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Subject[tip:x86/boot] x86/KASLR: Improve comments around the mem_avoid[] logic
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
+ * 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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