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Date: Fri, 30 Jun 2017 11:27:03 -0700
From: Kees Cook <keescook@...omium.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Russell King <linux@...linux.org.uk>, Will Deacon <will.deacon@....com>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] ARM: fix randomized task_struct
On Fri, Jun 30, 2017 at 9:03 AM, Arnd Bergmann <arnd@...db.de> 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@...aro.org>
> Fixes: c33d8b12fbbd ("task_struct: Allow randomized layout")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
Looks great, thanks!
Reviewed-by: Kees Cook <keescook@...omium.org>
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
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