Re: [PATCH v4 8/8] x86, vdso: mark vDSO read-only after init

[Andy Lutomirski]

On Tue, Jan 19, 2016 at 10:08 AM, 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>
> ---
>  arch/x86/entry/vdso/vdso2c.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/entry/vdso/vdso2c.h b/arch/x86/entry/vdso/vdso2c.h
> index 0224987556ce..eb93a3137ed2 100644
> --- a/arch/x86/entry/vdso/vdso2c.h
> +++ b/arch/x86/entry/vdso/vdso2c.h
> @@ -140,7 +140,7 @@ static void BITSFUNC(go)(void *raw_addr, size_t raw_len,
>         fprintf(outfile, "#include <asm/vdso.h>\n");
>         fprintf(outfile, "\n");
>         fprintf(outfile,
> -               "static unsigned char raw_data[%lu] __page_aligned_data = {",
> +               "static unsigned char raw_data[%lu] __ro_after_init __aligned(PAGE_SIZE) = {",
>                 mapping_size);
>         for (j = 0; j < stripped_len; j++) {
>                 if (j % 10 == 0)
> @@ -150,7 +150,7 @@ static void BITSFUNC(go)(void *raw_addr, size_t raw_len,
>         }
>         fprintf(outfile, "\n};\n\n");
>
> -       fprintf(outfile, "static struct page *pages[%lu];\n\n",
> +       fprintf(outfile, "static struct page *pages[%lu] __ro_after_init;\n\n",

I spoke a bit too soon.  This line of code is gone in -tip:

https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?h=x86/asm&id=05ef76b20fc4297b0d3f8a956f1c809a8a1b3f1d

My ack stands for the other change in here, though.

--Andy