diff options
Diffstat (limited to 'arch/x86/boot/compressed/kaslr.c')
| -rw-r--r-- | arch/x86/boot/compressed/kaslr.c | 77 |
1 files 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); |