On Fri, Mar 31, 2017 at 10:17 AM, Dave Jones <da...@codemonkey.org.uk> wrote:
> On Thu, Mar 30, 2017 at 12:52:31PM -0700, Kees Cook wrote:
> > On Thu, Mar 30, 2017 at 12:41 PM, Dave Jones <da...@codemonkey.org.uk>
> wrote:
> > > On Thu, Mar 30, 2017 at 09:45:26AM -0700, Kees Cook wrote:
> > > > On Wed, Mar 29, 2017 at 11:44 PM, Tommi Rantala
> > > > <tommi.t.rant...@nokia.com> wrote:
> > > > > Hi,
> > > > >
> > > > > Running:
> > > > >
> > > > > $ sudo x86info -a
> > > > >
> > > > > On this HP ZBook 15 G3 laptop kills the x86info process with
> segfault and
> > > > > produces the following kernel BUG.
> > > > >
> > > > > $ git describe
> > > > > v4.11-rc4-40-gfe82203
> > > > >
> > > > > It is also reproducible with the fedora kernel:
> 4.9.14-200.fc25.x86_64
> > > > >
> > > > > Full dmesg output here: https://pastebin.com/raw/Kur2mpZq
> > > > >
> > > > > [ 51.418954] usercopy: kernel memory exposure attempt detected
> from
> > > > > ffff880000090000 (dma-kmalloc-256) (4096 bytes)
> > > >
> > > > This seems like a real exposure: the copy is attempting to read 4096
> > > > bytes from a 256 byte object.
> > >
> > > The code[1] is doing a 4k read from /dev/mem in the range 0x90000 ->
> 0xa0000
> > > According to arch/x86/mm/init.c:devmem_is_allowed, that's still valid..
> > >
> > > Note that the printk is using the direct mapping address. Is that what's
> > > being passed down to devmem_is_allowed now ? If so, that's probably what
> broke.
> >
> > So this is attempting to read physical memory 0x90000 -> 0xa0000, but
> > that's somehow resolving to a virtual address that is claimed by
> > dma-kmalloc?? I'm confused how that's happening...
>
> /dev/mem is using physical addresses that the kernel translates through the
> direct mapping. __check_object_size seems to think that anything passed
> into it is always allocated by the kernel, but in this case, I think
> read_mem()
> is just passing through the direct mapping to copy_to_user.
How is ffff880000090000 both in the direct mapping and a slab object?
It would need to pass all of these checks, and be marked as PageSlab
before it could be evaluated by __check_heap_object:
if (is_vmalloc_or_module_addr(ptr))
return NULL;
if (!virt_addr_valid(ptr))
return NULL;
page = virt_to_head_page(ptr);
/* Check slab allocator for flags and size. */
if (PageSlab(page))
return __check_heap_object(ptr, n, page);
-Kees
--
Kees Cook
Pixel Security
