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

From: Roman Penyaev
Date: Fri Jan 04 2019 - 06:06:49 EST

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);
/* Do not care about mem leak */

return remap_vmalloc_range(vma, mem, 0);

And userspace simply mmaps size + PAGE_SIZE:


Possible candidates for oops which do not have any explicit size

*** 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);


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 {