Re: sudo x86info -a => kernel BUG at mm/usercopy.c:78!

[Dave Jones]

On Thu, Mar 30, 2017 at 12:52:31PM -0700, Kees Cook wrote:
 > On Thu, Mar 30, 2017 at 12:41 PM, Dave Jones <davej@xxxxxxxxxxxxxxxxx> 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.rantala@xxxxxxxxx> 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