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Subject: Re: [PATCH 2/2] arm64: Clear the stack
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Alexander Popov <alex.popov@linux.com>,
        Kees Cook <keescook@chromium.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20180221011303.20392-1-labbott@redhat.com>
 <20180221011303.20392-3-labbott@redhat.com>
 <20180221153850.ywpzsigfnz3etoun@salmiak>
From:   Laura Abbott <labbott@redhat.com>
Message-ID: <3b76d238-e10a-9abf-c9cb-6d3738eb7896@redhat.com>
Date:   Wed, 21 Feb 2018 15:53:35 -0800
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On 02/21/2018 07:38 AM, Mark Rutland wrote:
> Hi Laura,
> 
> On Tue, Feb 20, 2018 at 05:13:03PM -0800, Laura Abbott wrote:
>> Implementation of stackleak based heavily on the x86 version
> 
> Neat!
> 
>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
>> index ec2ee720e33e..b909b436293a 100644
>> --- a/arch/arm64/kernel/entry.S
>> +++ b/arch/arm64/kernel/entry.S
>> @@ -401,6 +401,11 @@ tsk	.req	x28		// current thread_info
>>   
>>   	.text
>>   
>> +	.macro	erase_kstack
>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>> +	bl	__erase_kstack
>> +#endif
>> +	.endm
>>   /*
>>    * Exception vectors.
>>    */
>> @@ -901,6 +906,7 @@ work_pending:
>>    */
>>   ret_to_user:
>>   	disable_daif
>> +	erase_kstack
> 
> I *think* this should happen in finish_ret_to_user a few lines down, since we
> can call C code if we branch to work_pending, dirtying the stack.
> 

I think you're right but this didn't immediately work when I tried it.
I'll have to dig into this some more.

>>   	ldr	x1, [tsk, #TSK_TI_FLAGS]
>>   	and	x2, x1, #_TIF_WORK_MASK
>>   	cbnz	x2, work_pending
>> @@ -1337,3 +1343,105 @@ alternative_else_nop_endif
>>   ENDPROC(__sdei_asm_handler)
>>   NOKPROBE(__sdei_asm_handler)
>>   #endif /* CONFIG_ARM_SDE_INTERFACE */
>> +
>> +/*
>> + * This is what the stack looks like
>> + *
>> + * +---+ <- task_stack_page(p) + THREAD_SIZE
>> + * |   |
>> + * +---+ <- task_stack_page(p) + THREAD_START_SP
>> + * |   |
>> + * |   |
>> + * +---+ <- task_pt_regs(p)
> 
> THREAD_START_SP got killed off in commit 34be98f4944f9907 as part of the
> VMAP_STACK rework, so this can be:
> 
>        +---+ <- task_stack_page(p) + THREAD_SIZE
>        |   |
>        |   |
>        +---+ <- task_pt_regs(p)
>         ...
> 

Good point.

>> + * |   |
>> + * |   |
>> + * |   | <- current_sp
>> + * ~~~~~
>> + *
>> + * ~~~~~
>> + * |   | <- lowest_stack
>> + * |   |
>> + * |   |
>> + * +---+ <- task_stack_page(p)
>> + *
>> + * This function is desgned to poison the memory between the lowest_stack
>> + * and the current stack pointer. After clearing the stack, the lowest
>> + * stack is reset.
>> + */
>> +
>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>> +ENTRY(__erase_kstack)
>> +	mov	x10, x0	// save x0 for the fast path
> 
> AFAICT, we only call this from ret_to_user, where x0 doesn't need to be
> preserved.
> 
> Is that for ret_fast_syscall? In some cases, ret_fast_syscall can bypass
> ret_to_user and calls kernel_exit directly, so we might need a call there.
> 

This was a hold over when I was experimenting with calling  erase_kstack
more places, one of which came through ret_fast_syscall. I realized
later that the erase was unnecessary but accidentally kept the saving
in. I'll see about removing it assuming we don't decide later to put
a call on the fast path.

>> +
>> +	get_thread_info	x0
>> +	ldr	x1, [x0, #TSK_TI_LOWEST_STACK]
>> +
>> +	/* get the number of bytes to check for lowest stack */
>> +	mov	x3, x1
>> +	and	x3, x3, #THREAD_SIZE - 1
>> +	lsr	x3, x3, #3
>> +
>> +	/* generate addresses from the bottom of the stack */
>> +	mov	x4, sp
>> +	movn	x2, #THREAD_SIZE - 1
>> +	and	x1, x4, x2
> 
> Can we replace the MOVN;AND with a single instruction to clear the low bits?
> e.g.
> 
> 	mov	x4, sp
> 	bic	x1, x4, #THREAD_SIZE - 1
> 
> ... IIUC BIC is an alias for the bitfield instructions, though I can't recall
> exactly which one(s).
> 

Good suggestion.

>> +
>> +	mov	x2, #STACKLEAK_POISON
>> +
>> +	mov	x5, #0
>> +1:
>> +	/*
>> +	 * As borrowed from the x86 logic, start from the lowest_stack
>> +	 * and go to the bottom to find the poison value.
>> +	 * The check of 16 is to hopefully avoid false positives.
>> +	 */
>> +	cbz	x3, 4f
>> +	ldr	x4, [x1, x3, lsl #3]
>> +	cmp	x4, x2
>> +	csinc	x5, xzr, x5, ne
>> +	tbnz	x5, #STACKLEAK_POISON_CHECK_DEPTH/4, 4f	// found 16 poisons?
>> +	sub	x3, x3, #1
>> +	b	1b
>> +
>> +4:
>> +	/* total number of bytes to poison */
>> +	add     x5, x1, x3, lsl #3
>> +	mov	x4, sp
>> +	sub     x8, x4, x5
>> +
>> +	cmp     x8, #THREAD_SIZE // sanity check the range
>> +	b.lo    5f
>> +	ASM_BUG()
>> +
>> +5:
>> +	/*
>> +	 * We may have hit a path where the stack did not get used,
>> +	 * no need to do anything here
>> +	 */
>> +	cbz	x8, 7f
>> +
>> +	sub	x8, x8, #1 // don't poison the current stack pointer
>> +
>> +	lsr     x8, x8, #3
>> +	add     x3, x3, x8
>> +
>> +	/*
>> +	 * The logic of this loop ensures the last stack word isn't
>> +	 * ovewritten.
>> +	 */
> 
> Is that to ensure that we don't clobber the word at the current sp value?
> 

Correct.

>> +6:
>> +	cbz     x8, 7f
>> +	str     x2, [x1, x3, lsl #3]
>> +	sub     x3, x3, #1
>> +	sub     x8, x8, #1
>> +	b	6b
>> +
>> +	/* Reset the lowest stack to the top of the stack */
>> +7:
>> +	mov	x1, sp
>> +	str	x1, [x0, #TSK_TI_LOWEST_STACK]
>> +
>> +	mov	x0, x10
>> +	ret
>> +ENDPROC(__erase_kstack)
>> +#endif
> 
> [...]
> 
>> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
>> index 7b3ba40f0745..35ebbc1b17ff 100644
>> --- a/drivers/firmware/efi/libstub/Makefile
>> +++ b/drivers/firmware/efi/libstub/Makefile
>> @@ -20,7 +20,8 @@ cflags-$(CONFIG_EFI_ARMSTUB)	+= -I$(srctree)/scripts/dtc/libfdt
>>   KBUILD_CFLAGS			:= $(cflags-y) -DDISABLE_BRANCH_PROFILING \
>>   				   -D__NO_FORTIFY \
>>   				   $(call cc-option,-ffreestanding) \
>> -				   $(call cc-option,-fno-stack-protector)
>> +				   $(call cc-option,-fno-stack-protector) \
>> +				   $(DISABLE_STACKLEAK_PLUGIN)
> 
> I believe the KVM hyp code will also need to opt-out of this.
> 

I'll double check that.

> Thanks,
> Mark.
> 

Thanks,
Laura