All of lore.kernel.org
 help / color / mirror / Atom feed
From: Mark Rutland <mark.rutland@arm.com>
To: Laura Abbott <labbott@redhat.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
Subject: Re: [PATCH 2/2] arm64: Clear the stack
Date: Thu, 3 May 2018 08:19:18 +0100	[thread overview]
Message-ID: <20180503071917.xm2xvgagvzkworay@salmiak> (raw)
In-Reply-To: <20180502203326.9491-3-labbott@redhat.com>

Hi Laura,

On Wed, May 02, 2018 at 01:33:26PM -0700, Laura Abbott wrote:
> 
> Implementation of stackleak based heavily on the x86 version
> 
> Signed-off-by: Laura Abbott <labbott@redhat.com>
> ---
> Now written in C instead of a bunch of assembly.

This looks neat!

I have a few minor comments below.

> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index bf825f38d206..0ceea613c65b 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -55,6 +55,9 @@ arm64-reloc-test-y := reloc_test_core.o reloc_test_syms.o
>  arm64-obj-$(CONFIG_CRASH_DUMP)		+= crash_dump.o
>  arm64-obj-$(CONFIG_ARM_SDE_INTERFACE)	+= sdei.o
>  
> +arm64-obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += erase.o
> +KASAN_SANITIZE_erase.o	:= n

I suspect we want to avoid the full set of instrumentation suspects here, e.g.
GKOV, KASAN, UBSAN, and KCOV.

> +
>  obj-y					+= $(arm64-obj-y) vdso/ probes/
>  obj-m					+= $(arm64-obj-m)
>  head-y					:= head.o
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index ec2ee720e33e..3144f1ebdc18 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

Nit: The rest of our asm macros are lower-case -- can we stick to that here?

>  /*
>   * Exception vectors.
>   */
> @@ -906,6 +911,7 @@ ret_to_user:
>  	cbnz	x2, work_pending
>  finish_ret_to_user:
>  	enable_step_tsk x1, x2
> +	ERASE_KSTACK
>  	kernel_exit 0
>  ENDPROC(ret_to_user)

I believe we also need this in ret_fast_syscall.

[...]

> +asmlinkage void erase_kstack(void)
> +{
> +	unsigned long p = current->thread.lowest_stack;
> +	unsigned long boundary = p & ~(THREAD_SIZE - 1);
> +	unsigned long poison = 0;
> +	const unsigned long check_depth = STACKLEAK_POISON_CHECK_DEPTH /
> +							sizeof(unsigned long);
> +
> +	/*
> +	 * Let's search for the poison value in the stack.
> +	 * Start from the lowest_stack and go to the bottom.
> +	 */
> +	while (p > boundary && poison <= check_depth) {
> +		if (*(unsigned long *)p == STACKLEAK_POISON)
> +			poison++;
> +		else
> +			poison = 0;
> +
> +		p -= sizeof(unsigned long);
> +	}
> +
> +	/*
> +	 * One long int at the bottom of the thread stack is reserved and
> +	 * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
> +	 */
> +	if (p == boundary)
> +		p += sizeof(unsigned long);

I wonder if end_of_stack() should be taught about CONFIG_SCHED_STACK_END_CHECK,
given that's supposed to return the last *usable* long on the stack, and we
don't account for this elsewhere.

If we did, then IIUC we could do:

	unsigned long boundary = (unsigned long)end_of_stack(current);

... at the start of the function, and not have to worry about this explicitly.

> +
> +#ifdef CONFIG_STACKLEAK_METRICS
> +	current->thread.prev_lowest_stack = p;
> +#endif
> +
> +	/*
> +	 * So let's write the poison value to the kernel stack.
> +	 * Start from the address in p and move up till the new boundary.
> +	 */
> +	boundary = current_stack_pointer;

I worry a little that the compiler can move the SP during a function's
lifetime, but maybe that's only the case when there are VLAs, or something like
that?

> +
> +	BUG_ON(boundary - p >= THREAD_SIZE);
> +
> +	while (p < boundary) {
> +		*(unsigned long *)p = STACKLEAK_POISON;
> +		p += sizeof(unsigned long);
> +	}
> +
> +	/* Reset the lowest_stack value for the next syscall */
> +	current->thread.lowest_stack = current_stack_pointer;
> +}

Once this function returns, its data is left on the stack. Is that not a problem?

No strong feelings either way, but it might be worth mentioning in the commit
message.

> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index f08a2ed9db0d..156fa0a0da19 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -364,6 +364,9 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
>  	p->thread.cpu_context.pc = (unsigned long)ret_from_fork;
>  	p->thread.cpu_context.sp = (unsigned long)childregs;
>  
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +	p->thread.lowest_stack = (unsigned long)task_stack_page(p);

Nit: end_of_stack(p) would be slightly better semantically, even though
currently equivalent to task_stack_page(p).

[...]

> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +void __used check_alloca(unsigned long size)
> +{
> +	unsigned long sp, stack_left;
> +
> +	sp = current_stack_pointer;
> +
> +	stack_left = sp & (THREAD_SIZE - 1);
> +	BUG_ON(stack_left < 256 || size >= stack_left - 256);
> +}

Is this arbitrary, or is there something special about 256?

Even if this is arbitrary, can we give it some mnemonic?

> +EXPORT_SYMBOL(check_alloca);
> +#endif
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index a34e9290a699..25dd2a14560d 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)
>  
>  GCOV_PROFILE			:= n
>  KASAN_SANITIZE			:= n

I believe we'll also need to do this for the KVM hyp code in arch/arm64/kvm/hyp/.

Thanks,
Mark.

WARNING: multiple messages have this Message-ID (diff)
From: mark.rutland@arm.com (Mark Rutland)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH 2/2] arm64: Clear the stack
Date: Thu, 3 May 2018 08:19:18 +0100	[thread overview]
Message-ID: <20180503071917.xm2xvgagvzkworay@salmiak> (raw)
In-Reply-To: <20180502203326.9491-3-labbott@redhat.com>

Hi Laura,

On Wed, May 02, 2018 at 01:33:26PM -0700, Laura Abbott wrote:
> 
> Implementation of stackleak based heavily on the x86 version
> 
> Signed-off-by: Laura Abbott <labbott@redhat.com>
> ---
> Now written in C instead of a bunch of assembly.

This looks neat!

I have a few minor comments below.

> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index bf825f38d206..0ceea613c65b 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -55,6 +55,9 @@ arm64-reloc-test-y := reloc_test_core.o reloc_test_syms.o
>  arm64-obj-$(CONFIG_CRASH_DUMP)		+= crash_dump.o
>  arm64-obj-$(CONFIG_ARM_SDE_INTERFACE)	+= sdei.o
>  
> +arm64-obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += erase.o
> +KASAN_SANITIZE_erase.o	:= n

I suspect we want to avoid the full set of instrumentation suspects here, e.g.
GKOV, KASAN, UBSAN, and KCOV.

> +
>  obj-y					+= $(arm64-obj-y) vdso/ probes/
>  obj-m					+= $(arm64-obj-m)
>  head-y					:= head.o
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index ec2ee720e33e..3144f1ebdc18 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

Nit: The rest of our asm macros are lower-case -- can we stick to that here?

>  /*
>   * Exception vectors.
>   */
> @@ -906,6 +911,7 @@ ret_to_user:
>  	cbnz	x2, work_pending
>  finish_ret_to_user:
>  	enable_step_tsk x1, x2
> +	ERASE_KSTACK
>  	kernel_exit 0
>  ENDPROC(ret_to_user)

I believe we also need this in ret_fast_syscall.

[...]

> +asmlinkage void erase_kstack(void)
> +{
> +	unsigned long p = current->thread.lowest_stack;
> +	unsigned long boundary = p & ~(THREAD_SIZE - 1);
> +	unsigned long poison = 0;
> +	const unsigned long check_depth = STACKLEAK_POISON_CHECK_DEPTH /
> +							sizeof(unsigned long);
> +
> +	/*
> +	 * Let's search for the poison value in the stack.
> +	 * Start from the lowest_stack and go to the bottom.
> +	 */
> +	while (p > boundary && poison <= check_depth) {
> +		if (*(unsigned long *)p == STACKLEAK_POISON)
> +			poison++;
> +		else
> +			poison = 0;
> +
> +		p -= sizeof(unsigned long);
> +	}
> +
> +	/*
> +	 * One long int at the bottom of the thread stack is reserved and
> +	 * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
> +	 */
> +	if (p == boundary)
> +		p += sizeof(unsigned long);

I wonder if end_of_stack() should be taught about CONFIG_SCHED_STACK_END_CHECK,
given that's supposed to return the last *usable* long on the stack, and we
don't account for this elsewhere.

If we did, then IIUC we could do:

	unsigned long boundary = (unsigned long)end_of_stack(current);

... at the start of the function, and not have to worry about this explicitly.

> +
> +#ifdef CONFIG_STACKLEAK_METRICS
> +	current->thread.prev_lowest_stack = p;
> +#endif
> +
> +	/*
> +	 * So let's write the poison value to the kernel stack.
> +	 * Start from the address in p and move up till the new boundary.
> +	 */
> +	boundary = current_stack_pointer;

I worry a little that the compiler can move the SP during a function's
lifetime, but maybe that's only the case when there are VLAs, or something like
that?

> +
> +	BUG_ON(boundary - p >= THREAD_SIZE);
> +
> +	while (p < boundary) {
> +		*(unsigned long *)p = STACKLEAK_POISON;
> +		p += sizeof(unsigned long);
> +	}
> +
> +	/* Reset the lowest_stack value for the next syscall */
> +	current->thread.lowest_stack = current_stack_pointer;
> +}

Once this function returns, its data is left on the stack. Is that not a problem?

No strong feelings either way, but it might be worth mentioning in the commit
message.

> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index f08a2ed9db0d..156fa0a0da19 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -364,6 +364,9 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
>  	p->thread.cpu_context.pc = (unsigned long)ret_from_fork;
>  	p->thread.cpu_context.sp = (unsigned long)childregs;
>  
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +	p->thread.lowest_stack = (unsigned long)task_stack_page(p);

Nit: end_of_stack(p) would be slightly better semantically, even though
currently equivalent to task_stack_page(p).

[...]

> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +void __used check_alloca(unsigned long size)
> +{
> +	unsigned long sp, stack_left;
> +
> +	sp = current_stack_pointer;
> +
> +	stack_left = sp & (THREAD_SIZE - 1);
> +	BUG_ON(stack_left < 256 || size >= stack_left - 256);
> +}

Is this arbitrary, or is there something special about 256?

Even if this is arbitrary, can we give it some mnemonic?

> +EXPORT_SYMBOL(check_alloca);
> +#endif
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index a34e9290a699..25dd2a14560d 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)
>  
>  GCOV_PROFILE			:= n
>  KASAN_SANITIZE			:= n

I believe we'll also need to do this for the KVM hyp code in arch/arm64/kvm/hyp/.

Thanks,
Mark.

  parent reply	other threads:[~2018-05-03  7:19 UTC|newest]

Thread overview: 86+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-06 14:22 [PATCH v11 0/6] Introduce the STACKLEAK feature and a test for it Alexander Popov
2018-04-06 14:22 ` [PATCH v11 1/6] gcc-plugins: Clean up the cgraph_create_edge* macros Alexander Popov
2018-04-06 14:22 ` [PATCH v11 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls Alexander Popov
2018-04-16 18:29   ` Kees Cook
2018-04-18 18:33     ` Laura Abbott
2018-04-18 18:50     ` Dave Hansen
2018-04-24  1:03       ` Kees Cook
2018-04-24  4:23   ` Dave Hansen
2018-04-30 23:48     ` Kees Cook
2018-05-02  8:42       ` Thomas Gleixner
2018-05-02 12:38         ` Kees Cook
2018-05-02 12:39           ` Thomas Gleixner
2018-05-02 12:51             ` Kees Cook
2018-05-02 21:02               ` Kees Cook
2018-05-06 10:04                 ` Thomas Gleixner
2018-04-06 14:22 ` [PATCH v11 3/6] gcc-plugins: Add STACKLEAK plugin for tracking the kernel stack Alexander Popov
2018-04-06 14:22 ` [PATCH v11 4/6] lkdtm: Add a test for STACKLEAK Alexander Popov
2018-04-06 14:22 ` [PATCH v11 5/6] fs/proc: Show STACKLEAK metrics in the /proc file system Alexander Popov
2018-04-06 14:22 ` [PATCH v11 6/6] doc: self-protection: Add information about STACKLEAK feature Alexander Popov
2018-05-02 20:33 ` [PATCH 0/2] Stackleak for arm64 Laura Abbott
2018-05-02 20:33   ` Laura Abbott
2018-05-02 20:33   ` [PATCH 1/2] stackleak: Update " Laura Abbott
2018-05-02 20:33     ` Laura Abbott
2018-05-02 20:33   ` [PATCH 2/2] arm64: Clear the stack Laura Abbott
2018-05-02 20:33     ` Laura Abbott
2018-05-02 21:31     ` Kees Cook
2018-05-02 21:31       ` Kees Cook
2018-05-02 23:07       ` Laura Abbott
2018-05-02 23:07         ` Laura Abbott
2018-05-02 23:37         ` Kees Cook
2018-05-02 23:37           ` Kees Cook
2018-05-03 16:05         ` Alexander Popov
2018-05-03 16:05           ` Alexander Popov
2018-05-03 16:45           ` Kees Cook
2018-05-03 16:45             ` Kees Cook
2018-05-03  7:19     ` Mark Rutland [this message]
2018-05-03  7:19       ` Mark Rutland
2018-05-03 11:37       ` Ard Biesheuvel
2018-05-03 11:37         ` Ard Biesheuvel
2018-05-03 17:33       ` Alexander Popov
2018-05-03 17:33         ` Alexander Popov
2018-05-03 19:09         ` Laura Abbott
2018-05-03 19:09           ` Laura Abbott
2018-05-04  8:30           ` Alexander Popov
2018-05-04  8:30             ` Alexander Popov
2018-05-04 11:09         ` Mark Rutland
2018-05-04 11:09           ` Mark Rutland
2018-05-06  8:22           ` Alexander Popov
2018-05-06  8:22             ` Alexander Popov
2018-05-11 15:50             ` Alexander Popov
2018-05-11 15:50               ` Alexander Popov
2018-05-11 16:13               ` Mark Rutland
2018-05-11 16:13                 ` Mark Rutland
2018-05-13  8:40                 ` Alexander Popov
2018-05-13  8:40                   ` Alexander Popov
2018-05-14  5:15                   ` Mark Rutland
2018-05-14  5:15                     ` Mark Rutland
2018-05-14  9:35                     ` Alexander Popov
2018-05-14  9:35                       ` Alexander Popov
2018-05-14 10:06                       ` Mark Rutland
2018-05-14 10:06                         ` Mark Rutland
2018-05-14 13:53                         ` Alexander Popov
2018-05-14 13:53                           ` Alexander Popov
2018-05-14 14:07                           ` Mark Rutland
2018-05-14 14:07                             ` Mark Rutland
2018-05-03 19:00       ` Laura Abbott
2018-05-03 19:00         ` Laura Abbott
2018-05-04 11:16         ` Mark Rutland
2018-05-04 11:16           ` Mark Rutland
2018-05-14 18:55 ` [PATCH v11 0/6] Introduce the STACKLEAK feature and a test for it Laura Abbott
  -- strict thread matches above, loose matches on Subject: below --
2018-07-18 21:10 [PATCH 0/2] Stackleak for arm64 Laura Abbott
2018-07-18 21:10 ` [PATCH 2/2] arm64: Clear the stack Laura Abbott
2018-07-18 21:10   ` Laura Abbott
2018-07-19  2:20   ` Kees Cook
2018-07-19  2:20     ` Kees Cook
2018-07-19 10:41   ` Alexander Popov
2018-07-19 10:41     ` Alexander Popov
2018-07-19 11:41   ` Mark Rutland
2018-07-19 11:41     ` Mark Rutland
2018-02-21  1:13 [PATCH 0/2] Stackleak for arm64 Laura Abbott
2018-02-21  1:13 ` [PATCH 2/2] arm64: Clear the stack Laura Abbott
2018-02-21  1:13   ` Laura Abbott
2018-02-21 15:38   ` Mark Rutland
2018-02-21 15:38     ` Mark Rutland
2018-02-21 23:53     ` Laura Abbott
2018-02-21 23:53       ` Laura Abbott
2018-02-22  1:35       ` Laura Abbott
2018-02-22  1:35         ` Laura Abbott

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20180503071917.xm2xvgagvzkworay@salmiak \
    --to=mark.rutland@arm.com \
    --cc=alex.popov@linux.com \
    --cc=ard.biesheuvel@linaro.org \
    --cc=keescook@chromium.org \
    --cc=kernel-hardening@lists.openwall.com \
    --cc=labbott@redhat.com \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-kernel@vger.kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.