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

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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>
This commit is contained in:
Kees Cook 2016-05-06 12:44:59 -07:00 committed by Ingo Molnar
parent 549f90db68
commit ed09acde44
1 changed files with 78 additions and 48 deletions
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126
arch/x86/boot/compressed/kaslr.c
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@ -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.
*
* 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 initrd, cmdline, and boot_params are trivial to identify for
* avoiding. The are MEM_AVOID_INITRD, MEM_AVOID_CMDLINE, and
* MEM_AVOID_BOOTPARAMS respectively below.
*
* The compressed vmlinux (ZO) plus relocs and the run space of ZO can't be
* overwritten by decompression output.
* 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 sits against the end of the decompression buffer, so we can calculate
* where text, data, bss, etc of ZO are positioned.
* 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.
*
* 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
* 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.
*
* 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
* 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
*
* 0 output input input+input_len output+init_size
* | | | | |
* | | | | |
* |-----|--------|--------|------------------|----|------------|----------|
* | | |
* | | |
* output+init_size-ZO_INIT_SIZE output+output_len output+kernel_total_size
* (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.)
*
* [output, output+init_size) is for the buffer for decompressing the
* compressed kernel (ZO).
* The above conditions can be illustrated by a diagram:
*
* [output, output+kernel_total_size) is for the uncompressed kernel (VO)
* and its bss, brk, etc.
* [output, output+output_len) is VO plus relocs
* 0 output input input+input_size output+init_size
* | | | | |
* | | | | |
* |-----|--------|--------|--------------|-----------|--|-------------|
* | | |
* | | |
* output+init_size-ZO_INIT_SIZE output+output_size output+kernel_total_size
*
* [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.
* [output, output+init_size) is the entire memory range used for
* extracting the compressed image.
*
* [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[].
* [output, output+kernel_total_size) is the range needed for the
* uncompressed kernel (VO) and its run size (bss, brk, etc).
*
* [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+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.)
*
* [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.
*
* [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.
*
* 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? */