspace simply mmaps size + PAGE_SIZE:

  mmap(NULL, 8192, PROT_WRITE|PROT_READ, MAP_PRIVATE, fd, 0);

Possible candidates for oops which do not have any explicit size
checks:

   *** drivers/media/usb/stkwebcam/stk-webcam.c:
   v4l_stk_mmap[789]   ret = remap_vmalloc_range(vma, sbuf->buffer, 0);

Or the following one:

   *** drivers/video/fbdev/core/fbmem.c
   static int
   fb_mmap(struct file *file, struct vm_area_struct * vma)
        ...
        res = fb->fb_mmap(info, vma);

Where fb_mmap callback calls remap_vmalloc_range() directly without any
explicit checks:

   *** drivers/video/fbdev/vfb.c
   static int vfb_mmap(struct fb_info *info,
             struct vm_area_struct *vma)
   {
       return remap_vmalloc_range(vma, (void *)info->fix.smem_start, vma->vm_pgoff);
   }

Link: http://lkml.kernel.org/r/20190103145954.16942-2-rpenyaev@xxxxxxx
Signed-off-by: Roman Penyaev <rpenyaev@xxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Joe Perches <joe@xxxxxxxxxxx>
Cc: "Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/vmalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2141,7 +2141,7 @@ int remap_vmalloc_range_partial(struct v
 	if (!(area->flags & VM_USERMAP))
 		return -EINVAL;
 
-	if (kaddr + size > area->addr + area->size)
+	if (kaddr + size > area->addr + get_vm_area_size(area))
 		return -EINVAL;
 
 	do {