Re: [PATCH v3] x86, kaslr: Prevent .bss from overlaping initrd

From: Kees Cook
Date: Fri Oct 31 2014 - 12:11:58 EST

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In reply to: Junjie Mao: "[PATCH v3] x86, kaslr: Prevent .bss from overlaping initrd"
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On Fri, Oct 31, 2014 at 6:40 AM, Junjie Mao <eternal.n08@xxxxxxxxx> wrote:
> When choosing a random address, the current implementation does not take into
> account the reversed space for .bss and .brk sections. Thus the relocated kernel
> may overlap other components in memory. Here is an example of the overlap from a
> x86_64 kernel in qemu (the ranges of physical addresses are presented):
>
> Physical Address
>
> 0x0fe00000 --+--------------------+ <-- randomized base
> / | relocated kernel |
> vmlinux.bin | (from vmlinux.bin) |
> 0x1336d000 (an ELF file) +--------------------+--
> \ | | \
> 0x1376d870 --+--------------------+ |
> | relocs table | |
> 0x13c1c2a8 +--------------------+ .bss and .brk
> | | |
> 0x13ce6000 +--------------------+ |
> | | /
> 0x13f77000 | initrd |--
> | |
> 0x13fef374 +--------------------+
>
> The initrd image will then be overwritten by the memset during early
> initialization:
>
> [ 1.655204] Unpacking initramfs...
> [ 1.662831] Initramfs unpacking failed: junk in compressed archive
>
> This patch prevents the above situation by requiring a larger space when looking
> for a random kernel base, so that existing logic can effectively avoids the
> overlap.
>
> Fixes: 82fa9637a2 ("x86, kaslr: Select random position from e820 maps")
> Reported-by: Fengguang Wu <fengguang.wu@xxxxxxxxx>
> Signed-off-by: Junjie Mao <eternal.n08@xxxxxxxxx>
> [kees: switched to perl to avoid hex translation pain in mawk vs gawk]
> [kees: calculated overlap without relocs table]
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> This version updates the commit log only.
>
> Kees, please help review the documentation. Thanks!

Looks great, thanks! Can someone from x86 pull this please?

-Kees

>
> Best Regards
> Junjie Mao
> ---
> arch/x86/boot/compressed/Makefile | 4 +++-
> arch/x86/boot/compressed/head_32.S | 5 +++--
> arch/x86/boot/compressed/head_64.S | 5 ++++-
> arch/x86/boot/compressed/misc.c | 13 ++++++++++---
> arch/x86/boot/compressed/mkpiggy.c | 9 +++++++--
> arch/x86/tools/calc_run_size.pl | 30 ++++++++++++++++++++++++++++++
> 6 files changed, 57 insertions(+), 9 deletions(-)
> create mode 100644 arch/x86/tools/calc_run_size.pl
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 0fcd9133790c..14fe7cba21d1 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -75,8 +75,10 @@ suffix-$(CONFIG_KERNEL_XZ) := xz
> suffix-$(CONFIG_KERNEL_LZO) := lzo
> suffix-$(CONFIG_KERNEL_LZ4) := lz4
>
> +RUN_SIZE = $(shell objdump -h vmlinux | \
> + perl $(srctree)/arch/x86/tools/calc_run_size.pl)
> quiet_cmd_mkpiggy = MKPIGGY $@
> - cmd_mkpiggy = $(obj)/mkpiggy $< > $@ || ( rm -f $@ ; false )
> + cmd_mkpiggy = $(obj)/mkpiggy $< $(RUN_SIZE) > $@ || ( rm -f $@ ; false )
>
> targets += piggy.S
> $(obj)/piggy.S: $(obj)/vmlinux.bin.$(suffix-y) $(obj)/mkpiggy FORCE
> diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
> index cbed1407a5cd..1d7fbbcc196d 100644
> --- a/arch/x86/boot/compressed/head_32.S
> +++ b/arch/x86/boot/compressed/head_32.S
> @@ -207,7 +207,8 @@ relocated:
> * Do the decompression, and jump to the new kernel..
> */
> /* push arguments for decompress_kernel: */
> - pushl $z_output_len /* decompressed length */
> + pushl $z_run_size /* size of kernel with .bss and .brk */
> + pushl $z_output_len /* decompressed length, end of relocs */
> leal z_extract_offset_negative(%ebx), %ebp
> pushl %ebp /* output address */
> pushl $z_input_len /* input_len */
> @@ -217,7 +218,7 @@ relocated:
> pushl %eax /* heap area */
> pushl %esi /* real mode pointer */
> call decompress_kernel /* returns kernel location in %eax */
> - addl $24, %esp
> + addl $28, %esp
>
> /*
> * Jump to the decompressed kernel.
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 2884e0c3e8a5..6b1766c6c082 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -402,13 +402,16 @@ relocated:
> * Do the decompression, and jump to the new kernel..
> */
> pushq %rsi /* Save the real mode argument */
> + movq $z_run_size, %r9 /* size of kernel with .bss and .brk */
> + pushq %r9
> movq %rsi, %rdi /* real mode address */
> leaq boot_heap(%rip), %rsi /* malloc area for uncompression */
> leaq input_data(%rip), %rdx /* input_data */
> movl $z_input_len, %ecx /* input_len */
> movq %rbp, %r8 /* output target address */
> - movq $z_output_len, %r9 /* decompressed length */
> + movq $z_output_len, %r9 /* decompressed length, end of relocs */
> call decompress_kernel /* returns kernel location in %rax */
> + popq %r9
> popq %rsi
>
> /*
> diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> index 57ab74df7eea..30dd59a9f0b4 100644
> --- a/arch/x86/boot/compressed/misc.c
> +++ b/arch/x86/boot/compressed/misc.c
> @@ -358,7 +358,8 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
> unsigned char *input_data,
> unsigned long input_len,
> unsigned char *output,
> - unsigned long output_len)
> + unsigned long output_len,
> + unsigned long run_size)
> {
> real_mode = rmode;
>
> @@ -381,8 +382,14 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
> free_mem_ptr = heap; /* Heap */
> free_mem_end_ptr = heap + BOOT_HEAP_SIZE;
>
> - output = choose_kernel_location(input_data, input_len,
> - output, output_len);
> + /*
> + * The memory hole needed for the kernel is the larger of either
> + * the entire decompressed kernel plus relocation table, or the
> + * entire decompressed kernel plus .bss and .brk sections.
> + */
> + output = choose_kernel_location(input_data, input_len, output,
> + output_len > run_size ? output_len
> + : run_size);
>
> /* Validate memory location choices. */
> if ((unsigned long)output & (MIN_KERNEL_ALIGN - 1))
> diff --git a/arch/x86/boot/compressed/mkpiggy.c b/arch/x86/boot/compressed/mkpiggy.c
> index b669ab65bf6c..d8222f213182 100644
> --- a/arch/x86/boot/compressed/mkpiggy.c
> +++ b/arch/x86/boot/compressed/mkpiggy.c
> @@ -36,11 +36,13 @@ int main(int argc, char *argv[])
> uint32_t olen;
> long ilen;
> unsigned long offs;
> + unsigned long run_size;
> FILE *f = NULL;
> int retval = 1;
>
> - if (argc < 2) {
> - fprintf(stderr, "Usage: %s compressed_file\n", argv[0]);
> + if (argc < 3) {
> + fprintf(stderr, "Usage: %s compressed_file run_size\n",
> + argv[0]);
> goto bail;
> }
>
> @@ -74,6 +76,7 @@ int main(int argc, char *argv[])
> offs += olen >> 12; /* Add 8 bytes for each 32K block */
> offs += 64*1024 + 128; /* Add 64K + 128 bytes slack */
> offs = (offs+4095) & ~4095; /* Round to a 4K boundary */
> + run_size = atoi(argv[2]);
>
> printf(".section \".rodata..compressed\",\"a\",@progbits\n");
> printf(".globl z_input_len\n");
> @@ -85,6 +88,8 @@ int main(int argc, char *argv[])
> /* z_extract_offset_negative allows simplification of head_32.S */
> printf(".globl z_extract_offset_negative\n");
> printf("z_extract_offset_negative = -0x%lx\n", offs);
> + printf(".globl z_run_size\n");
> + printf("z_run_size = %lu\n", run_size);
>
> printf(".globl input_data, input_data_end\n");
> printf("input_data:\n");
> diff --git a/arch/x86/tools/calc_run_size.pl b/arch/x86/tools/calc_run_size.pl
> new file mode 100644
> index 000000000000..0b0b124d3ece
> --- /dev/null
> +++ b/arch/x86/tools/calc_run_size.pl
> @@ -0,0 +1,30 @@
> +#!/usr/bin/perl
> +#
> +# Calculate the amount of space needed to run the kernel, including room for
> +# the .bss and .brk sections.
> +#
> +# Usage:
> +# objdump -h a.out | perl calc_run_size.pl
> +use strict;
> +
> +my $mem_size = 0;
> +my $file_offset = 0;
> +
> +my $sections=" *[0-9]+ \.(?:bss|brk) +";
> +while (<>) {
> + if (/^$sections([0-9a-f]+) +(?:[0-9a-f]+ +){2}([0-9a-f]+)/) {
> + my $size = hex($1);
> + my $offset = hex($2);
> + $mem_size += $size;
> + if ($file_offset == 0) {
> + $file_offset = $offset;
> + } elsif ($file_offset != $offset) {
> + die ".bss and .brk lack common file offset\n";
> + }
> + }
> +}
> +
> +if ($file_offset == 0) {
> + die "Never found .bss or .brk file offset\n";
> +}
> +printf("%d\n", $mem_size + $file_offset);
> --
> 1.9.3

--
Kees Cook
Chrome OS Security
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Next message: Yinghai Lu: "Re: Machine crashes right *after* ~successful resume"
Previous message: Matthias Brugger: "Re: [PATCH 3/3] I2C: mediatek: Add driver for MediaTek I2C controller"
In reply to: Junjie Mao: "[PATCH v3] x86, kaslr: Prevent .bss from overlaping initrd"
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