Re: [PATCH v2] ARM: fix randomized task_struct

[Kees Cook]

On Fri, Jun 30, 2017 at 9:03 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> With the new task struct randomization, we can run into a build
> failure for certain random seeds:
>
> arch/arm/kernel/entry-armv.S: Assembler messages:
> arch/arm/kernel/entry-armv.S:803: Error: bad immediate value for offset (4096)
>
> Only two constants in asm-offset.h are affected, and I'm changing
> both of them here to work correctly in all configurations.
>
> One more macro has the problem, but is currently unused, so this
> removes it instead of adding complexity.
>
> Suggested-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Fixes: c33d8b12fbbd ("task_struct: Allow randomized layout")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

Looks great, thanks!

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

Russell, if you're happy with it can you Ack this? I'd like to carry
the change in the randstruct tree (so all the dependencies are in the
same tree).

-Kees

> ---
> v2: define a more appropriate macro instead of PAGE_MASK,
>     fix always-true comparison to test the right thing
> ---
>  arch/arm/include/asm/assembler.h |  2 ++
>  arch/arm/kernel/entry-armv.S     |  5 ++++-
>  arch/arm/mm/proc-macros.S        | 10 ++++------
>  3 files changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> index 68b06f9c65de..ad301f107dd2 100644
> --- a/arch/arm/include/asm/assembler.h
> +++ b/arch/arm/include/asm/assembler.h
> @@ -87,6 +87,8 @@
>  #define CALGN(code...)
>  #endif
>
> +#define IMM12_MASK 0xfff
> +
>  /*
>   * Enable and disable interrupts
>   */
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index 9f157e7c51e7..c731f0d2b2af 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -797,7 +797,10 @@ ENTRY(__switch_to)
>  #if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP)
>         ldr     r7, [r2, #TI_TASK]
>         ldr     r8, =__stack_chk_guard
> -       ldr     r7, [r7, #TSK_STACK_CANARY]
> +       .if (TSK_STACK_CANARY > IMM12_MASK)
> +       add     r7, r7, #TSK_STACK_CANARY & ~IMM12_MASK
> +       .endif
> +       ldr     r7, [r7, #TSK_STACK_CANARY & IMM12_MASK]
>  #endif
>  #ifdef CONFIG_CPU_USE_DOMAINS
>         mcr     p15, 0, r6, c3, c0, 0           @ Set domain register
> diff --git a/arch/arm/mm/proc-macros.S b/arch/arm/mm/proc-macros.S
> index 0d40c285bd86..f944836da8a2 100644
> --- a/arch/arm/mm/proc-macros.S
> +++ b/arch/arm/mm/proc-macros.S
> @@ -25,11 +25,6 @@
>         ldr     \rd, [\rn, #VMA_VM_FLAGS]
>         .endm
>
> -       .macro  tsk_mm, rd, rn
> -       ldr     \rd, [\rn, #TI_TASK]
> -       ldr     \rd, [\rd, #TSK_ACTIVE_MM]
> -       .endm
> -
>  /*
>   * act_mm - get current->active_mm
>   */
> @@ -37,7 +32,10 @@
>         bic     \rd, sp, #8128
>         bic     \rd, \rd, #63
>         ldr     \rd, [\rd, #TI_TASK]
> -       ldr     \rd, [\rd, #TSK_ACTIVE_MM]
> +       .if (TSK_ACTIVE_MM > IMM12_MASK)
> +       add     \rd, \rd, #TSK_ACTIVE_MM & ~IMM12_MASK
> +       .endif
> +       ldr     \rd, [\rd, #TSK_ACTIVE_MM & IMM12_MASK]
>         .endm
>
>  /*
> --
> 2.9.0
>



-- 
Kees Cook
Pixel Security