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

[Michal Hocko <mhocko@kernel.org>]

On Thu 03-01-19 21:31:58, Roman Penyaev wrote:
> On 2019-01-03 20:40, Michal Hocko wrote:
> > On Thu 03-01-19 20:27:26, Roman Penyaev wrote:
> > > On 2019-01-03 16:13, Michal Hocko wrote:
> > > > On Thu 03-01-19 15:59:52, 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.
> > > >
> > > > Can this actually happen? I am not really familiar with all the callers
> > > > of this API but VM_NO_GUARD is not really used wildly in the kernel.
> > > 
> > > Exactly, by default (VM_NO_GUARD is not set) each area has guard page,
> > > thus the area->size will be bigger.  The bug is not reproduced if
> > > VM_NO_GUARD is set.
> > > 
> > > > All I can see is kasan na arm64 which doesn't really seem to use it
> > > > for vmalloc.
> > > >
> > > > So is the problem real or this is a mere cleanup?
> > > 
> > > This is the real problem, try this hunk for any file descriptor which
> > > provides
> > > mapping, or say modify epoll as example:
> > 
> > OK, my response was more confusing than I intended. I meant to say. Is
> > there any in kernel code that would allow the bug have had in mind?
> > In other words can userspace trick any existing code?
> 
> In theory any existing caller of remap_vmalloc_range() which does
> not have an explicit size check should trigger an oops, e.g. this is
> a good candidate:
> 
> *** drivers/media/usb/stkwebcam/stk-webcam.c:
> v4l_stk_mmap[789]              ret = remap_vmalloc_range(vma, sbuf->buffer,
> 0);

Hmm, sbuf->buffer is allocated in stk_setup_siobuf to have
buf->v4lbuf.length. mmap callback maps this buffer to the vma size and
that is indeed not enforced to be <= length AFAICS. So you are right!

Can we have an example in the changelog please?

Thanks!
-- 
Michal Hocko
SUSE Labs