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

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Date:   Fri, 31 Mar 2017 10:32:04 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Dave Jones <davej@...emonkey.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Tommi Rantala <tommi.t.rantala@...ia.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: sudo x86info -a => kernel BUG at mm/usercopy.c:78!

On Fri, Mar 31, 2017 at 10:17 AM, Dave Jones <davej@...emonkey.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@...emonkey.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@...ia.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