Re: [PATCH v3 8/8] x86, vdso: mark vDSO read-only after init

[Andy Lutomirski]

On Wed, Dec 9, 2015 at 1:43 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> The vDSO does not need to be writable after __init, so mark it as
> __ro_after_init. The result kills the exploit method of writing to the
> vDSO from kernel space resulting in userspace executing the modified code,
> as shown here to bypass SMEP restrictions: http://itszn.com/blog/?p=21
>
> The memory map (with added vDSO address reporting) shows the vDSO moving
> into read-only memory:
>
> Before:
> [    0.143067] vDSO @ ffffffff82004000
> [    0.143551] vDSO @ ffffffff82006000
> ---[ High Kernel Mapping ]---
> 0xffffffff80000000-0xffffffff81000000      16M                         pmd
> 0xffffffff81000000-0xffffffff81800000       8M   ro     PSE     GLB x  pmd
> 0xffffffff81800000-0xffffffff819f3000    1996K   ro             GLB x  pte
> 0xffffffff819f3000-0xffffffff81a00000      52K   ro                 NX pte
> 0xffffffff81a00000-0xffffffff81e00000       4M   ro     PSE     GLB NX pmd
> 0xffffffff81e00000-0xffffffff81e05000      20K   ro             GLB NX pte
> 0xffffffff81e05000-0xffffffff82000000    2028K   ro                 NX pte
> 0xffffffff82000000-0xffffffff8214f000    1340K   RW             GLB NX pte
> 0xffffffff8214f000-0xffffffff82281000    1224K   RW                 NX pte
> 0xffffffff82281000-0xffffffff82400000    1532K   RW             GLB NX pte
> 0xffffffff82400000-0xffffffff83200000      14M   RW     PSE     GLB NX pmd
> 0xffffffff83200000-0xffffffffc0000000     974M                         pmd
>
> After:
> [    0.145062] vDSO @ ffffffff81da1000
> [    0.146057] vDSO @ ffffffff81da4000
> ---[ High Kernel Mapping ]---
> 0xffffffff80000000-0xffffffff81000000      16M                         pmd
> 0xffffffff81000000-0xffffffff81800000       8M   ro     PSE     GLB x  pmd
> 0xffffffff81800000-0xffffffff819f3000    1996K   ro             GLB x  pte
> 0xffffffff819f3000-0xffffffff81a00000      52K   ro                 NX pte
> 0xffffffff81a00000-0xffffffff81e00000       4M   ro     PSE     GLB NX pmd
> 0xffffffff81e00000-0xffffffff81e0b000      44K   ro             GLB NX pte
> 0xffffffff81e0b000-0xffffffff82000000    2004K   ro                 NX pte
> 0xffffffff82000000-0xffffffff8214c000    1328K   RW             GLB NX pte
> 0xffffffff8214c000-0xffffffff8227e000    1224K   RW                 NX pte
> 0xffffffff8227e000-0xffffffff82400000    1544K   RW             GLB NX pte
> 0xffffffff82400000-0xffffffff83200000      14M   RW     PSE     GLB NX pmd
> 0xffffffff83200000-0xffffffffc0000000     974M                         pmd
>
> Based on work by PaX Team and Brad Spengler.
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>

--Andy
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