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Date: Fri, 6 May 2016 18:08:39 +0200
From: Borislav Petkov <bp@...en8.de>
To: keescook@...omium.org
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
luto@...nel.org, dyoung@...hat.com, hpa@...or.com,
luto@...capital.net, torvalds@...ux-foundation.org,
dvlasenk@...hat.com, tglx@...utronix.de, brgerst@...il.com,
yinghai@...nel.org, peterz@...radead.org, vgoyal@...hat.com,
bhe@...hat.com, mingo@...nel.org, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/boot] x86/KASLR: Consolidate mem_avoid[] entries
On Fri, May 06, 2016 at 12:46:05AM -0700, tip-bot for Yinghai Lu wrote:
> Commit-ID: 9dc1969c24eff8b7d7a9a565d1047b624921ba06
> Gitweb: http://git.kernel.org/tip/9dc1969c24eff8b7d7a9a565d1047b624921ba06
> Author: Yinghai Lu <yinghai@...nel.org>
> AuthorDate: Thu, 5 May 2016 15:13:47 -0700
> Committer: Ingo Molnar <mingo@...nel.org>
> CommitDate: Fri, 6 May 2016 09:00:59 +0200
>
> x86/KASLR: Consolidate mem_avoid[] entries
>
> The mem_avoid[] array is used to track positions that should be avoided (like
> the compressed kernel, decompression code, etc) when selecting a memory
> position for the randomly relocated kernel. Since ZO is now at the end of
Can we stop with those ZO(MG) obfuscated abbreviations and call it
simply the "compressed kernel image"?
> the decompression buffer and the decompression code (and its heap and
> stack) are at the front, we can safely consolidate the decompression entry,
> the heap entry, and the stack entry. The boot_params memory, however, could
> be elsewhere, so it should be explicitly included.
>
> Signed-off-by: Yinghai Lu <yinghai@...nel.org>
> Signed-off-by: Baoquan He <bhe@...hat.com>
> [ Rwrote changelog, cleaned up code comments. ]
> Signed-off-by: Kees Cook <keescook@...omium.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Brian Gerst <brgerst@...il.com>
> Cc: Dave Young <dyoung@...hat.com>
> Cc: Denys Vlasenko <dvlasenk@...hat.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Vivek Goyal <vgoyal@...hat.com>
> Cc: kernel-hardening@...ts.openwall.com
> Cc: lasse.collin@...aani.org
> Link: http://lkml.kernel.org/r/1462486436-3707-3-git-send-email-keescook@chromium.org
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
> arch/x86/boot/compressed/kaslr.c | 77 +++++++++++++++++++++++++++++++---------
> 1 file changed, 61 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index 2072d82..6392f00 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -121,7 +121,7 @@ struct mem_vector {
> unsigned long size;
> };
>
> -#define MEM_AVOID_MAX 5
> +#define MEM_AVOID_MAX 4
> static struct mem_vector mem_avoid[MEM_AVOID_MAX];
>
> static bool mem_contains(struct mem_vector *region, struct mem_vector *item)
> @@ -146,22 +146,71 @@ static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
> return true;
> }
>
> +/*
> + * In theroy, KASLR can put the kernel anywhere in area of [16M, 64T). The
^^^^^^
Spellchecker please.
in the range of [...
> + * mem_avoid array is used to store the ranges that need to be avoided when
> + * KASLR searches for a an appropriate random address. We must avoid any
s/a //
> + * regions that are unsafe to overlap with during decompression, and other
> + * things like the initrd, cmdline and boot_params.
> + *
> + * How to calculate the unsafe areas is detailed here, and is informed by
> + * the decompression calculations in header.S, and the diagram in misc.c.
> + *
> + * The compressed vmlinux (ZO) plus relocs and the run space of ZO can't be
> + * overwritten by decompression output.
> + *
> + * ZO sits against the end of the decompression buffer, so we can calculate
> + * where text, data, bss, etc of ZO are positioned.
> + *
> + * The follow are already enforced by the code:
" ... following conditions are ..."
> + * - init_size >= kernel_total_size
> + * - input + input_len >= output + output_len
> + * - kernel_total_size could be >= or < output_len
> + *
> + * From this, we can make several observations, illustrated by a diagram:
> + * - init_size >= kernel_total_size
You just said this above?!
> + * - input + input_len > output + output_len
> + * - kernel_total_size >= output_len
Ok, what is the point of the observations? To show that we're making the
conditions enforced by the code more concrete, stricter, ...?
> + *
> + * 0 output input input+input_len output+init_size
> + * | | | | |
> + * | | | | |
> + * |-----|--------|--------|------------------|----|------------|----------|
> + * | | |
> + * | | |
> + * output+init_size-ZO_INIT_SIZE output+output_len output+kernel_total_size
> + *
> + * [output, output+init_size) is for the buffer for decompressing the
> + * compressed kernel (ZO).
> + *
> + * [output, output+kernel_total_size) is for the uncompressed kernel (VO)
> + * and its bss, brk, etc.
> + * [output, output+output_len) is VO plus relocs
> + *
> + * [output+init_size-ZO_INIT_SIZE, output+init_size) is the copied ZO.
..., i.e., ZO_INIT_SIZE.
> + * [input, input+input_len) is the copied compressed (VO (vmlinux after
> + * objcopy) plus relocs), not the ZO.
> + *
> + * [input+input_len, output+init_size) is [_text, _end) for ZO. That was the
> + * first range in mem_avoid, which included ZO's heap and stack. Also
> + * [input, input+input_size) need be put in mem_avoid array, but since it
" ... needs to be in the mem_avoid ..."
> + * is adjacent to the first entry, they can be merged. This is how we get
> + * the first entry in mem_avoid[].
> + */
<--- my head smokes right about here. :-)
> static void mem_avoid_init(unsigned long input, unsigned long input_size,
> - unsigned long output, unsigned long output_size)
> + unsigned long output)
> {
> + unsigned long init_size = boot_params->hdr.init_size;
> u64 initrd_start, initrd_size;
> u64 cmd_line, cmd_line_size;
> - unsigned long unsafe, unsafe_len;
> char *ptr;
>
> /*
> * Avoid the region that is unsafe to overlap during
> - * decompression (see calculations in ../header.S).
> + * decompression.
> */
> - unsafe_len = (output_size >> 12) + 32768 + 18;
> - unsafe = (unsigned long)input + input_size - unsafe_len;
> - mem_avoid[0].start = unsafe;
> - mem_avoid[0].size = unsafe_len;
> + mem_avoid[0].start = input;
> + mem_avoid[0].size = (output + init_size) - input;
There's a define above called MEM_AVOID_MAX but the array elements are
adressed with naked numbers here. Perhaps if we had defines for them
too, it'll make the code a bit more understandable:
mem_avoid[MEM_AVOID_ZO_RANGE] ...
mem_avoid[MEM_AVOID_INITRD] ...
and so on.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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