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.
Dave
