Re: [PATCH 1/3] mm/vmalloc: fix size check for remap_vmalloc_range_partial()

[Roman Penyaev]

Hi Andrew,

Could you please update the commit message with something as
the following (I hope it will become a bit clearer now):

-------------
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);
  }
--------

--
Roman


On 2019-01-03 15:59, Roman Penyaev wrote:
> area->size can include adjacent guard page but get_vm_area_size()
> returns actual size of the area.
>
> This fixes possible kernel crash when userspace tries to map area
> on 1 page bigger: size check passes but the following vmalloc_to_page()
> returns NULL on last guard (non-existing) page.
>
> Signed-off-by: Roman Penyaev <rpenyaev@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Joe Perches <joe@xxxxxxxxxxx>
> Cc: "Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  mm/vmalloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 871e41c55e23..2cd24186ba84 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2248,7 +2248,7 @@ int remap_vmalloc_range_partial(struct
> vm_area_struct *vma, unsigned long uaddr,
>  	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 {