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

From: Kees Cook
Date: Mon Mar 09 2015 - 20:54:11 EST

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On Sat, Mar 7, 2015 at 2:07 PM, Yinghai Lu <yinghai@xxxxxxxxxx> wrote:
> Boris found data from boot stage can not be used kernel stage.

"... be used during kernel stage."

Also, can you give a specific example of this problem? (Which data, used how?)

> Bootloader allocate buffer according to init_size in hdr, and load the
> ZO (arch/x86/boot/compressed/vmlinux) from start of that buffer.
> During running of ZO, ZO move itself to the middle of buffer at
> z_extract_offset to make sure that decompressor would not have output
> overwrite input data before input data get consumed.
> After decompressor is called, VO (vmlinux) use whole buffer from start,
> and ZO code and data section is overlapped with VO bss section.
> And later VO/clear_bss() clear them before code in arch/x86/kernel/setup.c
> access them.
>
> To make the data survive that later, we should avoid the overlapping.
> At first move ZO close the end of buffer instead of middle of the buffer,
> that will move out ZO data out of VO bss area.
>
> Also after that we can find out where is data section of copied ZO
> instead of guessing. That will aslr mem_avoid array filling for

"That will make aslr mem_avoid array ..."

> new buffer seaching much simple.
>
> And rename z_extract_offset to z_min_extract_offset, as it is
> actually the minimum offset for extracting now.
>
> To keep the final real extract_offset to be page aligned like
> min_extract_offset, we need make VO _end and ZO _end both
> page aligned to make sure init_size always page aligned.
>
> Next patch will add ZO data size to init_size, so it will make sure
> ZO data is even out of VO brk area.

This seems like a reasonable idea, but I think the changes should be
noted/updated in misc.c since a lot of effort was made to make the
in-memory foot print as small as possible. These changes do expand the
size of the loaded kernel, IIUC. If not in this patch, maybe in 5/7?

-Kees

>
> Fixes: f47233c2d34f ("x86/mm/ASLR: Propagate base load address calculation")
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Matt Fleming <matt.fleming@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
> ---
> 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 cbed140..a9b56f1 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 2884e0c..69015b5 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 b669ab6..c03b009 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 34d047c..e24e0a0 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 ZO size page aligned */
> _end = .;
> }
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 16ef025..9bfab22 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 9f6b934..0e8e4f7 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 00bf300..ac25c7f 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 VO init size page aligned */
> _end = .;
>
> STABS_DEBUG
> --
> 1.8.4.5
>

--
Kees Cook
Chrome OS Security
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Next message: Yinghai Lu: "Re: [PATCH v3 0/7] x86, boot: clean up kasl"
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