Re: [tip:x86/boot] x86/KASLR: Improve comments around the mem_avoid[] logic

[Baoquan He <bhe@redhat.com>]

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@chromium.org>
> AuthorDate: Fri, 6 May 2016 12:44:59 -0700
> Committer:  Ingo Molnar <mingo@kernel.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@alien8.de>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Yinghai Lu <yinghai@kernel.org>
> Link: http://lkml.kernel.org/r/20160506194459.GA16480@www.outflux.net
> Signed-off-by: Ingo Molnar <mingo@kernel.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? */