[PATCH RFC v10 0/6] Introduce the STACKLEAK feature and a test for it

[PATCH RFC v10 0/6] Introduce the STACKLEAK feature and a test for it

[Alexander Popov]

This is the 10th version of the patch series introducing STACKLEAK to the
mainline kernel. The previous version raised a fervent discussion[0].
The assembly code introduced by v9 irritated the reviewers.

I've found the way to bypass the obstacles[1] of the C implementation.
So I dare come once again. Let me ask you to look at this code without
preconception.

Motivation
==========

STACKLEAK (initially developed by PaX Team):

 1. reduces the information that can be revealed through kernel stack leak bugs.
    The idea of erasing the thread stack at the end of syscalls is similar to
    CONFIG_PAGE_POISONING and memzero_explicit() in kernel crypto, which all
    comply with FDP_RIP.2 (Full Residual Information Protection) of the
    Common Criteria standard.

 2. blocks some uninitialized stack variable attacks (e.g. CVE-2017-17712,
    CVE-2010-2963). That kind of bugs should be killed by improving C compilers
    in future, which might take a long time.

 3. blocks stack depth overflow caused by alloca (aka Stack Clash attack).
    That is orthogonal to the mainline kernel VLA cleanup and protects
    un-upstreamed code.

Performance impact
==================

Hardware: Intel Core i7-4770, 16 GB RAM

Test #1: building the Linux kernel on a single core
	0.91% slowdown

Test #2: hackbench -s 4096 -l 2000 -g 15 -f 25 -P
	4.2% slowdown

So the STACKLEAK description in Kconfig includes:
"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".

Links
=====

[0] http://www.openwall.com/lists/kernel-hardening/2018/03/03/7
[1] http://www.openwall.com/lists/kernel-hardening/2018/03/21/4


Alexander Popov (6):
  gcc-plugins: Clean up the cgraph_create_edge* macros
  x86/entry: Add STACKLEAK erasing the kernel stack at the end of
    syscalls
  gcc-plugins: Add STACKLEAK plugin for tracking the kernel stack
  lkdtm: Add a test for STACKLEAK
  fs/proc: Show STACKLEAK metrics in the /proc file system
  doc: self-protection: Add information about STACKLEAK feature

 Documentation/security/self-protection.rst |  23 +-
 Documentation/x86/x86_64/mm.txt            |   2 +
 arch/Kconfig                               |  53 ++++
 arch/x86/Kconfig                           |   1 +
 arch/x86/entry/Makefile                    |   3 +
 arch/x86/entry/entry_32.S                  |  11 +
 arch/x86/entry/entry_64.S                  |  11 +
 arch/x86/entry/entry_64_compat.S           |  11 +
 arch/x86/entry/erase.c                     |  58 ++++
 arch/x86/include/asm/processor.h           |   7 +
 arch/x86/kernel/dumpstack.c                |  19 ++
 arch/x86/kernel/process_32.c               |   8 +
 arch/x86/kernel/process_64.c               |   8 +
 drivers/misc/Makefile                      |   3 +
 drivers/misc/lkdtm.h                       |   4 +
 drivers/misc/lkdtm_core.c                  |   2 +
 drivers/misc/lkdtm_stackleak.c             | 141 +++++++++
 fs/proc/base.c                             |  18 ++
 include/linux/compiler.h                   |   4 +
 mm/util.c                                  |  33 ++
 scripts/Makefile.gcc-plugins               |   3 +
 scripts/gcc-plugins/gcc-common.h           |  26 +-
 scripts/gcc-plugins/stackleak_plugin.c     | 470 +++++++++++++++++++++++++++++
 23 files changed, 900 insertions(+), 19 deletions(-)
 create mode 100644 arch/x86/entry/erase.c
 create mode 100644 drivers/misc/lkdtm_stackleak.c
 create mode 100644 scripts/gcc-plugins/stackleak_plugin.c

-- 
2.7.4

[PATCH RFC v10 1/6] gcc-plugins: Clean up the cgraph_create_edge* macros

[Alexander Popov]

Drop useless redefinitions of cgraph_create_edge* macros. Drop the unused
nest argument. Also support gcc-8, which doesn't have freq argument.

Signed-off-by: Alexander Popov <alex.popov@linux.com>
---
 scripts/gcc-plugins/gcc-common.h | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/scripts/gcc-plugins/gcc-common.h b/scripts/gcc-plugins/gcc-common.h
index f467500..552d5ef 100644
--- a/scripts/gcc-plugins/gcc-common.h
+++ b/scripts/gcc-plugins/gcc-common.h
@@ -392,13 +392,6 @@ static inline struct cgraph_node *cgraph_alias_target(struct cgraph_node *n)
 }
 #endif
 
-#if BUILDING_GCC_VERSION >= 4007 && BUILDING_GCC_VERSION <= 4009
-#define cgraph_create_edge(caller, callee, call_stmt, count, freq, nest) \
-	cgraph_create_edge((caller), (callee), (call_stmt), (count), (freq))
-#define cgraph_create_edge_including_clones(caller, callee, old_call_stmt, call_stmt, count, freq, nest, reason) \
-	cgraph_create_edge_including_clones((caller), (callee), (old_call_stmt), (call_stmt), (count), (freq), (reason))
-#endif
-
 #if BUILDING_GCC_VERSION <= 4008
 #define ENTRY_BLOCK_PTR_FOR_FN(FN)	ENTRY_BLOCK_PTR_FOR_FUNCTION(FN)
 #define EXIT_BLOCK_PTR_FOR_FN(FN)	EXIT_BLOCK_PTR_FOR_FUNCTION(FN)
@@ -723,10 +716,23 @@ static inline const char *get_decl_section_name(const_tree decl)
 #define varpool_get_node(decl) varpool_node::get(decl)
 #define dump_varpool_node(file, node) (node)->dump(file)
 
-#define cgraph_create_edge(caller, callee, call_stmt, count, freq, nest) \
+#if BUILDING_GCC_VERSION >= 8000
+#define cgraph_create_edge(caller, callee, call_stmt, count, freq) \
+	(caller)->create_edge((callee), (call_stmt), (count))
+
+#define cgraph_create_edge_including_clones(caller, callee,	\
+		old_call_stmt, call_stmt, count, freq, reason)	\
+	(caller)->create_edge_including_clones((callee),	\
+		(old_call_stmt), (call_stmt), (count), (reason))
+#else
+#define cgraph_create_edge(caller, callee, call_stmt, count, freq) \
 	(caller)->create_edge((callee), (call_stmt), (count), (freq))
-#define cgraph_create_edge_including_clones(caller, callee, old_call_stmt, call_stmt, count, freq, nest, reason) \
-	(caller)->create_edge_including_clones((callee), (old_call_stmt), (call_stmt), (count), (freq), (reason))
+
+#define cgraph_create_edge_including_clones(caller, callee,	\
+		old_call_stmt, call_stmt, count, freq, reason)	\
+	(caller)->create_edge_including_clones((callee),	\
+		(old_call_stmt), (call_stmt), (count), (freq), (reason))
+#endif
 
 typedef struct cgraph_node *cgraph_node_ptr;
 typedef struct cgraph_edge *cgraph_edge_p;
-- 
2.7.4

[PATCH RFC v10 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Alexander Popov]

The STACKLEAK feature erases the kernel stack before returning from
syscalls. That reduces the information which kernel stack leak bugs can
reveal and blocks some uninitialized stack variable attacks. Moreover,
STACKLEAK provides runtime checks for kernel stack overflow detection.

This commit introduces the architecture-specific code filling the used
part of the kernel stack with a poison value before returning to the
userspace. Full STACKLEAK feature also contains the gcc plugin which
comes in a separate commit.

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>
---
 Documentation/x86/x86_64/mm.txt  |  2 ++
 arch/Kconfig                     | 27 ++++++++++++++++++++
 arch/x86/Kconfig                 |  1 +
 arch/x86/entry/Makefile          |  3 +++
 arch/x86/entry/entry_32.S        | 11 ++++++++
 arch/x86/entry/entry_64.S        | 11 ++++++++
 arch/x86/entry/entry_64_compat.S | 11 ++++++++
 arch/x86/entry/erase.c           | 54 ++++++++++++++++++++++++++++++++++++++++
 arch/x86/include/asm/processor.h |  4 +++
 arch/x86/kernel/process_32.c     |  5 ++++
 arch/x86/kernel/process_64.c     |  5 ++++
 include/linux/compiler.h         |  4 +++
 12 files changed, 138 insertions(+)
 create mode 100644 arch/x86/entry/erase.c

diff --git a/Documentation/x86/x86_64/mm.txt b/Documentation/x86/x86_64/mm.txt
index ea91cb6..21ee7c5 100644
--- a/Documentation/x86/x86_64/mm.txt
+++ b/Documentation/x86/x86_64/mm.txt
@@ -24,6 +24,7 @@ ffffffffa0000000 - [fixmap start]   (~1526 MB) module mapping space (variable)
 [fixmap start]   - ffffffffff5fffff kernel-internal fixmap range
 ffffffffff600000 - ffffffffff600fff (=4 kB) legacy vsyscall ABI
 ffffffffffe00000 - ffffffffffffffff (=2 MB) unused hole
+STACKLEAK_POISON value in this last hole: ffffffffffff4111
 
 Virtual memory map with 5 level page tables:
 
@@ -50,6 +51,7 @@ ffffffffa0000000 - fffffffffeffffff (1520 MB) module mapping space
 [fixmap start]   - ffffffffff5fffff kernel-internal fixmap range
 ffffffffff600000 - ffffffffff600fff (=4 kB) legacy vsyscall ABI
 ffffffffffe00000 - ffffffffffffffff (=2 MB) unused hole
+STACKLEAK_POISON value in this last hole: ffffffffffff4111
 
 Architecture defines a 64-bit virtual address. Implementations can support
 less. Currently supported are 48- and 57-bit virtual addresses. Bits 63
diff --git a/arch/Kconfig b/arch/Kconfig
index 76c0b54..368e2fb 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -401,6 +401,13 @@ 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
@@ -531,6 +538,26 @@ 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
+	  kernel stack leak bugs 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 HAVE_CC_STACKPROTECTOR
 	bool
 	help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 0fa71a7..e700879 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -119,6 +119,7 @@ config X86
 	select HAVE_ARCH_COMPAT_MMAP_BASES	if MMU && COMPAT
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
+	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/Makefile b/arch/x86/entry/Makefile
index 06fc70c..abe4d92 100644
--- a/arch/x86/entry/Makefile
+++ b/arch/x86/entry/Makefile
@@ -15,3 +15,6 @@ obj-y				+= vsyscall/
 
 obj-$(CONFIG_IA32_EMULATION)	+= entry_64_compat.o syscall_32.o
 
+obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += erase.o
+KASAN_SANITIZE_erase.o		:= n
+
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 6ad064c..0dec4f3 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -77,6 +77,12 @@
 #endif
 .endm
 
+.macro ERASE_KSTACK
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	call erase_kstack
+#endif
+.endm
+
 /*
  * User gs save/restore
  *
@@ -298,6 +304,7 @@ ENTRY(ret_from_fork)
 	/* When we fork, we trace the syscall return in the child, too. */
 	movl    %esp, %eax
 	call    syscall_return_slowpath
+	ERASE_KSTACK
 	jmp     restore_all
 
 	/* kernel thread */
@@ -458,6 +465,8 @@ 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 */
@@ -544,6 +553,8 @@ 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 18ed349..c85af83 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -66,6 +66,14 @@ END(native_usergs_sysret64)
 	TRACE_IRQS_FLAGS EFLAGS(%rsp)
 .endm
 
+.macro ERASE_KSTACK
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	PUSH_AND_CLEAR_REGS
+	call erase_kstack
+	POP_REGS
+#endif
+.endm
+
 /*
  * When dynamic function tracer is enabled it will add a breakpoint
  * to all locations that it is about to modify, sync CPUs, update
@@ -323,6 +331,8 @@ syscall_return_via_sysret:
 	 * We are on the trampoline stack.  All regs except RDI are live.
 	 * We can do future final exit work right here.
 	 */
+	ERASE_KSTACK
+
 	SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
 
 	popq	%rdi
@@ -681,6 +691,7 @@ GLOBAL(swapgs_restore_regs_and_return_to_usermode)
 	 * We are on the trampoline stack.  All regs except RDI are live.
 	 * We can do future final exit work right here.
 	 */
+	ERASE_KSTACK
 
 	SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
 
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 08425c4..6c29df6 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -19,6 +19,12 @@
 
 	.section .entry.text, "ax"
 
+	.macro ERASE_KSTACK
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	call erase_kstack
+#endif
+	.endm
+
 /*
  * 32-bit SYSENTER entry.
  *
@@ -258,6 +264,11 @@ GLOBAL(entry_SYSCALL_compat_after_hwframe)
 
 	/* Opportunistic SYSRET */
 sysret32_from_system_call:
+	/*
+	 * We are not going to return to the userspace from the trampoline
+	 * stack. So let's erase the thread stack right now.
+	 */
+	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 */
diff --git a/arch/x86/entry/erase.c b/arch/x86/entry/erase.c
new file mode 100644
index 0000000..4892335
--- /dev/null
+++ b/arch/x86/entry/erase.c
@@ -0,0 +1,54 @@
+#include <linux/bug.h>
+#include <linux/sched.h>
+#include <asm/current.h>
+#include <asm/linkage.h>
+#include <asm/processor.h>
+
+asmlinkage void erase_kstack(void)
+{
+	register unsigned long p = current->thread.lowest_stack;
+	register 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);
+
+	/*
+	 * So let's write the poison value to the kernel stack.
+	 * Start from the address in p and move up till the new boundary.
+	 */
+	if (on_thread_stack())
+		boundary = current_stack_pointer;
+	else
+		boundary = current_top_of_stack();
+
+	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_top_of_stack() - 256;
+}
+
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index b0ccd48..0c87813 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -494,6 +494,10 @@ 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/process_32.c b/arch/x86/kernel/process_32.c
index 5224c60..1b0892e 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) +
+						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 9eb448c..82122af 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -281,6 +281,11 @@ 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) +
+						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/include/linux/compiler.h b/include/linux/compiler.h
index ab4711c..341b6cf8 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -342,4 +342,8 @@ unsigned long read_word_at_a_time(const void *addr)
 	compiletime_assert(__native_word(t),				\
 		"Need native word sized stores/loads for atomicity.")
 
+/* Poison value points to the unused hole in the virtual memory map */
+#define STACKLEAK_POISON -0xBEEF
+#define STACKLEAK_POISON_CHECK_DEPTH 128
+
 #endif /* __LINUX_COMPILER_H */
-- 
2.7.4

Re: [PATCH RFC v10 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Dave Hansen]

On 03/28/2018 12:57 PM, Alexander Popov wrote:
> +.macro ERASE_KSTACK
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +	PUSH_AND_CLEAR_REGS
> +	call erase_kstack
> +	POP_REGS
> +#endif
> +.endm

Thanks again for the hard work to trim down the complexity of the
assembly.  I noticed the 64-bit version saves/restores registers while
the 32-bit version doesn't.  What's the reasoning there?

Re: [PATCH RFC v10 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Alexander Popov]

On 29.03.2018 01:55, Dave Hansen wrote:
> On 03/28/2018 12:57 PM, Alexander Popov wrote:
>> +.macro ERASE_KSTACK
>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>> +	PUSH_AND_CLEAR_REGS
>> +	call erase_kstack
>> +	POP_REGS
>> +#endif
>> +.endm
> 
> Thanks again for the hard work to trim down the complexity of the
> assembly. 

Hello Dave, thanks!

> I noticed the 64-bit version saves/restores registers while
> the 32-bit version doesn't.  What's the reasoning there?

When erase_kstack() is called from the trampoline stack, it must save and
restore any modified registers, since all registers except RDI are live
(prepared for the userspace).

When erase_kstack() is called from the thread stack, it can clobber registers
according the function call convention without any harm.

Best regards,
Alexander

Re: [PATCH RFC v10 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Dave Hansen]

On 03/28/2018 11:58 PM, Alexander Popov wrote:
>> I noticed the 64-bit version saves/restores registers while
>> the 32-bit version doesn't.  What's the reasoning there?
> When erase_kstack() is called from the trampoline stack, it must save and
> restore any modified registers, since all registers except RDI are live
> (prepared for the userspace).
> 
> When erase_kstack() is called from the thread stack, it can clobber registers
> according the function call convention without any harm.

Oh, and since there's no 32-bit trampoline stack, we don't need it on
32-bit?

If end up reposting this set again, could you add a few comments about
this around the ERASE_KSTACK macro definitions, or perhaps the call
sites?  You might even want to call them ERASE_KSTACK_CLOBBER (for
32-bit) and ERASE_KSTACK_NOCLOBBER (for 64-bit) to make this more clear.

Re: [PATCH RFC v10 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Alexander Popov]

Hello Dave and Boris,

Thanks for your replies!

On 29.03.2018 18:09, Boris Lukashev wrote:
> On Thu, Mar 29, 2018 at 9:51 AM, Dave Hansen
> <dave.hansen@linux.intel.com> wrote:
>> On 03/28/2018 11:58 PM, Alexander Popov wrote:
>>>> I noticed the 64-bit version saves/restores registers while
>>>> the 32-bit version doesn't.  What's the reasoning there?
>>> When erase_kstack() is called from the trampoline stack, it must save and
>>> restore any modified registers, since all registers except RDI are live
>>> (prepared for the userspace).
>>>
>>> When erase_kstack() is called from the thread stack, it can clobber registers
>>> according the function call convention without any harm.
>>
>> Oh, and since there's no 32-bit trampoline stack, we don't need it on
>> 32-bit?
>>
>> If end up reposting this set again,

Hope so. Let's see...

>> could you add a few comments about
>> this around the ERASE_KSTACK macro definitions, or perhaps the call
>> sites?  You might even want to call them ERASE_KSTACK_CLOBBER (for
>> 32-bit) and ERASE_KSTACK_NOCLOBBER (for 64-bit) to make this more clear.
> 
> Not sure if the macro name differentiation is such a good idea, might
> entice improper use attempts.
> A more detailed explanation of this should probably go into the
> headers and doc/commit log for future implementation on architectures
> which may have their own weird semantics around the trampoline
> stack/not have one.

Ok, I see. Let me give the overview and propose the solution.

The current version has 3 separate ERASE_KSTACK definitions:

1. a simple one in entry_32.S, used only in that file:

+.macro ERASE_KSTACK
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	call erase_kstack
+#endif
+.endm


2. another one saving registers in entry_64.S, used only in that file for
erasing from the trampoline stack:

+.macro ERASE_KSTACK
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	PUSH_AND_CLEAR_REGS
+	call erase_kstack
+	POP_REGS
+#endif
+.endm

The call sights are already prepared and documented by Andy Lutomirski:

	/*
 	 * We are on the trampoline stack.  All regs except RDI are live.
 	 * We can do future final exit work right here.
 	 */
+	ERASE_KSTACK


3. a simple one in entry_64_compat.S (similar to case 1), used only in that file:

+	.macro ERASE_KSTACK
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	call erase_kstack
+#endif
+	.endm
+

The call sight is documented as well:

 sysret32_from_system_call:
+	/*
+	 * We are not going to return to the userspace from the trampoline
+	 * stack. So let's erase the thread stack right now.
+	 */
+	ERASE_KSTACK


If STACKLEAK is not banned, would you like me to introduce ERASE_KSTACK (for
cases 1 and 3) and ERASE_KSTACK_NOCLOBBER (for case 2) in
arch/x86/entry/calling.h?

Best regards,
Alexander

Re: [PATCH RFC v10 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Dave Hansen]

On 03/29/2018 01:56 PM, Alexander Popov wrote:
> 
> If STACKLEAK is not banned, would you like me to introduce ERASE_KSTACK (for
> cases 1 and 3) and ERASE_KSTACK_NOCLOBBER (for case 2) in
> arch/x86/entry/calling.h?

Yes, please.  But, whatever you do, I think having each name have
constant behavior is a must.  Please don't have "ERASE_KSTACK" in one
file clobber and another not clobber (even if one is 32-bit and the
other is not).

Re: [PATCH RFC v10 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Alexander Popov]

On 29.03.2018 21:38, Shivappa Vikas wrote:
> On Wed, 28 Mar 2018, Alexander Popov wrote:
> 
>> +	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);
>> +
>> +	/*
>> +	 * So let's write the poison value to the kernel stack.
>> +	 * Start from the address in p and move up till the new boundary.
>> +	 */
>> +	if (on_thread_stack())
>> +		boundary = current_stack_pointer;
>> +	else
>> +		boundary = current_top_of_stack();
>> +
>> +	BUG_ON(boundary - p >= THREAD_SIZE);
>> +
>> +	while (p < boundary) {
>> +		*(unsigned long *)p = STACKLEAK_POISON;
>> +		p += sizeof(unsigned long);
>> +	}
> 
> Sorry catching up late on this. 

Hello Vikas,

You are just in time! Thank you for looking at the code.

> can we use a stosq or something here ? wondering 
> if that helps get more performance. since you seem to copy the whole stack with 
> the poison value. If that was already discussed sorry for the spam :)

Yes, the previous version of the patch series had this function written in
assembly. It used scasq for the poison searching and then stosq for writing:
http://www.openwall.com/lists/kernel-hardening/2018/03/03/9

That amount of assembly was blamed by the maintainers. So I've done my best to
rework it in C bypassing the pitfalls:
http://www.openwall.com/lists/kernel-hardening/2018/03/21/4

In fact, this implementation is not much slower than the assembly one, since we
erase only the used part of the thread stack. This is achieved by the
lowest_stack variable, which is updated during the syscall (please see the next
patch of the series).

Best regards,
Alexander

[PATCH RFC v10 3/6] gcc-plugins: Add STACKLEAK plugin for tracking the kernel stack

[Alexander Popov]

The STACKLEAK feature erases the kernel stack before returning from
syscalls. That reduces the information which kernel stack leak bugs can
reveal and blocks some uninitialized stack variable attacks. Moreover,
STACKLEAK provides runtime checks for kernel stack overflow detection.

This commit introduces the STACKLEAK gcc plugin. It is needed for:
 - tracking the lowest border of the kernel stack, which is important
    for the code erasing the used part of the kernel stack at the end
    of syscalls (comes in a separate commit);
 - checking that alloca calls don't cause stack overflow.

So this plugin instruments the kernel code inserting:
 - the check_alloca() call before alloca and the track_stack() call
    after it;
 - the track_stack() call for the functions with a stack frame size
    greater than or equal to CONFIG_STACKLEAK_TRACK_MIN_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                           |  14 +
 arch/x86/kernel/dumpstack.c            |  19 ++
 mm/util.c                              |  33 +++
 scripts/Makefile.gcc-plugins           |   3 +
 scripts/gcc-plugins/stackleak_plugin.c | 470 +++++++++++++++++++++++++++++++++
 5 files changed, 539 insertions(+)
 create mode 100644 scripts/gcc-plugins/stackleak_plugin.c

diff --git a/arch/Kconfig b/arch/Kconfig
index 368e2fb..a4a8fba 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -542,6 +542,8 @@ config GCC_PLUGIN_STACKLEAK
 	bool "Erase the kernel stack before returning from syscalls"
 	depends on GCC_PLUGINS
 	depends on HAVE_ARCH_STACKLEAK
+	imply VMAP_STACK
+	imply SCHED_STACK_END_CHECK
 	help
 	  This option makes the kernel erase the kernel stack before it
 	  returns from a system call. That reduces the information which
@@ -558,6 +560,18 @@ config GCC_PLUGIN_STACKLEAK
 	   * 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/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index a2d8a39..97ed2a8 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -375,3 +375,22 @@ static int __init code_bytes_setup(char *s)
 	return 1;
 }
 __setup("code_bytes=", code_bytes_setup);
+
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+#define MIN_STACK_LEFT 256
+
+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 < MIN_STACK_LEFT ||
+				size >= stack_left - MIN_STACK_LEFT);
+}
+EXPORT_SYMBOL(check_alloca);
+#endif
diff --git a/mm/util.c b/mm/util.c
index c125050..f846015 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -757,3 +757,36 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen)
 out:
 	return res;
 }
+
+#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;
+
+	/*
+	 * Having CONFIG_STACKLEAK_TRACK_MIN_SIZE larger than
+	 * STACKLEAK_POISON_CHECK_DEPTH makes the poison search in
+	 * erase_kstack() unreliable. Let's prevent that.
+	 */
+	BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MIN_SIZE >
+						STACKLEAK_POISON_CHECK_DEPTH);
+
+	if (sp < current->thread.lowest_stack &&
+	    sp >= (unsigned long)task_stack_page(current) +
+					sizeof(unsigned long)) {
+		current->thread.lowest_stack = sp;
+	}
+}
+EXPORT_SYMBOL(track_stack);
+#endif /* CONFIG_GCC_PLUGIN_STACKLEAK */
diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
index b2a95af..8d6070f 100644
--- a/scripts/Makefile.gcc-plugins
+++ b/scripts/Makefile.gcc-plugins
@@ -35,6 +35,9 @@ 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..6ac2a05
--- /dev/null
+++ b/scripts/gcc-plugins/stackleak_plugin.c
@@ -0,0 +1,470 @@
+/*
+ * Copyright 2011-2017 by the PaX Team <pageexec@freemail.hu>
+ * Modified by Alexander Popov <alex.popov@linux.com>
+ * 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 the 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(),
+ *     print_rtl() and print_simple_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"
+		"disable\t\tdo not activate the plugin\n"
+};
+
+static void stackleak_add_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);
+}
+
+static void stackleak_add_track_stack(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);
+}
+
+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_instrument_execute(void)
+{
+	basic_block bb, entry_bb;
+	bool prologue_instrumented = false, is_leaf = true;
+	gimple_stmt_iterator gsi;
+
+	/*
+	 * 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) {
+		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;
+
+			if (!is_alloca(stmt))
+				continue;
+
+			/* 2. Insert stack overflow check before alloca call */
+			stackleak_add_check_alloca(&gsi);
+
+			/* 3. Insert track_stack() call after alloca call */
+			stackleak_add_track_stack(&gsi, true);
+			if (bb == entry_bb)
+				prologue_instrumented = true;
+		}
+	}
+
+	if (prologue_instrumented)
+		return 0;
+
+	/*
+	 * Special cases to skip the instrumentation.
+	 *
+	 * Taking the address of static inline functions materializes 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;
+	}
+
+	/* 4. Insert track_stack() call at the function beginning */
+	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_track_stack(&gsi, false);
+
+	return 0;
+}
+
+static bool large_stack_frame(void)
+{
+#if BUILDING_GCC_VERSION >= 8000
+	return maybe_ge(get_frame_size(), track_frame_size);
+#else
+	return (get_frame_size() >= track_frame_size);
+#endif
+}
+
+/*
+ * Work with the RTL representation of the code.
+ * Remove the unneeded track_stack() calls from the functions which don't
+ * call alloca and don't have a large enough stack frame size.
+ */
+static unsigned int stackleak_cleanup_execute(void)
+{
+	rtx_insn *insn, *next;
+
+	if (cfun->calls_alloca)
+		return 0;
+
+	if (large_stack_frame())
+		return 0;
+
+	/*
+	 * 1. Find track_stack() calls. Loop through the chain of insns,
+	 * which is an RTL representation of the code for a function.
+	 *
+	 * The example of a matching insn:
+	 *    (call_insn 8 4 10 2 (call (mem (symbol_ref ("track_stack")
+	 *    [flags 0x41] <function_decl 0x7f7cd3302a80 track_stack>)
+	 *    [0 track_stack S1 A8]) (0)) 675 {*call} (expr_list
+	 *    (symbol_ref ("track_stack") [flags 0x41] <function_decl
+	 *    0x7f7cd3302a80 track_stack>) (expr_list (0) (nil))) (nil))
+	 */
+	for (insn = get_insns(); insn; insn = next) {
+		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 the track_stack() call */
+		delete_insn_and_edges(insn);
+#if BUILDING_GCC_VERSION >= 4007 && BUILDING_GCC_VERSION < 8000
+		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
+	}
+
+	return 0;
+}
+
+static bool stackleak_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_instrument_gate(void)
+{
+	return stackleak_gate();
+}
+
+#define PASS_NAME stackleak_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_cleanup_gate(void)
+{
+	return stackleak_gate();
+}
+
+#define PASS_NAME stackleak_cleanup
+#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 = 0;
+
+	/* 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_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_instrument, "optimized", 1,
+						PASS_POS_INSERT_BEFORE);
+
+	/*
+	 * The stackleak_cleanup pass should be executed after the
+	 * "reload" pass, when the stack frame size is final.
+	 */
+	PASS_INFO(stackleak_cleanup, "reload", 1, PASS_POS_INSERT_AFTER);
+
+	if (!plugin_default_version_check(version, &gcc_version)) {
+		error(G_("incompatible gcc/plugin versions"));
+		return 1;
+	}
+
+	/* Parse the plugin arguments */
+	for (i = 0; i < argc; i++) {
+		if (!strcmp(argv[i].key, "disable"))
+			return 0;
+
+		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);
+				return 1;
+			}
+
+			track_frame_size = atoi(argv[i].value);
+			if (track_frame_size < 0) {
+				error(G_("invalid option argument '-fplugin-arg-%s-%s=%s'"),
+					plugin_name, argv[i].key, argv[i].value);
+				return 1;
+			}
+		} else {
+			error(G_("unknown option '-fplugin-arg-%s-%s'"),
+					plugin_name, argv[i].key);
+			return 1;
+		}
+	}
+
+	/* Give the information about the plugin */
+	register_callback(plugin_name, PLUGIN_INFO, NULL,
+						&stackleak_plugin_info);
+
+	/* 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_instrument_pass_info);
+	register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL,
+					&stackleak_cleanup_pass_info);
+
+	return 0;
+}
-- 
2.7.4

[PATCH RFC v10 4/6] lkdtm: Add a test for STACKLEAK

[Alexander Popov]

Introduce two lkdtm tests for the STACKLEAK feature: STACKLEAK_ALLOCA
and STACKLEAK_DEEP_RECURSION. Both of them check that the current task
stack is properly erased (filled with STACKLEAK_POISON).

STACKLEAK_ALLOCA tests that:
 - check_alloca() allows alloca calls which don't exhaust the kernel stack;
 - alloca calls which exhaust/overflow the kernel stack hit BUG() in
    check_alloca().

STACKLEAK_DEEP_RECURSION tests that exhausting the current task stack
with a deep recursion is detected by CONFIG_VMAP_STACK (which is implied
by CONFIG_GCC_PLUGIN_STACKLEAK).

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Signed-off-by: Alexander Popov <alex.popov@linux.com>
---
 drivers/misc/Makefile          |   3 +
 drivers/misc/lkdtm.h           |   4 ++
 drivers/misc/lkdtm_core.c      |   2 +
 drivers/misc/lkdtm_stackleak.c | 141 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 150 insertions(+)
 create mode 100644 drivers/misc/lkdtm_stackleak.c

diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
index c3c8624..2b11823 100644
--- a/drivers/misc/Makefile
+++ b/drivers/misc/Makefile
@@ -65,6 +65,9 @@ lkdtm-$(CONFIG_LKDTM)		+= lkdtm_perms.o
 lkdtm-$(CONFIG_LKDTM)		+= lkdtm_refcount.o
 lkdtm-$(CONFIG_LKDTM)		+= lkdtm_rodata_objcopy.o
 lkdtm-$(CONFIG_LKDTM)		+= lkdtm_usercopy.o
+lkdtm-$(CONFIG_LKDTM)		+= lkdtm_stackleak.o
+
+KASAN_SANITIZE_lkdtm_stackleak.o := n
 
 KCOV_INSTRUMENT_lkdtm_rodata.o	:= n
 
diff --git a/drivers/misc/lkdtm.h b/drivers/misc/lkdtm.h
index 9e513dc..4b2b8e3 100644
--- a/drivers/misc/lkdtm.h
+++ b/drivers/misc/lkdtm.h
@@ -83,4 +83,8 @@ void lkdtm_USERCOPY_STACK_FRAME_FROM(void);
 void lkdtm_USERCOPY_STACK_BEYOND(void);
 void lkdtm_USERCOPY_KERNEL(void);
 
+/* lkdtm_stackleak.c */
+void lkdtm_STACKLEAK_ALLOCA(void);
+void lkdtm_STACKLEAK_DEEP_RECURSION(void);
+
 #endif
diff --git a/drivers/misc/lkdtm_core.c b/drivers/misc/lkdtm_core.c
index 2154d1b..c37fd85 100644
--- a/drivers/misc/lkdtm_core.c
+++ b/drivers/misc/lkdtm_core.c
@@ -183,6 +183,8 @@ static const struct crashtype crashtypes[] = {
 	CRASHTYPE(USERCOPY_STACK_FRAME_FROM),
 	CRASHTYPE(USERCOPY_STACK_BEYOND),
 	CRASHTYPE(USERCOPY_KERNEL),
+	CRASHTYPE(STACKLEAK_ALLOCA),
+	CRASHTYPE(STACKLEAK_DEEP_RECURSION),
 };
 
 
diff --git a/drivers/misc/lkdtm_stackleak.c b/drivers/misc/lkdtm_stackleak.c
new file mode 100644
index 0000000..4706595
--- /dev/null
+++ b/drivers/misc/lkdtm_stackleak.c
@@ -0,0 +1,141 @@
+/*
+ * This code tests several aspects of the STACKLEAK feature:
+ *  - the current task stack is properly erased (filled with STACKLEAK_POISON);
+ *  - check_alloca() allows alloca calls which don't exhaust the kernel stack;
+ *  - alloca calls which exhaust/overflow the kernel stack hit BUG() in
+ *     check_alloca();
+ *  - exhausting the current task stack with a deep recursion is detected by
+ *     CONFIG_VMAP_STACK (which is implied by CONFIG_GCC_PLUGIN_STACKLEAK).
+ *
+ * Authors:
+ *   Tycho Andersen <tycho@tycho.ws>
+ *   Alexander Popov <alex.popov@linux.com>
+ */
+
+#include "lkdtm.h"
+#include <linux/sched.h>
+#include <linux/compiler.h>
+
+#ifndef CONFIG_STACKLEAK_TRACK_MIN_SIZE
+# define CONFIG_STACKLEAK_TRACK_MIN_SIZE 0
+#endif
+
+static noinline bool stack_is_erased(void)
+{
+	unsigned long *sp, left, found, i;
+	const unsigned long check_depth = STACKLEAK_POISON_CHECK_DEPTH /
+							sizeof(unsigned long);
+
+	/*
+	 * For the details about the alignment of the poison values, see
+	 * the comment in track_stack().
+	 */
+	sp = PTR_ALIGN(&i, sizeof(unsigned long));
+
+	left = ((unsigned long)sp & (THREAD_SIZE - 1)) / sizeof(unsigned long);
+	sp--;
+
+	/*
+	 * One long int at the bottom of the thread stack is reserved
+	 * and not poisoned.
+	 */
+	if (left > 1)
+		left--;
+	else
+		return false;
+
+	pr_info("checking unused part of the thread stack (%lu bytes)...\n",
+					left * sizeof(unsigned long));
+
+	/*
+	 * Search for check_depth poison values in a row (just like
+	 * erase_kstack() does).
+	 */
+	for (i = 0, found = 0; i < left && found <= check_depth; i++) {
+		if (*(sp - i) == STACKLEAK_POISON)
+			found++;
+		else
+			found = 0;
+	}
+
+	if (found <= check_depth) {
+		pr_err("FAIL: thread stack is not erased (checked %lu bytes)\n",
+						i * sizeof(unsigned long));
+		return false;
+	}
+
+	pr_info("first %lu bytes are unpoisoned\n",
+				(i - found) * sizeof(unsigned long));
+
+	/* The rest of thread stack should be erased */
+	for (; i < left; i++) {
+		if (*(sp - i) != STACKLEAK_POISON) {
+			pr_err("FAIL: thread stack is NOT properly erased\n");
+			return false;
+		}
+	}
+
+	pr_info("the rest of the thread stack is properly erased\n");
+	return true;
+}
+
+static noinline void do_alloca(unsigned long size)
+{
+	char buf[size];
+
+	/* So this doesn't get inlined or optimized out */
+	snprintf(buf, size, "testing alloca...\n");
+}
+
+void lkdtm_STACKLEAK_ALLOCA(void)
+{
+	unsigned long left = (unsigned long)&left & (THREAD_SIZE - 1);
+
+	if (!stack_is_erased())
+		return;
+
+	/* Try a small alloca to see if it works */
+	pr_info("try a small alloca of 16 bytes...\n");
+	do_alloca(16);
+	pr_info("small alloca is successful\n");
+
+	/* Try to hit the BUG() in check_alloca() */
+	pr_info("try a large alloca of %lu bytes (stack overflow)...\n", left);
+	do_alloca(left);
+	pr_err("FAIL: large alloca overstepped the thread stack boundary\n");
+}
+
+/*
+ * The stack frame size of recursion() is bigger than the
+ * CONFIG_STACKLEAK_TRACK_MIN_SIZE, hence that function is instrumented
+ * by the STACKLEAK gcc plugin and it calls track_stack() at the beginning.
+ */
+static noinline unsigned long recursion(unsigned long prev_sp)
+{
+	char buf[CONFIG_STACKLEAK_TRACK_MIN_SIZE + 42];
+	unsigned long sp = (unsigned long)&sp;
+
+	snprintf(buf, sizeof(buf), "testing deep recursion...\n");
+
+	if (prev_sp < sp + THREAD_SIZE)
+		sp = recursion(prev_sp);
+
+	return sp;
+}
+
+void lkdtm_STACKLEAK_DEEP_RECURSION(void)
+{
+	unsigned long sp = (unsigned long)&sp;
+
+	if (!stack_is_erased())
+		return;
+
+	/*
+	 * Exhaust the thread stack with a deep recursion. It should hit the
+	 * guard page provided by CONFIG_VMAP_STACK (which is implied by
+	 * CONFIG_GCC_PLUGIN_STACKLEAK).
+	 */
+	pr_info("try to exhaust the thread stack with a deep recursion...\n");
+	pr_err("FAIL: thread stack exhaustion (%lu bytes) is not detected\n",
+							sp - recursion(sp));
+}
-- 
2.7.4

[PATCH RFC v10 5/6] fs/proc: Show STACKLEAK metrics in the /proc file system

[Alexander Popov]

Introduce CONFIG_STACKLEAK_METRICS providing STACKLEAK information about
tasks via the /proc file system. In particular, /proc/<pid>/stack_depth
shows the maximum kernel stack consumption for the current and previous
syscalls. Although this information is not precise, it  can be useful for
estimating the STACKLEAK performance impact for your workloads.

Signed-off-by: Alexander Popov <alex.popov@linux.com>
---
 arch/Kconfig                     | 12 ++++++++++++
 arch/x86/entry/erase.c           |  4 ++++
 arch/x86/include/asm/processor.h |  3 +++
 arch/x86/kernel/process_32.c     |  3 +++
 arch/x86/kernel/process_64.c     |  3 +++
 fs/proc/base.c                   | 18 ++++++++++++++++++
 6 files changed, 43 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index a4a8fba..42ebfb9 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -572,6 +572,18 @@ config STACKLEAK_TRACK_MIN_SIZE
 	  frame size greater than or equal to this parameter.
 	  If unsure, leave the default value 100.
 
+config STACKLEAK_METRICS
+	bool "Show STACKLEAK metrics in the /proc file system"
+	depends on GCC_PLUGIN_STACKLEAK
+	depends on PROC_FS
+	help
+	  If this is set, STACKLEAK metrics for every task are available in
+	  the /proc file system. In particular, /proc/<pid>/stack_depth
+	  shows the maximum kernel stack consumption for the current and
+	  previous syscalls. Although this information is not precise, it
+	  can be useful for estimating the STACKLEAK performance impact for
+	  your workloads.
+
 config HAVE_CC_STACKPROTECTOR
 	bool
 	help
diff --git a/arch/x86/entry/erase.c b/arch/x86/entry/erase.c
index 4892335..be09cc6 100644
--- a/arch/x86/entry/erase.c
+++ b/arch/x86/entry/erase.c
@@ -32,6 +32,10 @@ asmlinkage void erase_kstack(void)
 	if (p == boundary)
 		p += sizeof(unsigned long);
 
+#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.
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 0c87813..bca1074 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -496,6 +496,9 @@ struct thread_struct {
 
 #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
 	unsigned long		lowest_stack;
+# ifdef CONFIG_STACKLEAK_METRICS
+	unsigned long		prev_lowest_stack;
+# endif
 #endif
 
 	unsigned int		sig_on_uaccess_err:1;
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 1b0892e..577c104 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -139,6 +139,9 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
 #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
 	p->thread.lowest_stack = (unsigned long)task_stack_page(p) +
 						sizeof(unsigned long);
+# ifdef CONFIG_STACKLEAK_METRICS
+	p->thread.prev_lowest_stack = p->thread.lowest_stack;
+# endif
 #endif
 
 	if (unlikely(p->flags & PF_KTHREAD)) {
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 82122af..7a82f0d 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -284,6 +284,9 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
 #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
 	p->thread.lowest_stack = (unsigned long)task_stack_page(p) +
 						sizeof(unsigned long);
+# ifdef CONFIG_STACKLEAK_METRICS
+	p->thread.prev_lowest_stack = p->thread.lowest_stack;
+# endif
 #endif
 
 	savesegment(gs, p->thread.gsindex);
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 9298324..6a7f9bd 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2914,6 +2914,21 @@ static int proc_pid_patch_state(struct seq_file *m, struct pid_namespace *ns,
 }
 #endif /* CONFIG_LIVEPATCH */
 
+#ifdef CONFIG_STACKLEAK_METRICS
+static int proc_stack_depth(struct seq_file *m, struct pid_namespace *ns,
+				struct pid *pid, struct task_struct *task)
+{
+	unsigned long prev_depth = THREAD_SIZE -
+			(task->thread.prev_lowest_stack & (THREAD_SIZE - 1));
+	unsigned long depth = THREAD_SIZE -
+			(task->thread.lowest_stack & (THREAD_SIZE - 1));
+
+	seq_printf(m, "previous stack depth: %lu\nstack depth: %lu\n",
+							prev_depth, depth);
+	return 0;
+}
+#endif /* CONFIG_STACKLEAK_METRICS */
+
 /*
  * Thread groups
  */
@@ -3018,6 +3033,9 @@ static const struct pid_entry tgid_base_stuff[] = {
 #ifdef CONFIG_LIVEPATCH
 	ONE("patch_state",  S_IRUSR, proc_pid_patch_state),
 #endif
+#ifdef CONFIG_STACKLEAK_METRICS
+	ONE("stack_depth", S_IRUGO, proc_stack_depth),
+#endif
 };
 
 static int proc_tgid_base_readdir(struct file *file, struct dir_context *ctx)
-- 
2.7.4

[PATCH RFC v10 6/6] doc: self-protection: Add information about STACKLEAK feature

[Alexander Popov]

Add information about STACKLEAK feature to "Stack depth overflow" and
"Memory poisoning" sections of self-protection.rst.

Signed-off-by: Alexander Popov <alex.popov@linux.com>
---
 Documentation/security/self-protection.rst | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/Documentation/security/self-protection.rst b/Documentation/security/self-protection.rst
index 0f53826..b685f18 100644
--- a/Documentation/security/self-protection.rst
+++ b/Documentation/security/self-protection.rst
@@ -165,10 +165,15 @@ Stack depth overflow
 A less well understood attack is using a bug that triggers the
 kernel to consume stack memory with deep function calls or large stack
 allocations. With this attack it is possible to write beyond the end of
-the kernel's preallocated stack space and into sensitive structures. Two
-important changes need to be made for better protections: moving the
-sensitive thread_info structure elsewhere, and adding a faulting memory
-hole at the bottom of the stack to catch these overflows.
+the kernel's preallocated stack space and into sensitive structures.
+The combination of the following measures gives better protection:
+
+* moving the sensitive thread_info structure off the stack
+  (``CONFIG_THREAD_INFO_IN_TASK``);
+* adding a faulting memory hole at the bottom of the stack to catch
+  these overflows (``CONFIG_VMAP_STACK``);
+* runtime checking that alloca() calls don't overstep the stack boundary
+  (``CONFIG_GCC_PLUGIN_STACKLEAK``).
 
 Heap memory integrity
 ---------------------
@@ -302,11 +307,11 @@ sure structure holes are cleared.
 Memory poisoning
 ----------------
 
-When releasing memory, it is best to poison the contents (clear stack on
-syscall return, wipe heap memory on a free), to avoid reuse attacks that
-rely on the old contents of memory. This frustrates many uninitialized
-variable attacks, stack content exposures, heap content exposures, and
-use-after-free attacks.
+When releasing memory, it is best to poison the contents, to avoid reuse
+attacks that rely on the old contents of memory. E.g., clear stack on a
+syscall return (``CONFIG_GCC_PLUGIN_STACKLEAK``), wipe heap memory on a
+free. This frustrates many uninitialized variable attacks, stack content
+exposures, heap content exposures, and use-after-free attacks.
 
 Destination tracking
 --------------------
-- 
2.7.4

Re: [PATCH RFC v10 0/6] Introduce the STACKLEAK feature and a test for it

[Kees Cook]

On Wed, Mar 28, 2018 at 12:57 PM, Alexander Popov <alex.popov@linux.com> wrote:
> This is the 10th version of the patch series introducing STACKLEAK to the
> mainline kernel. The previous version raised a fervent discussion[0].
> The assembly code introduced by v9 irritated the reviewers.

Thanks for persisting!

> I've found the way to bypass the obstacles[1] of the C implementation.
> So I dare come once again. Let me ask you to look at this code without
> preconception.

The assembly changes are now very minimal; thanks for reworking this.
I hope this addresses both Dave Hansen and Linus's (similar)
objections.

>  1. reduces the information that can be revealed through kernel stack leak bugs.
>     The idea of erasing the thread stack at the end of syscalls is similar to
>     CONFIG_PAGE_POISONING and memzero_explicit() in kernel crypto, which all
>     comply with FDP_RIP.2 (Full Residual Information Protection) of the
>     Common Criteria standard.

Agreed: I continue to believe this is meaningful even if just for
reducing the lifetime of sensitive data on the stack.

-Kees

-- 
Kees Cook
Pixel Security