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SubjectRe: sudo x86info -a => kernel BUG at mm/usercopy.c:78!
On Fri, Mar 31, 2017 at 10:17 AM, Dave Jones <davej@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 <davej@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.rantala@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

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