Re: [PATCH v3 2/7] x86, boot: Move ZO to end of buffer

Kees Cook
2015-03-10T16:59:42Z

On Tue, Mar 10, 2015 at 1:00 AM, Borislav Petkov <bp@suse.de> wrote:
> Final patch:
>
> ---
> From: Yinghai Lu <yinghai@kernel.org>
> Date: Sat, 7 Mar 2015 14:07:16 -0800
> Subject: [PATCH] x86/setup: Move compressed kernel to the end of the buffer
>
> Boris found that passing KASLR status through setup_data from the boot
> stage cannot be used later in the kernel stage, see commit
>
> f47233c2d34f ("x86/mm/ASLR: Propagate base load address calculation")
>
> Here's some background:
>
> The boot loader allocates a buffer of size init_size in concordance with
> the value passed in the setup header and it loads the compressed, i.e.
> first kernel (arch/x86/boot/compressed/vmlinux) in it.
>
> First kernel then moves itself somewhere around the middle of the
> buffer at z_extract_offset to make sure that the decompressor does not
> overwrite input data.
>
> After the decompressor is finished, kernel proper (vmlinux) uses the
> whole buffer from the beginning and the compressed kernel's code and
> data section is overlapped with the kernel proper's bss section.
>
> Later on, clear_bss() in kernel proper clears .bss before code in
> arch/x86/kernel/setup.c can access setup_data passed in the first,
> compressed kernel.
>
> To make sure that data survives, we should avoid the overlapping.
>
> As a first step, move the first kernel closer to the end of the buffer
> instead of the middle. As a result, this will place first kernel's data
> area out of kernel proper's .bss area.
>
> This way we can find out where the data section of the copied first
> kernel is instead of guessing. In addition, it will make the KASLR
> mem_avoid array preparation for the search of a fitting buffer much
> simpler.
>
> While at it, rename z_extract_offset to z_min_extract_offset as it is
> actually the minimum extract offset now.
>
> In order to keep the final extract offset page-aligned we need to
> make both kernels' _end markers page-aligned too so that init_size is
> page-aligned as a result.
>
> Signed-off-by: Yinghai Lu <yinghai@kernel.org>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Matt Fleming <matt.fleming@intel.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Jiri Kosina <jkosina@suse.cz>
> Cc: linux-efi@vger.kernel.org
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Baoquan He <bhe@redhat.com>
> Fixes: f47233c2d34f ("x86/mm/ASLR: Propagate base load address calculation")
> Link: http://lkml.kernel.org/r/1425766041-6551-3-git-send-email-yinghai@kernel.org
> [ Commit message massively rewritten ]

Acked-by: Kees Cook <keescook@chromium.org>

Thanks!

-Kees

> Signed-off-by:
> ---
> arch/x86/boot/compressed/head_32.S | 11 +++++++++--
> arch/x86/boot/compressed/head_64.S | 8 ++++++--
> arch/x86/boot/compressed/mkpiggy.c | 7 ++-----
> arch/x86/boot/compressed/vmlinux.lds.S | 1 +
> arch/x86/boot/header.S | 2 +-
> arch/x86/kernel/asm-offsets.c | 1 +
> arch/x86/kernel/vmlinux.lds.S | 1 +
> 7 files changed, 21 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
> index cbed1407a5cd..a9b56f1d8e75 100644
> --- a/arch/x86/boot/compressed/head_32.S
> +++ b/arch/x86/boot/compressed/head_32.S
> @@ -147,7 +147,9 @@ preferred_addr:
> 1:
>
> /* Target address to relocate to for decompression */
> - addl $z_extract_offset, %ebx
> + movl BP_init_size(%esi), %eax
> + subl $_end, %eax
> + addl %eax, %ebx
>
> /* Set up the stack */
> leal boot_stack_end(%ebx), %esp
> @@ -208,8 +210,13 @@ relocated:
> */
> /* push arguments for decompress_kernel: */
> pushl $z_output_len /* decompressed length */
> - leal z_extract_offset_negative(%ebx), %ebp
> +
> + movl BP_init_size(%esi), %eax
> + subl $_end, %eax
> + movl %ebx, %ebp
> + subl %eax, %ebp
> pushl %ebp /* output address */
> +
> pushl $z_input_len /* input_len */
> leal input_data(%ebx), %eax
> pushl %eax /* input_data */
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 2884e0c3e8a5..69015b576cf6 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -101,7 +101,9 @@ ENTRY(startup_32)
> 1:
>
> /* Target address to relocate to for decompression */
> - addl $z_extract_offset, %ebx
> + movl BP_init_size(%esi), %eax
> + subl $_end, %eax
> + addl %eax, %ebx
>
> /*
> * Prepare for entering 64 bit mode
> @@ -329,7 +331,9 @@ preferred_addr:
> 1:
>
> /* Target address to relocate to for decompression */
> - leaq z_extract_offset(%rbp), %rbx
> + movl BP_init_size(%rsi), %ebx
> + subl $_end, %ebx
> + addq %rbp, %rbx
>
> /* Set up the stack */
> leaq boot_stack_end(%rbx), %rsp
> diff --git a/arch/x86/boot/compressed/mkpiggy.c b/arch/x86/boot/compressed/mkpiggy.c
> index b669ab65bf6c..c03b0097ce58 100644
> --- a/arch/x86/boot/compressed/mkpiggy.c
> +++ b/arch/x86/boot/compressed/mkpiggy.c
> @@ -80,11 +80,8 @@ int main(int argc, char *argv[])
> printf("z_input_len = %lu\n", ilen);
> printf(".globl z_output_len\n");
> printf("z_output_len = %lu\n", (unsigned long)olen);
> - printf(".globl z_extract_offset\n");
> - printf("z_extract_offset = 0x%lx\n", offs);
> - /* 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_min_extract_offset\n");
> + printf("z_min_extract_offset = 0x%lx\n", offs);
>
> printf(".globl input_data, input_data_end\n");
> printf("input_data:\n");
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 34d047c98284..a80acabb80ec 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -70,5 +70,6 @@ SECTIONS
> _epgtable = . ;
> }
> #endif
> + . = ALIGN(PAGE_SIZE); /* keep size page-aligned */
> _end = .;
> }
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 16ef02596db2..9bfab22efdf7 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -440,7 +440,7 @@ setup_data: .quad 0 # 64-bit physical pointer to
>
> pref_address: .quad LOAD_PHYSICAL_ADDR # preferred load addr
>
> -#define ZO_INIT_SIZE (ZO__end - ZO_startup_32 + ZO_z_extract_offset)
> +#define ZO_INIT_SIZE (ZO__end - ZO_startup_32 + ZO_z_min_extract_offset)
> #define VO_INIT_SIZE (VO__end - VO__text)
> #if ZO_INIT_SIZE > VO_INIT_SIZE
> #define INIT_SIZE ZO_INIT_SIZE
> diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
> index 9f6b9341950f..0e8e4f7a31ce 100644
> --- a/arch/x86/kernel/asm-offsets.c
> +++ b/arch/x86/kernel/asm-offsets.c
> @@ -66,6 +66,7 @@ void common(void) {
> OFFSET(BP_hardware_subarch, boot_params, hdr.hardware_subarch);
> OFFSET(BP_version, boot_params, hdr.version);
> OFFSET(BP_kernel_alignment, boot_params, hdr.kernel_alignment);
> + OFFSET(BP_init_size, boot_params, hdr.init_size);
> OFFSET(BP_pref_address, boot_params, hdr.pref_address);
> OFFSET(BP_code32_start, boot_params, hdr.code32_start);
>
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 00bf300fd846..a92d3dc2812a 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -325,6 +325,7 @@ SECTIONS
> __brk_limit = .;
> }
>
> + . = ALIGN(PAGE_SIZE); /* keep init size page-aligned */
> _end = .;
>
> STABS_DEBUG
> --
> 2.2.0.33.gc18b867
>
>
> --
> Regards/Gruss,
> Boris.
>
> ECO tip #101: Trim your mails when you reply.
> --

--
Kees Cook
Chrome OS Security