[RFC,v4,1/3] gcc-plugins: Add STACKLEAK erasing the kernel stack at the end of syscalls
diff mbox

Message ID 1507157703-14972-2-git-send-email-alex.popov@linux.com
State New
Headers show

Commit Message

Alexander Popov Oct. 4, 2017, 10:55 p.m. UTC
The STACKLEAK feature erases the kernel stack before returning from
syscalls. That reduces the information which a kernel stack leak bug can
reveal and blocks some uninitialized stack variable attacks. Moreover,
STACKLEAK provides runtime checks for kernel stack overflow detection.

This feature consists of:
 - the architecture-specific code filling the used part of the kernel
    stack with a poison value before returning to the userspace;
 - the STACKLEAK gcc plugin. It instruments the kernel code inserting
    the track_stack() call for tracking the lowest border of the kernel
    stack and check_alloca() call for checking alloca size.

The STACKLEAK feature is ported from grsecurity/PaX. More information at:
  https://grsecurity.net/
  https://pax.grsecurity.net/

This code is modified from Brad Spengler/PaX Team's code in the last
public patch of grsecurity/PaX based on our understanding of the code.
Changes or omissions from the original code are ours and don't reflect
the original grsecurity/PaX code.

Signed-off-by: Alexander Popov <alex.popov@linux.com>
---
 arch/Kconfig                           |  39 ++++
 arch/x86/Kconfig                       |   1 +
 arch/x86/entry/common.c                |  17 +-
 arch/x86/entry/entry_32.S              |  69 ++++++
 arch/x86/entry/entry_64.S              |  95 ++++++++
 arch/x86/entry/entry_64_compat.S       |   8 +
 arch/x86/include/asm/processor.h       |   4 +
 arch/x86/kernel/asm-offsets.c          |   9 +
 arch/x86/kernel/dumpstack_32.c         |  12 +
 arch/x86/kernel/dumpstack_64.c         |  15 ++
 arch/x86/kernel/process_32.c           |   5 +
 arch/x86/kernel/process_64.c           |   5 +
 fs/exec.c                              |  30 +++
 include/linux/compiler.h               |   4 +
 scripts/Makefile.gcc-plugins           |   3 +
 scripts/gcc-plugins/stackleak_plugin.c | 397 +++++++++++++++++++++++++++++++++
 16 files changed, 710 insertions(+), 3 deletions(-)
 create mode 100644 scripts/gcc-plugins/stackleak_plugin.c

Comments

Kees Cook Oct. 4, 2017, 11:31 p.m. UTC | #1
On Wed, Oct 4, 2017 at 3:55 PM, Alexander Popov <alex.popov@linux.com> wrote:
> The STACKLEAK feature erases the kernel stack before returning from
> syscalls. That reduces the information which a kernel stack leak bug can
> reveal and blocks some uninitialized stack variable attacks. Moreover,
> STACKLEAK provides runtime checks for kernel stack overflow detection.
>
> This feature consists of:
>  - the architecture-specific code filling the used part of the kernel
>     stack with a poison value before returning to the userspace;
>  - the STACKLEAK gcc plugin. It instruments the kernel code inserting
>     the track_stack() call for tracking the lowest border of the kernel
>     stack and check_alloca() call for checking alloca size.
>
> The STACKLEAK feature is ported from grsecurity/PaX. More information at:
>   https://grsecurity.net/
>   https://pax.grsecurity.net/
>
> This code is modified from Brad Spengler/PaX Team's code in the last
> public patch of grsecurity/PaX based on our understanding of the code.
> Changes or omissions from the original code are ours and don't reflect
> the original grsecurity/PaX code.
>
> Signed-off-by: Alexander Popov <alex.popov@linux.com>

Thanks for the continuing work on this!

If I can get some review from Andy or other x86 folks, I'd appreciate
it. If they're happy, I'll add this to the gcc-plugins tree...

-Kees
Ingo Molnar Oct. 5, 2017, 7:27 a.m. UTC | #2
* Alexander Popov <alex.popov@linux.com> wrote:

> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index 8a13d46..06bc57b 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -75,6 +75,71 @@
>  #endif
>  .endm
>  
> +.macro erase_kstack
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +	call erase_kstack
> +#endif
> +.endm
> +
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +/* For the detailed comments, see erase_kstack in entry_64.S */
> +ENTRY(erase_kstack)
> +	pushl	%edi
> +	pushl	%ecx
> +	pushl	%eax
> +	pushl	%ebp
> +
> +	movl	PER_CPU_VAR(current_task), %ebp
> +	mov	TASK_lowest_stack(%ebp), %edi
> +	mov	$STACKLEAK_POISON, %eax
> +	std
> +
> +1:
> +	mov	%edi, %ecx
> +	and	$THREAD_SIZE_asm - 1, %ecx
> +	shr	$2, %ecx
> +	repne	scasl
> +	jecxz	2f
> +
> +	cmp	$2*16, %ecx
> +	jc	2f
> +
> +	mov	$2*16, %ecx
> +	repe	scasl
> +	jecxz	2f
> +	jne	1b
> +
> +2:
> +	cld
> +	or	$2*4, %edi
> +	mov	%esp, %ecx
> +	sub	%edi, %ecx
> +
> +	cmp	$THREAD_SIZE_asm, %ecx
> +	jb	3f
> +	ud2
> +
> +3:
> +	shr	$2, %ecx
> +	rep	stosl
> +
> +	/*
> +	 * TODO: sp0 on x86_32 is not reliable, right?
> +	 * Doubt because of the definition of cpu_current_top_of_stack
> +	 * in arch/x86/kernel/cpu/common.c.
> +	 */
> +	mov	TASK_thread_sp0(%ebp), %edi
> +	sub	$128, %edi
> +	mov	%edi, TASK_lowest_stack(%ebp)
> +
> +	popl	%ebp
> +	popl	%eax
> +	popl	%ecx
> +	popl	%edi
> +	ret
> +ENDPROC(erase_kstack)
> +#endif
> +
>  /*
>   * User gs save/restore
>   *
> @@ -445,6 +510,8 @@ ENTRY(entry_SYSENTER_32)
>  	ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
>  		    "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
>  
> +	erase_kstack
> +
>  /* Opportunistic SYSEXIT */
>  	TRACE_IRQS_ON			/* User mode traces as IRQs on. */
>  	movl	PT_EIP(%esp), %edx	/* pt_regs->ip */
> @@ -531,6 +598,8 @@ ENTRY(entry_INT80_32)
>  	call	do_int80_syscall_32
>  .Lsyscall_32_done:
>  
> +	erase_kstack
> +
>  restore_all:
>  	TRACE_IRQS_IRET
>  .Lrestore_all_notrace:
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 4916725..189d843 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -59,6 +59,90 @@ END(native_usergs_sysret64)
>  #endif
>  .endm
>  
> +.macro erase_kstack
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +	call erase_kstack
> +#endif
> +.endm
> +
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +ENTRY(erase_kstack)
> +	pushq	%rdi
> +	pushq	%rcx
> +	pushq	%rax
> +	pushq	%r11
> +
> +	movq	PER_CPU_VAR(current_task), %r11
> +	mov	TASK_lowest_stack(%r11), %rdi
> +	mov	$STACKLEAK_POISON, %rax
> +	std
> +
> +	/*
> +	 * Let's search for the poison value in the stack.
> +	 * Start from the lowest_stack and go to the bottom (see std above).
> +	 */
> +1:
> +	mov	%edi, %ecx
> +	and	$THREAD_SIZE_asm - 1, %ecx
> +	shr	$3, %ecx
> +	repne	scasq
> +	jecxz	2f	/* Didn't find it. Go to poisoning. */
> +
> +	/*
> +	 * Found the poison value in the stack. Go to poisoning if there are
> +	 * less than 16 qwords left.
> +	 */
> +	cmp	$2*8, %ecx
> +	jc	2f
> +
> +	/*
> +	 * Check that 16 further qwords contain poison (avoid false positives).
> +	 * If so, the part of the stack below the address in %rdi is likely
> +	 * to be poisoned. Otherwise we need to search deeper.
> +	 */
> +	mov	$2*8, %ecx
> +	repe	scasq
> +	jecxz	2f	/* Poison the upper part of the stack. */
> +	jne	1b	/* Search deeper. */
> +
> +2:
> +	/*
> +	 * Prepare the counter for poisoning the kernel stack between
> +	 * %rdi and %rsp. Two qwords at the bottom of the stack are reserved
> +	 * and should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
> +	 */
> +	cld
> +	or	$2*8, %rdi
> +	mov	%esp, %ecx
> +	sub	%edi, %ecx
> +
> +	/* Check that the counter value is sane. */
> +	cmp	$THREAD_SIZE_asm, %rcx
> +	jb	3f
> +	ud2
> +
> +3:
> +	/*
> +	 * So let's write the poison value to the kernel stack. Start from the
> +	 * address in %rdi and move up (see cld above) to the address in %rsp
> +	 * (not included, used memory).
> +	 */
> +	shr	$3, %ecx
> +	rep	stosq
> +
> +	/* Set the lowest_stack value to the top_of_stack - 256. */
> +	mov	TASK_thread_sp0(%r11), %rdi
> +	sub	$256, %rdi
> +	mov	%rdi, TASK_lowest_stack(%r11)
> +
> +	popq	%r11
> +	popq	%rax
> +	popq	%rcx
> +	popq	%rdi
> +	ret
> +ENDPROC(erase_kstack)

A couple of (first round) review observations:

- Why is the erase_kstack() function written in assembly, instead of plain C?
  The complexity and fragility of this patch could be reduced if it was moved to C.

- The GCC plugin adds instrumentation in form of extra 'track_stack()' and
  'check_alloca()' calls. Could you please provide a frequency analysis of the
  impact of this: x86-64 defconfig vmlinux size before/after the patch, and the
  number of instrumentation function calls inserted, compared to the number of
  functions?

- Is there a debug facility to query the current (latest) lowest_stack value of
  any task in the system, via /proc? Observing this threshold over time would give 
  a good idea about the typical work the clearing function has to perform for
  every system call.

- Please clean up the GCC plugin code to follow proper kernel coding style.
  The '//' comment lines in particular are a big eyesore, plus there are a lot of 
  other stylistic variations as well that make the code unnecessarily difficult to 
  read.

- Also, this patch is way too big - there's no reason why the GCC plugin and
  the stack erasure features should be introduced in the same patch, etc.

Thanks,

	Ingo
Alexander Popov Oct. 5, 2017, 12:31 p.m. UTC | #3
On 05.10.2017 10:27, Ingo Molnar wrote:
> 
> * Alexander Popov <alex.popov@linux.com> wrote:
> 
>> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
>> index 8a13d46..06bc57b 100644
>> --- a/arch/x86/entry/entry_32.S
>> +++ b/arch/x86/entry/entry_32.S
>> @@ -75,6 +75,71 @@
>>  #endif
>>  .endm
>>  
>> +.macro erase_kstack
>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>> +	call erase_kstack
>> +#endif
>> +.endm
>> +
>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>> +/* For the detailed comments, see erase_kstack in entry_64.S */
>> +ENTRY(erase_kstack)
>> +	pushl	%edi
>> +	pushl	%ecx
>> +	pushl	%eax
>> +	pushl	%ebp
>> +
>> +	movl	PER_CPU_VAR(current_task), %ebp
>> +	mov	TASK_lowest_stack(%ebp), %edi
>> +	mov	$STACKLEAK_POISON, %eax
>> +	std
>> +
>> +1:
>> +	mov	%edi, %ecx
>> +	and	$THREAD_SIZE_asm - 1, %ecx
>> +	shr	$2, %ecx
>> +	repne	scasl
>> +	jecxz	2f
>> +
>> +	cmp	$2*16, %ecx
>> +	jc	2f
>> +
>> +	mov	$2*16, %ecx
>> +	repe	scasl
>> +	jecxz	2f
>> +	jne	1b
>> +
>> +2:
>> +	cld
>> +	or	$2*4, %edi
>> +	mov	%esp, %ecx
>> +	sub	%edi, %ecx
>> +
>> +	cmp	$THREAD_SIZE_asm, %ecx
>> +	jb	3f
>> +	ud2
>> +
>> +3:
>> +	shr	$2, %ecx
>> +	rep	stosl
>> +
>> +	/*
>> +	 * TODO: sp0 on x86_32 is not reliable, right?
>> +	 * Doubt because of the definition of cpu_current_top_of_stack
>> +	 * in arch/x86/kernel/cpu/common.c.
>> +	 */
>> +	mov	TASK_thread_sp0(%ebp), %edi
>> +	sub	$128, %edi
>> +	mov	%edi, TASK_lowest_stack(%ebp)
>> +
>> +	popl	%ebp
>> +	popl	%eax
>> +	popl	%ecx
>> +	popl	%edi
>> +	ret
>> +ENDPROC(erase_kstack)
>> +#endif
>> +
>>  /*
>>   * User gs save/restore
>>   *
>> @@ -445,6 +510,8 @@ ENTRY(entry_SYSENTER_32)
>>  	ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
>>  		    "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
>>  
>> +	erase_kstack
>> +
>>  /* Opportunistic SYSEXIT */
>>  	TRACE_IRQS_ON			/* User mode traces as IRQs on. */
>>  	movl	PT_EIP(%esp), %edx	/* pt_regs->ip */
>> @@ -531,6 +598,8 @@ ENTRY(entry_INT80_32)
>>  	call	do_int80_syscall_32
>>  .Lsyscall_32_done:
>>  
>> +	erase_kstack
>> +
>>  restore_all:
>>  	TRACE_IRQS_IRET
>>  .Lrestore_all_notrace:
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index 4916725..189d843 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -59,6 +59,90 @@ END(native_usergs_sysret64)
>>  #endif
>>  .endm
>>  
>> +.macro erase_kstack
>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>> +	call erase_kstack
>> +#endif
>> +.endm
>> +
>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>> +ENTRY(erase_kstack)
>> +	pushq	%rdi
>> +	pushq	%rcx
>> +	pushq	%rax
>> +	pushq	%r11
>> +
>> +	movq	PER_CPU_VAR(current_task), %r11
>> +	mov	TASK_lowest_stack(%r11), %rdi
>> +	mov	$STACKLEAK_POISON, %rax
>> +	std
>> +
>> +	/*
>> +	 * Let's search for the poison value in the stack.
>> +	 * Start from the lowest_stack and go to the bottom (see std above).
>> +	 */
>> +1:
>> +	mov	%edi, %ecx
>> +	and	$THREAD_SIZE_asm - 1, %ecx
>> +	shr	$3, %ecx
>> +	repne	scasq
>> +	jecxz	2f	/* Didn't find it. Go to poisoning. */
>> +
>> +	/*
>> +	 * Found the poison value in the stack. Go to poisoning if there are
>> +	 * less than 16 qwords left.
>> +	 */
>> +	cmp	$2*8, %ecx
>> +	jc	2f
>> +
>> +	/*
>> +	 * Check that 16 further qwords contain poison (avoid false positives).
>> +	 * If so, the part of the stack below the address in %rdi is likely
>> +	 * to be poisoned. Otherwise we need to search deeper.
>> +	 */
>> +	mov	$2*8, %ecx
>> +	repe	scasq
>> +	jecxz	2f	/* Poison the upper part of the stack. */
>> +	jne	1b	/* Search deeper. */
>> +
>> +2:
>> +	/*
>> +	 * Prepare the counter for poisoning the kernel stack between
>> +	 * %rdi and %rsp. Two qwords at the bottom of the stack are reserved
>> +	 * and should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
>> +	 */
>> +	cld
>> +	or	$2*8, %rdi
>> +	mov	%esp, %ecx
>> +	sub	%edi, %ecx
>> +
>> +	/* Check that the counter value is sane. */
>> +	cmp	$THREAD_SIZE_asm, %rcx
>> +	jb	3f
>> +	ud2
>> +
>> +3:
>> +	/*
>> +	 * So let's write the poison value to the kernel stack. Start from the
>> +	 * address in %rdi and move up (see cld above) to the address in %rsp
>> +	 * (not included, used memory).
>> +	 */
>> +	shr	$3, %ecx
>> +	rep	stosq
>> +
>> +	/* Set the lowest_stack value to the top_of_stack - 256. */
>> +	mov	TASK_thread_sp0(%r11), %rdi
>> +	sub	$256, %rdi
>> +	mov	%rdi, TASK_lowest_stack(%r11)
>> +
>> +	popq	%r11
>> +	popq	%rax
>> +	popq	%rcx
>> +	popq	%rdi
>> +	ret
>> +ENDPROC(erase_kstack)


Hello Ingo,

Thanks a lot for your review.

> A couple of (first round) review observations:
> 
> - Why is the erase_kstack() function written in assembly, instead of plain C?
>   The complexity and fragility of this patch could be reduced if it was moved to C.

Let me shortly describe my tactics.

Initially the erase_kstack() function is written in assembly in Grsecurity/PaX
patch. I've extracted the STACKLEAK feature from that huge patch and carefully
learned it bit by bit (it's quite complex). There are several bugs which I've
found and fixed in it (they are listed in the cover letter), but generally I
stick to the initial implementation in order not to accidentally break it.

I've added the detailed comments describing erase_kstack() for x86_64. IMO this
code is really cool (my respect to PaX Team). However, if you think that
rewriting it in C is obligatory, I'll do that. But let me at first fix the other
issues which you listed below.

> - The GCC plugin adds instrumentation in form of extra 'track_stack()' and
>   'check_alloca()' calls. Could you please provide a frequency analysis of the
>   impact of this: x86-64 defconfig vmlinux size before/after the patch, and the
>   number of instrumentation function calls inserted, compared to the number of
>   functions?

Ok, I'll provide that information.

> - Is there a debug facility to query the current (latest) lowest_stack value of
>   any task in the system, via /proc? Observing this threshold over time would give 
>   a good idea about the typical work the clearing function has to perform for
>   every system call.

Yes, I'll create a PoC for it and maybe return with some questions.

> - Please clean up the GCC plugin code to follow proper kernel coding style.
>   The '//' comment lines in particular are a big eyesore, plus there are a lot of 
>   other stylistic variations as well that make the code unnecessarily difficult to 
>   read.

Yes, sure, I'll fix it.

Which line length limit should I use? I'm asking because GCC plugins are written
in C++ and, as I see, other plugins in scripts/gcc-plugins/ have some very long
lines.

> - Also, this patch is way too big - there's no reason why the GCC plugin and
>   the stack erasure features should be introduced in the same patch, etc.

Ok, I'll split it.

Thanks again.
Best regards,
Alexander
Laura Abbott Oct. 10, 2017, 10:33 p.m. UTC | #4
On 10/05/2017 05:31 AM, Alexander Popov wrote:
> On 05.10.2017 10:27, Ingo Molnar wrote:
>>
>> * Alexander Popov <alex.popov@linux.com> wrote:
>>
>>> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
>>> index 8a13d46..06bc57b 100644
>>> --- a/arch/x86/entry/entry_32.S
>>> +++ b/arch/x86/entry/entry_32.S
>>> @@ -75,6 +75,71 @@
>>>  #endif
>>>  .endm
>>>  
>>> +.macro erase_kstack
>>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>>> +	call erase_kstack
>>> +#endif
>>> +.endm
>>> +
>>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>>> +/* For the detailed comments, see erase_kstack in entry_64.S */
>>> +ENTRY(erase_kstack)
>>> +	pushl	%edi
>>> +	pushl	%ecx
>>> +	pushl	%eax
>>> +	pushl	%ebp
>>> +
>>> +	movl	PER_CPU_VAR(current_task), %ebp
>>> +	mov	TASK_lowest_stack(%ebp), %edi
>>> +	mov	$STACKLEAK_POISON, %eax
>>> +	std
>>> +
>>> +1:
>>> +	mov	%edi, %ecx
>>> +	and	$THREAD_SIZE_asm - 1, %ecx
>>> +	shr	$2, %ecx
>>> +	repne	scasl
>>> +	jecxz	2f
>>> +
>>> +	cmp	$2*16, %ecx
>>> +	jc	2f
>>> +
>>> +	mov	$2*16, %ecx
>>> +	repe	scasl
>>> +	jecxz	2f
>>> +	jne	1b
>>> +
>>> +2:
>>> +	cld
>>> +	or	$2*4, %edi
>>> +	mov	%esp, %ecx
>>> +	sub	%edi, %ecx
>>> +
>>> +	cmp	$THREAD_SIZE_asm, %ecx
>>> +	jb	3f
>>> +	ud2
>>> +
>>> +3:
>>> +	shr	$2, %ecx
>>> +	rep	stosl
>>> +
>>> +	/*
>>> +	 * TODO: sp0 on x86_32 is not reliable, right?
>>> +	 * Doubt because of the definition of cpu_current_top_of_stack
>>> +	 * in arch/x86/kernel/cpu/common.c.
>>> +	 */
>>> +	mov	TASK_thread_sp0(%ebp), %edi
>>> +	sub	$128, %edi
>>> +	mov	%edi, TASK_lowest_stack(%ebp)
>>> +
>>> +	popl	%ebp
>>> +	popl	%eax
>>> +	popl	%ecx
>>> +	popl	%edi
>>> +	ret
>>> +ENDPROC(erase_kstack)
>>> +#endif
>>> +
>>>  /*
>>>   * User gs save/restore
>>>   *
>>> @@ -445,6 +510,8 @@ ENTRY(entry_SYSENTER_32)
>>>  	ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
>>>  		    "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
>>>  
>>> +	erase_kstack
>>> +
>>>  /* Opportunistic SYSEXIT */
>>>  	TRACE_IRQS_ON			/* User mode traces as IRQs on. */
>>>  	movl	PT_EIP(%esp), %edx	/* pt_regs->ip */
>>> @@ -531,6 +598,8 @@ ENTRY(entry_INT80_32)
>>>  	call	do_int80_syscall_32
>>>  .Lsyscall_32_done:
>>>  
>>> +	erase_kstack
>>> +
>>>  restore_all:
>>>  	TRACE_IRQS_IRET
>>>  .Lrestore_all_notrace:
>>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>>> index 4916725..189d843 100644
>>> --- a/arch/x86/entry/entry_64.S
>>> +++ b/arch/x86/entry/entry_64.S
>>> @@ -59,6 +59,90 @@ END(native_usergs_sysret64)
>>>  #endif
>>>  .endm
>>>  
>>> +.macro erase_kstack
>>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>>> +	call erase_kstack
>>> +#endif
>>> +.endm
>>> +
>>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>>> +ENTRY(erase_kstack)
>>> +	pushq	%rdi
>>> +	pushq	%rcx
>>> +	pushq	%rax
>>> +	pushq	%r11
>>> +
>>> +	movq	PER_CPU_VAR(current_task), %r11
>>> +	mov	TASK_lowest_stack(%r11), %rdi
>>> +	mov	$STACKLEAK_POISON, %rax
>>> +	std
>>> +
>>> +	/*
>>> +	 * Let's search for the poison value in the stack.
>>> +	 * Start from the lowest_stack and go to the bottom (see std above).
>>> +	 */
>>> +1:
>>> +	mov	%edi, %ecx
>>> +	and	$THREAD_SIZE_asm - 1, %ecx
>>> +	shr	$3, %ecx
>>> +	repne	scasq
>>> +	jecxz	2f	/* Didn't find it. Go to poisoning. */
>>> +
>>> +	/*
>>> +	 * Found the poison value in the stack. Go to poisoning if there are
>>> +	 * less than 16 qwords left.
>>> +	 */
>>> +	cmp	$2*8, %ecx
>>> +	jc	2f
>>> +
>>> +	/*
>>> +	 * Check that 16 further qwords contain poison (avoid false positives).
>>> +	 * If so, the part of the stack below the address in %rdi is likely
>>> +	 * to be poisoned. Otherwise we need to search deeper.
>>> +	 */
>>> +	mov	$2*8, %ecx
>>> +	repe	scasq
>>> +	jecxz	2f	/* Poison the upper part of the stack. */
>>> +	jne	1b	/* Search deeper. */
>>> +
>>> +2:
>>> +	/*
>>> +	 * Prepare the counter for poisoning the kernel stack between
>>> +	 * %rdi and %rsp. Two qwords at the bottom of the stack are reserved
>>> +	 * and should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
>>> +	 */
>>> +	cld
>>> +	or	$2*8, %rdi
>>> +	mov	%esp, %ecx
>>> +	sub	%edi, %ecx
>>> +
>>> +	/* Check that the counter value is sane. */
>>> +	cmp	$THREAD_SIZE_asm, %rcx
>>> +	jb	3f
>>> +	ud2
>>> +
>>> +3:
>>> +	/*
>>> +	 * So let's write the poison value to the kernel stack. Start from the
>>> +	 * address in %rdi and move up (see cld above) to the address in %rsp
>>> +	 * (not included, used memory).
>>> +	 */
>>> +	shr	$3, %ecx
>>> +	rep	stosq
>>> +
>>> +	/* Set the lowest_stack value to the top_of_stack - 256. */
>>> +	mov	TASK_thread_sp0(%r11), %rdi
>>> +	sub	$256, %rdi
>>> +	mov	%rdi, TASK_lowest_stack(%r11)
>>> +
>>> +	popq	%r11
>>> +	popq	%rax
>>> +	popq	%rcx
>>> +	popq	%rdi
>>> +	ret
>>> +ENDPROC(erase_kstack)
> 
> 
> Hello Ingo,
> 
> Thanks a lot for your review.
> 
>> A couple of (first round) review observations:
>>
>> - Why is the erase_kstack() function written in assembly, instead of plain C?
>>   The complexity and fragility of this patch could be reduced if it was moved to C.
> 
> Let me shortly describe my tactics.
> 
> Initially the erase_kstack() function is written in assembly in Grsecurity/PaX
> patch. I've extracted the STACKLEAK feature from that huge patch and carefully
> learned it bit by bit (it's quite complex). There are several bugs which I've
> found and fixed in it (they are listed in the cover letter), but generally I
> stick to the initial implementation in order not to accidentally break it.
> 
> I've added the detailed comments describing erase_kstack() for x86_64. IMO this
> code is really cool (my respect to PaX Team). However, if you think that
> rewriting it in C is obligatory, I'll do that. But let me at first fix the other
> issues which you listed below.
> 

I played around with reworking my arm64 version in C. I had to do some
tricks to save x0 which is used on the syscall fastpath. I'm concerned
about relying on gcc to not place anything on the stack when we clear
it. This might be mitigated if we don't make function calls, or maybe
I should have more faith in gcc?

Thanks,
Laura
Alexander Popov Oct. 13, 2017, 5:03 p.m. UTC | #5
Hello Ingo,

On 05.10.2017 10:27, Ingo Molnar wrote:
> - The GCC plugin adds instrumentation in form of extra 'track_stack()' and
>   'check_alloca()' calls. Could you please provide a frequency analysis of the
>   impact of this: x86-64 defconfig vmlinux size before/after the patch, and the
>   number of instrumentation function calls inserted, compared to the number of
>   functions?

Size of vmlinux (x86_64_defconfig):
 file size:
  - STACKLEAK disabled: 35014784 bytes
  - STACKLEAK enabled: 35044952 bytes (+0.086%)
 .text section size (calculated by size utility):
  - STACKLEAK disabled: 10752983
  - STACKLEAK enabled: 11062221 (+2.876%)

The readelf utility shows 45602 functions in vmlinux.

The STACKLEAK gcc plugin inserted 36 check_alloca() calls and 1265 track_stack()
calls (42274 calls are inserted during GIMPLE pass and 41009 calls are deleted
during RTL pass). So 2.853% of functions are instrumented.

I will add this information to the cover letter of the 5'th version, which I'm
currently preparing.

Best regards,
Alexander

Patch
diff mbox

diff --git a/arch/Kconfig b/arch/Kconfig
index 1aafb4e..f5a30cc 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -386,6 +386,13 @@  config SECCOMP_FILTER
 
 	  See Documentation/prctl/seccomp_filter.txt for details.
 
+config HAVE_ARCH_STACKLEAK
+	bool
+	help
+	  An architecture should select this if it has the code which
+	  fills the used part of the kernel stack with the STACKLEAK_POISON
+	  value before returning from system calls.
+
 config HAVE_GCC_PLUGINS
 	bool
 	help
@@ -516,6 +523,38 @@  config GCC_PLUGIN_RANDSTRUCT_PERFORMANCE
 	  in structures.  This reduces the performance hit of RANDSTRUCT
 	  at the cost of weakened randomization.
 
+config GCC_PLUGIN_STACKLEAK
+	bool "Erase the kernel stack before returning from syscalls"
+	depends on GCC_PLUGINS
+	depends on HAVE_ARCH_STACKLEAK
+	help
+	  This option makes the kernel erase the kernel stack before it
+	  returns from a system call. That reduces the information which
+	  a kernel stack leak bug can reveal and blocks some uninitialized
+	  stack variable attacks. This option also provides runtime checks
+	  for kernel stack overflow detection.
+
+	  The tradeoff is the performance impact: on a single CPU system kernel
+	  compilation sees a 1% slowdown, other systems and workloads may vary
+	  and you are advised to test this feature on your expected workload
+	  before deploying it.
+
+	  This plugin was ported from grsecurity/PaX. More information at:
+	   * https://grsecurity.net/
+	   * https://pax.grsecurity.net/
+
+config STACKLEAK_TRACK_MIN_SIZE
+	int "Minimum stack frame size of functions tracked by STACKLEAK"
+	default 100
+	range 0 4096
+	depends on GCC_PLUGIN_STACKLEAK
+	help
+	  The STACKLEAK gcc plugin instruments the kernel code for tracking
+	  the lowest border of the kernel stack (and for some other purposes).
+	  It inserts the track_stack() call for the functions with a stack
+	  frame size greater than or equal to this parameter. If unsure,
+	  leave the default value 100.
+
 config HAVE_CC_STACKPROTECTOR
 	bool
 	help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 971feac..b7da58f 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -114,6 +114,7 @@  config X86
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if MMU && COMPAT
 	select HAVE_ARCH_COMPAT_MMAP_BASES	if MMU && COMPAT
 	select HAVE_ARCH_SECCOMP_FILTER
+	select HAVE_ARCH_STACKLEAK
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if X86_64
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 03505ff..075487e 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -45,6 +45,12 @@  __visible inline void enter_from_user_mode(void)
 static inline void enter_from_user_mode(void) {}
 #endif
 
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+asmlinkage void erase_kstack(void);
+#else
+static void erase_kstack(void) {}
+#endif
+
 static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
 {
 #ifdef CONFIG_X86_64
@@ -81,8 +87,10 @@  static long syscall_trace_enter(struct pt_regs *regs)
 		emulated = true;
 
 	if ((emulated || (work & _TIF_SYSCALL_TRACE)) &&
-	    tracehook_report_syscall_entry(regs))
+	    tracehook_report_syscall_entry(regs)) {
+		erase_kstack();
 		return -1L;
+	}
 
 	if (emulated)
 		return -1L;
@@ -116,9 +124,11 @@  static long syscall_trace_enter(struct pt_regs *regs)
 			sd.args[5] = regs->bp;
 		}
 
-		ret = __secure_computing(&sd);
-		if (ret == -1)
+		ret = secure_computing(&sd);
+		if (ret == -1) {
+			erase_kstack();
 			return ret;
+		}
 	}
 #endif
 
@@ -127,6 +137,7 @@  static long syscall_trace_enter(struct pt_regs *regs)
 
 	do_audit_syscall_entry(regs, arch);
 
+	erase_kstack();
 	return ret ?: regs->orig_ax;
 }
 
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 8a13d46..06bc57b 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -75,6 +75,71 @@ 
 #endif
 .endm
 
+.macro erase_kstack
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	call erase_kstack
+#endif
+.endm
+
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+/* For the detailed comments, see erase_kstack in entry_64.S */
+ENTRY(erase_kstack)
+	pushl	%edi
+	pushl	%ecx
+	pushl	%eax
+	pushl	%ebp
+
+	movl	PER_CPU_VAR(current_task), %ebp
+	mov	TASK_lowest_stack(%ebp), %edi
+	mov	$STACKLEAK_POISON, %eax
+	std
+
+1:
+	mov	%edi, %ecx
+	and	$THREAD_SIZE_asm - 1, %ecx
+	shr	$2, %ecx
+	repne	scasl
+	jecxz	2f
+
+	cmp	$2*16, %ecx
+	jc	2f
+
+	mov	$2*16, %ecx
+	repe	scasl
+	jecxz	2f
+	jne	1b
+
+2:
+	cld
+	or	$2*4, %edi
+	mov	%esp, %ecx
+	sub	%edi, %ecx
+
+	cmp	$THREAD_SIZE_asm, %ecx
+	jb	3f
+	ud2
+
+3:
+	shr	$2, %ecx
+	rep	stosl
+
+	/*
+	 * TODO: sp0 on x86_32 is not reliable, right?
+	 * Doubt because of the definition of cpu_current_top_of_stack
+	 * in arch/x86/kernel/cpu/common.c.
+	 */
+	mov	TASK_thread_sp0(%ebp), %edi
+	sub	$128, %edi
+	mov	%edi, TASK_lowest_stack(%ebp)
+
+	popl	%ebp
+	popl	%eax
+	popl	%ecx
+	popl	%edi
+	ret
+ENDPROC(erase_kstack)
+#endif
+
 /*
  * User gs save/restore
  *
@@ -445,6 +510,8 @@  ENTRY(entry_SYSENTER_32)
 	ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
 		    "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
 
+	erase_kstack
+
 /* Opportunistic SYSEXIT */
 	TRACE_IRQS_ON			/* User mode traces as IRQs on. */
 	movl	PT_EIP(%esp), %edx	/* pt_regs->ip */
@@ -531,6 +598,8 @@  ENTRY(entry_INT80_32)
 	call	do_int80_syscall_32
 .Lsyscall_32_done:
 
+	erase_kstack
+
 restore_all:
 	TRACE_IRQS_IRET
 .Lrestore_all_notrace:
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 4916725..189d843 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -59,6 +59,90 @@  END(native_usergs_sysret64)
 #endif
 .endm
 
+.macro erase_kstack
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	call erase_kstack
+#endif
+.endm
+
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ENTRY(erase_kstack)
+	pushq	%rdi
+	pushq	%rcx
+	pushq	%rax
+	pushq	%r11
+
+	movq	PER_CPU_VAR(current_task), %r11
+	mov	TASK_lowest_stack(%r11), %rdi
+	mov	$STACKLEAK_POISON, %rax
+	std
+
+	/*
+	 * Let's search for the poison value in the stack.
+	 * Start from the lowest_stack and go to the bottom (see std above).
+	 */
+1:
+	mov	%edi, %ecx
+	and	$THREAD_SIZE_asm - 1, %ecx
+	shr	$3, %ecx
+	repne	scasq
+	jecxz	2f	/* Didn't find it. Go to poisoning. */
+
+	/*
+	 * Found the poison value in the stack. Go to poisoning if there are
+	 * less than 16 qwords left.
+	 */
+	cmp	$2*8, %ecx
+	jc	2f
+
+	/*
+	 * Check that 16 further qwords contain poison (avoid false positives).
+	 * If so, the part of the stack below the address in %rdi is likely
+	 * to be poisoned. Otherwise we need to search deeper.
+	 */
+	mov	$2*8, %ecx
+	repe	scasq
+	jecxz	2f	/* Poison the upper part of the stack. */
+	jne	1b	/* Search deeper. */
+
+2:
+	/*
+	 * Prepare the counter for poisoning the kernel stack between
+	 * %rdi and %rsp. Two qwords at the bottom of the stack are reserved
+	 * and should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
+	 */
+	cld
+	or	$2*8, %rdi
+	mov	%esp, %ecx
+	sub	%edi, %ecx
+
+	/* Check that the counter value is sane. */
+	cmp	$THREAD_SIZE_asm, %rcx
+	jb	3f
+	ud2
+
+3:
+	/*
+	 * So let's write the poison value to the kernel stack. Start from the
+	 * address in %rdi and move up (see cld above) to the address in %rsp
+	 * (not included, used memory).
+	 */
+	shr	$3, %ecx
+	rep	stosq
+
+	/* Set the lowest_stack value to the top_of_stack - 256. */
+	mov	TASK_thread_sp0(%r11), %rdi
+	sub	$256, %rdi
+	mov	%rdi, TASK_lowest_stack(%r11)
+
+	popq	%r11
+	popq	%rax
+	popq	%rcx
+	popq	%rdi
+	ret
+ENDPROC(erase_kstack)
+#endif
+
 /*
  * When dynamic function tracer is enabled it will add a breakpoint
  * to all locations that it is about to modify, sync CPUs, update
@@ -216,6 +300,8 @@  entry_SYSCALL_64_fastpath:
 	testl	$_TIF_ALLWORK_MASK, TASK_TI_flags(%r11)
 	jnz	1f
 
+	erase_kstack
+
 	LOCKDEP_SYS_EXIT
 	TRACE_IRQS_ON		/* user mode is traced as IRQs on */
 	movq	RIP(%rsp), %rcx
@@ -245,6 +331,8 @@  entry_SYSCALL64_slow_path:
 	call	do_syscall_64		/* returns with IRQs disabled */
 
 return_from_SYSCALL_64:
+	erase_kstack
+
 	RESTORE_EXTRA_REGS
 	TRACE_IRQS_IRETQ		/* we're about to change IF */
 
@@ -421,6 +509,7 @@  ENTRY(ret_from_fork)
 	UNWIND_HINT_REGS
 	movq	%rsp, %rdi
 	call	syscall_return_slowpath	/* returns with IRQs disabled */
+	erase_kstack
 	TRACE_IRQS_ON			/* user mode is traced as IRQS on */
 	SWAPGS
 	jmp	restore_regs_and_iret
@@ -610,6 +699,12 @@  ret_from_intr:
 GLOBAL(retint_user)
 	mov	%rsp,%rdi
 	call	prepare_exit_to_usermode
+
+	/*
+	 * TODO: Do we need to call erase_kstack here?
+	 * The PaX patch has it here commented out.
+	 */
+
 	TRACE_IRQS_IRETQ
 	SWAPGS
 	jmp	restore_regs_and_iret
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index e26c25c..f79cbf4 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -18,6 +18,12 @@ 
 
 	.section .entry.text, "ax"
 
+	.macro erase_kstack
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	call erase_kstack
+#endif
+	.endm
+
 /*
  * 32-bit SYSENTER entry.
  *
@@ -228,6 +234,7 @@  GLOBAL(entry_SYSCALL_compat_after_hwframe)
 
 	/* Opportunistic SYSRET */
 sysret32_from_system_call:
+	erase_kstack
 	TRACE_IRQS_ON			/* User mode traces as IRQs on. */
 	movq	RBX(%rsp), %rbx		/* pt_regs->rbx */
 	movq	RBP(%rsp), %rbp		/* pt_regs->rbp */
@@ -335,6 +342,7 @@  ENTRY(entry_INT80_compat)
 .Lsyscall_32_done:
 
 	/* Go back to user mode. */
+	erase_kstack
 	TRACE_IRQS_ON
 	SWAPGS
 	jmp	restore_regs_and_iret
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index b390ff7..c6eaf2d 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -477,6 +477,10 @@  struct thread_struct {
 
 	mm_segment_t		addr_limit;
 
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	unsigned long		lowest_stack;
+#endif
+
 	unsigned int		sig_on_uaccess_err:1;
 	unsigned int		uaccess_err:1;	/* uaccess failed */
 
diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
index de827d6..4ed7451 100644
--- a/arch/x86/kernel/asm-offsets.c
+++ b/arch/x86/kernel/asm-offsets.c
@@ -37,6 +37,10 @@  void common(void) {
 	BLANK();
 	OFFSET(TASK_TI_flags, task_struct, thread_info.flags);
 	OFFSET(TASK_addr_limit, task_struct, thread.addr_limit);
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	OFFSET(TASK_lowest_stack, task_struct, thread.lowest_stack);
+	OFFSET(TASK_thread_sp0, task_struct, thread.sp0);
+#endif
 
 	BLANK();
 	OFFSET(crypto_tfm_ctx_offset, crypto_tfm, __crt_ctx);
@@ -73,6 +77,11 @@  void common(void) {
 	OFFSET(PV_MMU_read_cr2, pv_mmu_ops, read_cr2);
 #endif
 
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	BLANK();
+	DEFINE(THREAD_SIZE_asm, THREAD_SIZE);
+#endif
+
 #ifdef CONFIG_XEN
 	BLANK();
 	OFFSET(XEN_vcpu_info_mask, vcpu_info, evtchn_upcall_mask);
diff --git a/arch/x86/kernel/dumpstack_32.c b/arch/x86/kernel/dumpstack_32.c
index 4f04814..4627543 100644
--- a/arch/x86/kernel/dumpstack_32.c
+++ b/arch/x86/kernel/dumpstack_32.c
@@ -162,3 +162,15 @@  void show_regs(struct pt_regs *regs)
 	}
 	pr_cont("\n");
 }
+
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+void __used check_alloca(unsigned long size)
+{
+	unsigned long sp = (unsigned long)&sp, stack_left;
+
+	/* all kernel stacks are of the same size */
+	stack_left = sp & (THREAD_SIZE - 1);
+	BUG_ON(stack_left < 256 || size >= stack_left - 256);
+}
+EXPORT_SYMBOL(check_alloca);
+#endif
diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
index 225af41..ba82699 100644
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -178,3 +178,18 @@  void show_regs(struct pt_regs *regs)
 	}
 	pr_cont("\n");
 }
+
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+void __used check_alloca(unsigned long size)
+{
+	unsigned long sp = (unsigned long)&sp;
+	struct stack_info stack_info = {0};
+	unsigned long visit_mask = 0;
+	unsigned long stack_left;
+
+	BUG_ON(get_stack_info(&sp, current, &stack_info, &visit_mask));
+	stack_left = sp - (unsigned long)stack_info.begin;
+	BUG_ON(stack_left < 256 || size >= stack_left - 256);
+}
+EXPORT_SYMBOL(check_alloca);
+#endif
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 1196625..c7345d2 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -136,6 +136,11 @@  int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
 	p->thread.sp0 = (unsigned long) (childregs+1);
 	memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));
 
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	p->thread.lowest_stack = (unsigned long)task_stack_page(p) +
+						2 * sizeof(unsigned long);
+#endif
+
 	if (unlikely(p->flags & PF_KTHREAD)) {
 		/* kernel thread */
 		memset(childregs, 0, sizeof(struct pt_regs));
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 302e7b2..65ba73f 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -283,6 +283,11 @@  int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
 	p->thread.sp = (unsigned long) fork_frame;
 	p->thread.io_bitmap_ptr = NULL;
 
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	p->thread.lowest_stack = (unsigned long)task_stack_page(p) +
+						2 * sizeof(unsigned long);
+#endif
+
 	savesegment(gs, p->thread.gsindex);
 	p->thread.gsbase = p->thread.gsindex ? 0 : me->thread.gsbase;
 	savesegment(fs, p->thread.fsindex);
diff --git a/fs/exec.c b/fs/exec.c
index ac34d97..fb215e3 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1957,3 +1957,33 @@  COMPAT_SYSCALL_DEFINE5(execveat, int, fd,
 				  argv, envp, flags);
 }
 #endif
+
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+void __used track_stack(void)
+{
+	/*
+	 * N.B. The arch-specific part of the STACKLEAK feature fills the
+	 * kernel stack with the poison value, which has the register width.
+	 * That code assumes that the value of thread.lowest_stack is aligned
+	 * on the register width boundary.
+	 *
+	 * That is true for x86 and x86_64 because of the kernel stack
+	 * alignment on these platforms (for details, see cc_stack_align in
+	 * arch/x86/Makefile). Take care of that when you port STACKLEAK to
+	 * new platforms.
+	 */
+	unsigned long sp = (unsigned long)&sp;
+
+	if (sp < current->thread.lowest_stack &&
+	    sp >= (unsigned long)task_stack_page(current) +
+					2 * sizeof(unsigned long)) {
+		current->thread.lowest_stack = sp;
+	}
+
+#ifndef CONFIG_VMAP_STACK
+	if (unlikely((sp & (THREAD_SIZE - 1)) < (THREAD_SIZE / 16)))
+		BUG();
+#endif /* !CONFIG_VMAP_STACK */
+}
+EXPORT_SYMBOL(track_stack);
+#endif /* CONFIG_GCC_PLUGIN_STACKLEAK */
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index e95a263..916f02d 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -624,4 +624,8 @@  static __always_inline void __write_once_size(volatile void *p, void *res, int s
 	(_________p1); \
 })
 
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+# define STACKLEAK_POISON -0xBEEF
+#endif
+
 #endif /* __LINUX_COMPILER_H */
diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
index d1f7b0d..3793c41 100644
--- a/scripts/Makefile.gcc-plugins
+++ b/scripts/Makefile.gcc-plugins
@@ -34,6 +34,9 @@  ifdef CONFIG_GCC_PLUGINS
   gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_RANDSTRUCT)	+= -DRANDSTRUCT_PLUGIN
   gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_RANDSTRUCT_PERFORMANCE)	+= -fplugin-arg-randomize_layout_plugin-performance-mode
 
+  gcc-plugin-$(CONFIG_GCC_PLUGIN_STACKLEAK)	+= stackleak_plugin.so
+  gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK)	+= -DSTACKLEAK_PLUGIN -fplugin-arg-stackleak_plugin-track-min-size=$(CONFIG_STACKLEAK_TRACK_MIN_SIZE)
+
   GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))
 
   export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGIN GCC_PLUGIN_SUBDIR
diff --git a/scripts/gcc-plugins/stackleak_plugin.c b/scripts/gcc-plugins/stackleak_plugin.c
new file mode 100644
index 0000000..321ca80
--- /dev/null
+++ b/scripts/gcc-plugins/stackleak_plugin.c
@@ -0,0 +1,397 @@ 
+/*
+ * Copyright 2011-2017 by the PaX Team <pageexec@freemail.hu>
+ * Licensed under the GPL v2
+ *
+ * Note: the choice of the license means that the compilation process is
+ *       NOT 'eligible' as defined by gcc's library exception to the GPL v3,
+ *       but for the kernel it doesn't matter since it doesn't link against
+ *       any of the gcc libraries
+ *
+ * This gcc plugin is needed for tracking the lowest border of the kernel stack
+ * and checking that alloca calls don't cause stack overflow. It instruments
+ * the kernel code inserting:
+ *  - the check_alloca() call before alloca and track_stack() call after it;
+ *  - the track_stack() call for the functions with a stack frame size greater
+ *     than or equal to the "track-min-size" plugin parameter.
+ *
+ * This plugin is ported from grsecurity/PaX. For more information see:
+ *   https://grsecurity.net/
+ *   https://pax.grsecurity.net/
+ *
+ * Debugging:
+ *  - use fprintf() to stderr, debug_generic_expr(), debug_gimple_stmt()
+ *     and print_rtl();
+ *  - add "-fdump-tree-all -fdump-rtl-all" to the plugin CFLAGS in
+ *     Makefile.gcc-plugins to see the verbose dumps of the gcc passes;
+ *  - use gcc -E to understand the preprocessing shenanigans;
+ *  - use gcc with enabled CFG/GIMPLE/SSA verification (--enable-checking).
+ */
+
+#include "gcc-common.h"
+
+__visible int plugin_is_GPL_compatible;
+
+static int track_frame_size = -1;
+static const char track_function[] = "track_stack";
+static const char check_function[] = "check_alloca";
+
+// Mark these global variables (roots) for gcc garbage collector since
+// they point to the garbage-collected memory.
+static GTY(()) tree track_function_decl;
+static GTY(()) tree check_function_decl;
+
+static struct plugin_info stackleak_plugin_info = {
+	.version	= "201707101337",
+	.help		= "track-min-size=nn\ttrack stack for functions with a stack frame size >= nn bytes\n"
+};
+
+static void stackleak_check_alloca(gimple_stmt_iterator *gsi)
+{
+	gimple stmt;
+	gcall *check_alloca;
+	tree alloca_size;
+	cgraph_node_ptr node;
+	int frequency;
+	basic_block bb;
+
+	// insert call to void check_alloca(unsigned long size)
+	alloca_size = gimple_call_arg(gsi_stmt(*gsi), 0);
+	stmt = gimple_build_call(check_function_decl, 1, alloca_size);
+	check_alloca = as_a_gcall(stmt);
+	gsi_insert_before(gsi, check_alloca, GSI_SAME_STMT);
+
+	// update the cgraph
+	bb = gimple_bb(check_alloca);
+	node = cgraph_get_create_node(check_function_decl);
+	gcc_assert(node);
+	frequency = compute_call_stmt_bb_frequency(current_function_decl, bb);
+	cgraph_create_edge(cgraph_get_node(current_function_decl), node, check_alloca, bb->count, frequency, bb->loop_depth);
+}
+
+static void stackleak_add_instrumentation(gimple_stmt_iterator *gsi, bool after)
+{
+	gimple stmt;
+	gcall *track_stack;
+	cgraph_node_ptr node;
+	int frequency;
+	basic_block bb;
+
+	// insert call to void track_stack(void)
+	stmt = gimple_build_call(track_function_decl, 0);
+	track_stack = as_a_gcall(stmt);
+	if (after)
+		gsi_insert_after(gsi, track_stack, GSI_CONTINUE_LINKING);
+	else
+		gsi_insert_before(gsi, track_stack, GSI_SAME_STMT);
+
+	// update the cgraph
+	bb = gimple_bb(track_stack);
+	node = cgraph_get_create_node(track_function_decl);
+	gcc_assert(node);
+	frequency = compute_call_stmt_bb_frequency(current_function_decl, bb);
+	cgraph_create_edge(cgraph_get_node(current_function_decl), node, track_stack, bb->count, frequency, bb->loop_depth);
+}
+
+static bool is_alloca(gimple stmt)
+{
+	if (gimple_call_builtin_p(stmt, BUILT_IN_ALLOCA))
+		return true;
+
+#if BUILDING_GCC_VERSION >= 4007
+	if (gimple_call_builtin_p(stmt, BUILT_IN_ALLOCA_WITH_ALIGN))
+		return true;
+#endif
+
+	return false;
+}
+
+// Work with the GIMPLE representation of the code.
+// Insert the check_alloca() call before alloca and track_stack() call after
+// it. Also insert track_stack() call into the beginning of the function
+// if it is not instrumented.
+static unsigned int stackleak_tree_instrument_execute(void)
+{
+	basic_block bb, entry_bb;
+	bool prologue_instrumented = false, is_leaf = true;
+
+	// ENTRY_BLOCK_PTR is a basic block which represents possible entry
+	// point of a function. This block does not contain any code and
+	// has a CFG edge to its successor.
+	gcc_assert(single_succ_p(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
+	entry_bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
+
+	// 1. Loop through the GIMPLE statements in each of cfun basic blocks.
+	// cfun is a global variable which represents the function that is
+	// currently processed.
+	FOR_EACH_BB_FN(bb, cfun) {
+		gimple_stmt_iterator gsi;
+
+		for (gsi = gsi_start_bb(bb); !gsi_end_p(gsi); gsi_next(&gsi)) {
+			gimple stmt;
+
+			stmt = gsi_stmt(gsi);
+
+			// Leaf function is a function which makes no calls
+			if (is_gimple_call(stmt))
+				is_leaf = false;
+
+			// gimple match: align 8 built-in BUILT_IN_NORMAL:BUILT_IN_ALLOCA attributes <tree_list 0xb7576450>
+			if (!is_alloca(stmt))
+				continue;
+
+			// 2. insert stack overflow check before each alloca call
+			stackleak_check_alloca(&gsi);
+
+			// 3. insert track_stack() call after each alloca call
+			stackleak_add_instrumentation(&gsi, true);
+			if (bb == entry_bb)
+				prologue_instrumented = true;
+		}
+	}
+
+	// special cases for some Linux code: taking the address of static inline functions will materialize them
+	// but we mustn't instrument some of them as the resulting stack alignment required by the function call ABI
+	// will break other assumptions regarding the expected (but not otherwise enforced) register clobbering ABI.
+	// case in point: native_save_fl on amd64 when optimized for size clobbers rdx if it were instrumented here.
+	//
+	// TODO: any more special cases?
+	if (is_leaf && !TREE_PUBLIC(current_function_decl) && DECL_DECLARED_INLINE_P(current_function_decl))
+		return 0;
+	if (is_leaf && !strncmp(IDENTIFIER_POINTER(DECL_NAME(current_function_decl)), "_paravirt_", 10))
+		return 0;
+
+	if (!prologue_instrumented) {
+		// 4. insert track_stack() call at the beginning of the function
+		gimple_stmt_iterator gsi;
+
+		bb = entry_bb;
+		if (!single_pred_p(bb)) {
+//			gcc_assert(bb_loop_depth(bb) || (bb->flags & BB_IRREDUCIBLE_LOOP));
+			split_edge(single_succ_edge(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
+			gcc_assert(single_succ_p(ENTRY_BLOCK_PTR_FOR_FN(cfun)));
+			bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
+		}
+		gsi = gsi_after_labels(bb);
+		stackleak_add_instrumentation(&gsi, false);
+	}
+
+	return 0;
+}
+
+// Work with the RTL representation of the code.
+// Remove the unneeded track_stack() calls from the functions which
+// don't call alloca and have the stack frame size less than track_frame_size.
+static unsigned int stackleak_final_execute(void)
+{
+	rtx_insn *insn, *next;
+
+	if (cfun->calls_alloca)
+		return 0;
+
+	if (get_frame_size() >= track_frame_size)
+		return 0;
+
+	// 1. Find track_stack() calls. Loop through the chain of insns,
+	// which is an RTL representation of the code for a function.
+	for (insn = get_insns(); insn; insn = next) {
+		// rtl match: (call_insn 8 7 9 3 (call (mem (symbol_ref ("track_stack") [flags 0x41] <function_decl 0xb7470e80 track_stack>) [0 S1 A8]) (4)) -1 (nil) (nil))
+		rtx body;
+
+		next = NEXT_INSN(insn);
+
+		// Check the expression code of the insn.
+		if (!CALL_P(insn))
+			continue;
+
+		// Check the expression code of the insn body, which is an RTL
+		// Expression (RTX) describing the side effect performed by
+		// that insn.
+		body = PATTERN(insn);
+		if (GET_CODE(body) != CALL)
+			continue;
+
+		// Check the first operand of the call expression. It should
+		// be a mem RTX describing the needed subroutine with a
+		// symbol_ref RTX.
+		body = XEXP(body, 0);
+		if (GET_CODE(body) != MEM)
+			continue;
+
+		body = XEXP(body, 0);
+		if (GET_CODE(body) != SYMBOL_REF)
+			continue;
+
+		if (SYMBOL_REF_DECL(body) != track_function_decl)
+			continue;
+
+		// 2. delete call
+		delete_insn_and_edges(insn);
+#if BUILDING_GCC_VERSION >= 4007
+		if (GET_CODE(next) == NOTE && NOTE_KIND(next) == NOTE_INSN_CALL_ARG_LOCATION) {
+			insn = next;
+			next = NEXT_INSN(insn);
+			delete_insn_and_edges(insn);
+		}
+#endif
+	}
+
+	// Uncomment the following to see the code which was cleaned in this
+	// function. It should not contain check_alloca() and track_stack() calls.
+	// The stack frame size should be less than track_frame_size.
+	//
+	// warning(0, "cleaned from check_alloca and track_stack calls, stack frame size: %ld", get_frame_size());
+	// print_simple_rtl(stderr, get_insns());
+
+	return 0;
+}
+
+static bool stackleak_track_stack_gate(void)
+{
+	tree section;
+
+	section = lookup_attribute("section", DECL_ATTRIBUTES(current_function_decl));
+	if (section && TREE_VALUE(section)) {
+		section = TREE_VALUE(TREE_VALUE(section));
+
+		if (!strncmp(TREE_STRING_POINTER(section), ".init.text", 10))
+			return false;
+		if (!strncmp(TREE_STRING_POINTER(section), ".devinit.text", 13))
+			return false;
+		if (!strncmp(TREE_STRING_POINTER(section), ".cpuinit.text", 13))
+			return false;
+		if (!strncmp(TREE_STRING_POINTER(section), ".meminit.text", 13))
+			return false;
+	}
+
+	return track_frame_size >= 0;
+}
+
+// Build function declarations for track_stack() and check_alloca()
+static void stackleak_start_unit(void *gcc_data __unused, void *user_data __unused)
+{
+	tree fntype;
+
+	// void track_stack(void)
+	fntype = build_function_type_list(void_type_node, NULL_TREE);
+	track_function_decl = build_fn_decl(track_function, fntype);
+	DECL_ASSEMBLER_NAME(track_function_decl); // for LTO
+	TREE_PUBLIC(track_function_decl) = 1;
+	TREE_USED(track_function_decl) = 1;
+	DECL_EXTERNAL(track_function_decl) = 1;
+	DECL_ARTIFICIAL(track_function_decl) = 1;
+	DECL_PRESERVE_P(track_function_decl) = 1;
+
+	// void check_alloca(unsigned long)
+	fntype = build_function_type_list(void_type_node, long_unsigned_type_node, NULL_TREE);
+	check_function_decl = build_fn_decl(check_function, fntype);
+	DECL_ASSEMBLER_NAME(check_function_decl); // for LTO
+	TREE_PUBLIC(check_function_decl) = 1;
+	TREE_USED(check_function_decl) = 1;
+	DECL_EXTERNAL(check_function_decl) = 1;
+	DECL_ARTIFICIAL(check_function_decl) = 1;
+	DECL_PRESERVE_P(check_function_decl) = 1;
+}
+
+// Pass gate function is a predicate function that gets executed before the
+// corresponding pass. If the return value is 'true' the pass gets executed,
+// otherwise, it is skipped.
+static bool stackleak_tree_instrument_gate(void)
+{
+	return stackleak_track_stack_gate();
+}
+
+#define PASS_NAME stackleak_tree_instrument
+#define PROPERTIES_REQUIRED PROP_gimple_leh | PROP_cfg
+#define TODO_FLAGS_START TODO_verify_ssa | TODO_verify_flow | TODO_verify_stmts
+#define TODO_FLAGS_FINISH TODO_verify_ssa | TODO_verify_stmts | TODO_dump_func | TODO_update_ssa | TODO_rebuild_cgraph_edges
+#include "gcc-generate-gimple-pass.h"
+
+static bool stackleak_final_gate(void)
+{
+	return stackleak_track_stack_gate();
+}
+
+#define PASS_NAME stackleak_final
+#define TODO_FLAGS_FINISH TODO_dump_func
+#include "gcc-generate-rtl-pass.h"
+
+// Every gcc plugin exports a plugin_init() function that is called right
+// after the plugin is loaded. This function is responsible for registering
+// the plugin callbacks and doing other required initialization.
+__visible int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version)
+{
+	const char * const plugin_name = plugin_info->base_name;
+	const int argc = plugin_info->argc;
+	const struct plugin_argument * const argv = plugin_info->argv;
+	int i;
+
+	// Extra GGC root tables describing our GTY-ed data.
+	static const struct ggc_root_tab gt_ggc_r_gt_stackleak[] = {
+		{
+			.base = &track_function_decl,
+			.nelt = 1,
+			.stride = sizeof(track_function_decl),
+			.cb = &gt_ggc_mx_tree_node,
+			.pchw = &gt_pch_nx_tree_node
+		},
+		{
+			.base = &check_function_decl,
+			.nelt = 1,
+			.stride = sizeof(check_function_decl),
+			.cb = &gt_ggc_mx_tree_node,
+			.pchw = &gt_pch_nx_tree_node
+		},
+		LAST_GGC_ROOT_TAB
+	};
+
+	// The stackleak_tree_instrument pass should be executed before the
+	// "optimized" pass, which is the control flow graph cleanup that is
+	// performed just before expanding gcc trees to the RTL. In former
+	// versions of the plugin this new pass was inserted before the
+	// "tree_profile" pass, which is currently called "profile".
+	PASS_INFO(stackleak_tree_instrument, "optimized", 1, PASS_POS_INSERT_BEFORE);
+
+	// The stackleak_final pass should be executed before the "final" pass,
+	// which turns the RTL (Register Transfer Language) into assembly.
+	PASS_INFO(stackleak_final, "final", 1, PASS_POS_INSERT_BEFORE);
+
+	if (!plugin_default_version_check(version, &gcc_version)) {
+		error(G_("incompatible gcc/plugin versions"));
+		return 1;
+	}
+
+	// Give the information about the plugin
+	register_callback(plugin_name, PLUGIN_INFO, NULL, &stackleak_plugin_info);
+
+	// Parse the plugin arguments
+	for (i = 0; i < argc; ++i) {
+		if (!strcmp(argv[i].key, "track-min-size")) {
+			if (!argv[i].value) {
+				error(G_("no value supplied for option '-fplugin-arg-%s-%s'"), plugin_name, argv[i].key);
+				continue;
+			}
+			track_frame_size = atoi(argv[i].value);
+			if (argv[i].value[0] < '0' || argv[i].value[0] > '9' || track_frame_size < 0)
+				error(G_("invalid option argument '-fplugin-arg-%s-%s=%s'"), plugin_name, argv[i].key, argv[i].value);
+			continue;
+		}
+		error(G_("unknown option '-fplugin-arg-%s-%s'"), plugin_name, argv[i].key);
+	}
+
+	// Register to be called before processing a translation unit
+	register_callback(plugin_name, PLUGIN_START_UNIT, &stackleak_start_unit, NULL);
+
+	// Register an extra GCC garbage collector (GGC) root table
+	register_callback(plugin_name, PLUGIN_REGISTER_GGC_ROOTS, NULL, (void *)&gt_ggc_r_gt_stackleak);
+
+	// Hook into the Pass Manager to register new gcc passes.
+	//
+	// The stack frame size info is available only at the last RTL pass,
+	// when it's too late to insert complex code like a function call.
+	// So we register two gcc passes to instrument every function at first
+	// and remove the unneeded instrumentation later.
+	register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &stackleak_tree_instrument_pass_info);
+	register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &stackleak_final_pass_info);
+
+	return 0;
+}