[PATCH v6 02/11] x86/KASLR: Consolidate mem_avoid entries

[Kees Cook]

From: Yinghai Lu <yinghai@xxxxxxxxxx>

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
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.

Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
[kees: rewrote changelog, cleaned up code comment]
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 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 2072d82c1911..6392f0041b8a 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
+ * 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
+ * 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:
+ *  - 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
+ *  - input + input_len > output + output_len
+ *  - kernel_total_size >= output_len
+ *
+ * 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.
+ * [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
+ * is adjacent to the first entry, they can be merged. This is how we get
+ * the first entry in mem_avoid[].
+ */
 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;
 
 	/* Avoid initrd. */
 	initrd_start  = (u64)boot_params->ext_ramdisk_image << 32;
@@ -181,13 +230,9 @@ static void mem_avoid_init(unsigned long input, unsigned long input_size,
 	mem_avoid[2].start = cmd_line;
 	mem_avoid[2].size = cmd_line_size;
 
-	/* Avoid heap memory. */
-	mem_avoid[3].start = (unsigned long)free_mem_ptr;
-	mem_avoid[3].size = BOOT_HEAP_SIZE;
-
-	/* Avoid stack memory. */
-	mem_avoid[4].start = (unsigned long)free_mem_end_ptr;
-	mem_avoid[4].size = BOOT_STACK_SIZE;
+	/* Avoid params */
+	mem_avoid[3].start = (unsigned long)boot_params;
+	mem_avoid[3].size = sizeof(*boot_params);
 }
 
 /* Does this memory vector overlap a known avoided area? */
@@ -337,7 +382,7 @@ unsigned char *choose_random_location(unsigned char *input_ptr,
 	boot_params->hdr.loadflags |= KASLR_FLAG;
 
 	/* Record the various known unsafe memory ranges. */
-	mem_avoid_init(input, input_size, output, output_size);
+	mem_avoid_init(input, input_size, output);
 
 	/* Walk e820 and find a random address. */
 	random_addr = find_random_addr(output, output_size);
-- 
2.6.3