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Date: Mon, 9 May 2016 16:49:38 +0800
From: Baoquan He <bhe@...hat.com>
To: linux-kernel@...r.kernel.org, luto@...capital.net,
tglx@...utronix.de, bp@...e.de, torvalds@...ux-foundation.org,
keescook@...omium.org, luto@...nel.org, dvlasenk@...hat.com,
brgerst@...il.com, peterz@...radead.org, yinghai@...nel.org,
bp@...en8.de, mingo@...nel.org, hpa@...or.com
Cc: linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/boot] x86/KASLR: Improve comments around the
mem_avoid[] logic
On 05/06/16 at 11:36pm, tip-bot for Kees Cook wrote:
> Commit-ID: ed09acde44e301b5c13755ab84821fa44b188b5e
> Gitweb: http://git.kernel.org/tip/ed09acde44e301b5c13755ab84821fa44b188b5e
> Author: Kees Cook <keescook@...omium.org>
> AuthorDate: Fri, 6 May 2016 12:44:59 -0700
> Committer: Ingo Molnar <mingo@...nel.org>
> CommitDate: Sat, 7 May 2016 07:38:38 +0200
>
> x86/KASLR: Improve comments around the mem_avoid[] logic
>
> This attempts to improve the comments that describe how the memory
> range used for decompression is avoided. Additionally uses an enum
> instead of raw numbers for the mem_avoid[] indexing.
>
> Suggested-by: Borislav Petkov <bp@...en8.de>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Baoquan He <bhe@...hat.com>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Brian Gerst <brgerst@...il.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: Yinghai Lu <yinghai@...nel.org>
> Link: http://lkml.kernel.org/r/20160506194459.GA16480@www.outflux.net
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
> arch/x86/boot/compressed/kaslr.c | 126 ++++++++++++++++++++++++---------------
> 1 file changed, 78 insertions(+), 48 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index ff12277..8ef1186 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -121,7 +121,14 @@ struct mem_vector {
> unsigned long size;
> };
>
> -#define MEM_AVOID_MAX 4
> +enum mem_avoid_index {
> + MEM_AVOID_ZO_RANGE = 0,
> + MEM_AVOID_INITRD,
> + MEM_AVOID_CMDLINE,
> + MEM_AVOID_BOOTPARAMS,
> + MEM_AVOID_MAX,
> +};
> +
> static struct mem_vector mem_avoid[MEM_AVOID_MAX];
>
> static bool mem_contains(struct mem_vector *region, struct mem_vector *item)
> @@ -147,55 +154,78 @@ static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
> }
>
> /*
> - * In theroy, KASLR can put the kernel anywhere in area of [16M, 64T). The
> - * 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
> + * In theory, KASLR can put the kernel anywhere in the range of [16M, 64T).
> + * The mem_avoid array is used to store the ranges that need to be avoided
> + * when KASLR searches for an appropriate random address. We must avoid any
> * regions that are unsafe to overlap with during decompression, and other
> - * things like the initrd, cmdline and boot_params.
> + * things like the initrd, cmdline and boot_params. This comment seeks to
> + * explain mem_avoid as clearly as possible since incorrect mem_avoid
> + * memory ranges lead to really hard to debug boot failures.
> + *
> + * The initrd, cmdline, and boot_params are trivial to identify for
> + * avoiding. The are MEM_AVOID_INITRD, MEM_AVOID_CMDLINE, and
~~~a typo, should be "They"
> + * MEM_AVOID_BOOTPARAMS respectively below.
> + *
> + * What is not obvious how to avoid is the range of memory that is used
> + * during decompression (MEM_AVOID_ZO_RANGE below). This range must cover
> + * the compressed kernel (ZO) and its run space, which is used to extract
> + * the uncompressed kernel (VO) and relocs.
> + *
> + * ZO's full run size sits against the end of the decompression buffer, so
> + * we can calculate where text, data, bss, etc of ZO are positioned more
> + * easily.
> + *
> + * For additional background, the decompression calculations can be found
> + * in header.S, and the memory diagram is based on the one found in misc.c.
> + *
> + * The following conditions are already enforced by the image layouts and
> + * associated code:
> + * - input + input_size >= output + output_size
> + * - kernel_total_size <= init_size
> + * - kernel_total_size <= output_size (see Note below)
> + * - output + init_size >= output + output_size
> *
> - * 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.
> + * (Note that kernel_total_size and output_size have no fundamental
> + * relationship, but output_size is passed to choose_random_location
> + * as a maximum of the two. The diagram is showing a case where
> + * kernel_total_size is larger than output_size, but this case is
> + * handled by bumping output_size.)
> *
> - * The compressed vmlinux (ZO) plus relocs and the run space of ZO can't be
> - * overwritten by decompression output.
> + * The above conditions can be illustrated by a diagram:
> *
> - * ZO sits against the end of the decompression buffer, so we can calculate
> - * where text, data, bss, etc of ZO are positioned.
> + * 0 output input input+input_size output+init_size
> + * | | | | |
> + * | | | | |
> + * |-----|--------|--------|--------------|-----------|--|-------------|
> + * | | |
> + * | | |
> + * output+init_size-ZO_INIT_SIZE output+output_size output+kernel_total_size
> *
> - * The follow are already enforced by the code:
> - * - init_size >= kernel_total_size
> - * - input + input_len >= output + output_len
> - * - kernel_total_size could be >= or < output_len
> + * [output, output+init_size) is the entire memory range used for
> + * extracting the compressed image.
> *
> - * From this, we can make several observations, illustrated by a diagram:
> - * - init_size >= kernel_total_size
> - * - input + input_len > output + output_len
> - * - kernel_total_size >= output_len
> + * [output, output+kernel_total_size) is the range needed for the
> + * uncompressed kernel (VO) and its run size (bss, brk, etc).
> *
> - * 0 output input input+input_len output+init_size
> - * | | | | |
> - * | | | | |
> - * |-----|--------|--------|------------------|----|------------|----------|
> - * | | |
> - * | | |
> - * output+init_size-ZO_INIT_SIZE output+output_len output+kernel_total_size
> + * [output, output+output_size) is VO plus relocs (i.e. the entire
> + * uncompressed payload contained by ZO). This is the area of the buffer
> + * written to during decompression.
> *
> - * [output, output+init_size) is for the buffer for decompressing the
> - * compressed kernel (ZO).
> + * [output+init_size-ZO_INIT_SIZE, output+init_size) is the worst-case
> + * range of the copied ZO and decompression code. (i.e. the range
> + * covered backwards of size ZO_INIT_SIZE, starting from output+init_size.)
> *
> - * [output, output+kernel_total_size) is for the uncompressed kernel (VO)
> - * and its bss, brk, etc.
> - * [output, output+output_len) is VO plus relocs
> + * [input, input+input_size) is the original copied compressed image (ZO)
> + * (i.e. it does not include its run size). This range must be avoided
> + * because it contains the data used for decompression.
> *
> - * [output+init_size-ZO_INIT_SIZE, output+init_size) is the copied ZO.
> - * [input, input+input_len) is the copied compressed (VO (vmlinux after
> - * objcopy) plus relocs), not the ZO.
> + * [input+input_size, output+init_size) is [_text, _end) for ZO. This
> + * range includes ZO's heap and stack, and must be avoided since it
> + * performs the decompression.
> *
> - * [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
> - * is adjacent to the first entry, they can be merged. This is how we get
> - * the first entry in mem_avoid[].
> + * Since the above two ranges need to be avoided and they are adjacent,
> + * they can be merged, resulting in: [input, output+init_size) which
> + * becomes the MEM_AVOID_ZO_RANGE below.
> */
> static void mem_avoid_init(unsigned long input, unsigned long input_size,
> unsigned long output)
> @@ -209,16 +239,16 @@ static void mem_avoid_init(unsigned long input, unsigned long input_size,
> * Avoid the region that is unsafe to overlap during
> * decompression.
> */
> - mem_avoid[0].start = input;
> - mem_avoid[0].size = (output + init_size) - input;
> + mem_avoid[MEM_AVOID_ZO_RANGE].start = input;
> + mem_avoid[MEM_AVOID_ZO_RANGE].size = (output + init_size) - input;
>
> /* Avoid initrd. */
> initrd_start = (u64)boot_params->ext_ramdisk_image << 32;
> initrd_start |= boot_params->hdr.ramdisk_image;
> initrd_size = (u64)boot_params->ext_ramdisk_size << 32;
> initrd_size |= boot_params->hdr.ramdisk_size;
> - mem_avoid[1].start = initrd_start;
> - mem_avoid[1].size = initrd_size;
> + mem_avoid[MEM_AVOID_INITRD].start = initrd_start;
> + mem_avoid[MEM_AVOID_INITRD].size = initrd_size;
>
> /* Avoid kernel command line. */
> cmd_line = (u64)boot_params->ext_cmd_line_ptr << 32;
> @@ -227,12 +257,12 @@ static void mem_avoid_init(unsigned long input, unsigned long input_size,
> ptr = (char *)(unsigned long)cmd_line;
> for (cmd_line_size = 0; ptr[cmd_line_size++]; )
> ;
> - mem_avoid[2].start = cmd_line;
> - mem_avoid[2].size = cmd_line_size;
> + mem_avoid[MEM_AVOID_CMDLINE].start = cmd_line;
> + mem_avoid[MEM_AVOID_CMDLINE].size = cmd_line_size;
>
> - /* Avoid params */
> - mem_avoid[3].start = (unsigned long)boot_params;
> - mem_avoid[3].size = sizeof(*boot_params);
> + /* Avoid boot parameters. */
> + mem_avoid[MEM_AVOID_BOOTPARAMS].start = (unsigned long)boot_params;
> + mem_avoid[MEM_AVOID_BOOTPARAMS].size = sizeof(*boot_params);
> }
>
> /* Does this memory vector overlap a known avoided area? */
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