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Date: Tue, 19 Jan 2016 18:56:38 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Kees Cook <keescook@...omium.org>
Cc: Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
Michael Ellerman <mpe@...erman.id.au>, Mathias Krause <minipli@...glemail.com>,
Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
PaX Team <pageexec@...email.hu>, Emese Revfy <re.emese@...il.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v4 8/8] x86, vdso: mark vDSO read-only after init
On Tue, Jan 19, 2016 at 10:08 AM, Kees Cook <keescook@...omium.org> 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@...omium.org>
> Acked-by: Andy Lutomirski <luto@...nel.org>
> ---
> 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
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