[PATCH v12 0/6] Introduce the STACKLEAK feature and a test for it

[PATCH v12 0/6] Introduce the STACKLEAK feature and a test for it

[Alexander Popov]

This is the 12th version of the patch series introducing STACKLEAK to the
mainline kernel for x86. Some code is made common for easier porting to
arm64 (will be done by Laura Abbott).

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".

Changes in v12
==============

1. Some code is made common for easier porting to other platforms.
    Also introduced lowest_stack structure according to Kees' feedback.

2. Changes according to the feedback from Mark Rutland (kudos!):
    - improved stack depth overflow detection and reporting in check_alloca();
    - disabled KCOV instrumentation for erase_kstack() and track_stack();
    - added comments with assumptions about the compiler behaviour in
       erase_kstack().

3. Added a new STACKLEAK_RECURSION_WITH_ALLOCA test.

4. Included Laura's patch for the RTL traversal in the STACKLEAK gcc plugin.

5. Added missing SPDX-License-Identifiers.

Previous version: http://www.openwall.com/lists/kernel-hardening/2018/04/06/2


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/calling.h                   |  14 +
 arch/x86/entry/entry_32.S                  |   7 +
 arch/x86/entry/entry_64.S                  |   3 +
 arch/x86/entry/entry_64_compat.S           |   5 +
 arch/x86/include/asm/processor.h           |   3 +
 arch/x86/kernel/dumpstack.c                |  31 ++
 arch/x86/kernel/process_32.c               |   8 +
 arch/x86/kernel/process_64.c               |   8 +
 drivers/misc/lkdtm/Makefile                |   2 +
 drivers/misc/lkdtm/core.c                  |   3 +
 drivers/misc/lkdtm/lkdtm.h                 |   5 +
 drivers/misc/lkdtm/stackleak.c             | 147 +++++++++
 fs/proc/base.c                             |  18 ++
 include/linux/stackleak.h                  |  22 ++
 kernel/Makefile                            |   4 +
 kernel/stackleak.c                         | 106 +++++++
 scripts/Makefile.gcc-plugins               |   3 +
 scripts/gcc-plugins/gcc-common.h           |  26 +-
 scripts/gcc-plugins/stackleak_plugin.c     | 474 +++++++++++++++++++++++++++++
 23 files changed, 949 insertions(+), 19 deletions(-)
 create mode 100644 drivers/misc/lkdtm/stackleak.c
 create mode 100644 include/linux/stackleak.h
 create mode 100644 kernel/stackleak.c
 create mode 100644 scripts/gcc-plugins/stackleak_plugin.c

-- 
2.7.4

[PATCH v12 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 v12 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 blocks kernel stack depth overflow caused by alloca (aka
Stack Clash attack).

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>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
---
 Documentation/x86/x86_64/mm.txt  |  2 ++
 arch/Kconfig                     | 27 +++++++++++++++
 arch/x86/Kconfig                 |  1 +
 arch/x86/entry/calling.h         | 14 ++++++++
 arch/x86/entry/entry_32.S        |  7 ++++
 arch/x86/entry/entry_64.S        |  3 ++
 arch/x86/entry/entry_64_compat.S |  5 +++
 arch/x86/include/asm/processor.h |  3 ++
 arch/x86/kernel/process_32.c     |  5 +++
 arch/x86/kernel/process_64.c     |  5 +++
 include/linux/stackleak.h        | 19 +++++++++++
 kernel/Makefile                  |  4 +++
 kernel/stackleak.c               | 72 ++++++++++++++++++++++++++++++++++++++++
 13 files changed, 167 insertions(+)
 create mode 100644 include/linux/stackleak.h
 create mode 100644 kernel/stackleak.c

diff --git a/Documentation/x86/x86_64/mm.txt b/Documentation/x86/x86_64/mm.txt
index 5432a96..600bc2a 100644
--- a/Documentation/x86/x86_64/mm.txt
+++ b/Documentation/x86/x86_64/mm.txt
@@ -24,6 +24,7 @@ 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
 
 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 8e0d665..a4d7794 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
+	  returning from system calls. That reduces the information which
+	  kernel stack leak bugs can reveal and blocks some uninitialized
+	  stack variable attacks. This option also blocks kernel stack depth
+	  overflow caused by alloca (aka Stack Clash attack).
+
+	  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 c07f492..74b8f5c 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -121,6 +121,7 @@ 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/calling.h b/arch/x86/entry/calling.h
index 352e70c..43c79e7 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -329,8 +329,22 @@ For 32-bit we have the following conventions - kernel is built with
 
 #endif
 
+.macro ERASE_KSTACK_NOCLOBBER
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	PUSH_AND_CLEAR_REGS
+	call erase_kstack
+	POP_REGS
+#endif
+.endm
+
 #endif /* CONFIG_X86_64 */
 
+.macro ERASE_KSTACK
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	call erase_kstack
+#endif
+.endm
+
 /*
  * This does 'call enter_from_user_mode' unless we can avoid it based on
  * kernel config or using the static jump infrastructure.
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index bef8e2b..bb4f540 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -46,6 +46,8 @@
 #include <asm/frame.h>
 #include <asm/nospec-branch.h>
 
+#include "calling.h"
+
 	.section .entry.text, "ax"
 
 /*
@@ -298,6 +300,7 @@ 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 +461,8 @@ 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 +549,8 @@ 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 3166b96..c9648b2 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -329,6 +329,8 @@ 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_NOCLOBBER
+
 	SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
 
 	popq	%rdi
@@ -687,6 +689,7 @@ 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_NOCLOBBER
 
 	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 9de7f1e..2d10f72 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -261,6 +261,11 @@ 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/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 21a1149..7729996 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -32,6 +32,7 @@ struct vm86;
 #include <linux/err.h>
 #include <linux/irqflags.h>
 #include <linux/mem_encrypt.h>
+#include <linux/stackleak.h>
 
 /*
  * We handle most unaligned accesses in hardware.  On the other hand
@@ -504,6 +505,8 @@ struct thread_struct {
 
 	mm_segment_t		addr_limit;
 
+	struct lowest_stack	lowest_stack;
+
 	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..a19ea44 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.val = (unsigned long)end_of_stack(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 4b100fe..f7412c6 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -295,6 +295,11 @@ 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.val = (unsigned long)end_of_stack(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/stackleak.h b/include/linux/stackleak.h
new file mode 100644
index 0000000..458c73b
--- /dev/null
+++ b/include/linux/stackleak.h
@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_STACKLEAK_H
+#define _LINUX_STACKLEAK_H
+
+/*
+ * Check that the poison value points to the unused hole in the
+ * virtual memory map for your platform.
+ */
+#define STACKLEAK_POISON -0xBEEF
+
+#define STACKLEAK_POISON_CHECK_DEPTH 128
+
+struct lowest_stack {
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	unsigned long		val;
+#endif
+};
+
+#endif
diff --git a/kernel/Makefile b/kernel/Makefile
index f85ae5d..a530f77 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -114,6 +114,10 @@ obj-$(CONFIG_TORTURE_TEST) += torture.o
 
 obj-$(CONFIG_HAS_IOMEM) += memremap.o
 
+obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += stackleak.o
+KASAN_SANITIZE_stackleak.o := n
+KCOV_INSTRUMENT_stackleak.o := n
+
 $(obj)/configs.o: $(obj)/config_data.h
 
 targets += config_data.gz
diff --git a/kernel/stackleak.c b/kernel/stackleak.c
new file mode 100644
index 0000000..2ebfbaa
--- /dev/null
+++ b/kernel/stackleak.c
@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This code fills the used part of the kernel stack with a poison value
+ * before returning to the userspace. It's a part of the STACKLEAK feature
+ * ported from grsecurity/PaX.
+ *
+ * Author: Alexander Popov <alex.popov@linux.com>
+ *
+ * STACKLEAK reduces the information which kernel stack leak bugs can
+ * reveal and blocks some uninitialized stack variable attacks. Moreover,
+ * STACKLEAK blocks stack depth overflow caused by alloca (aka Stack Clash
+ * attack).
+ */
+
+#include <linux/bug.h>
+#include <linux/sched.h>
+#include <linux/stackleak.h>
+#include <asm/linkage.h>
+
+asmlinkage void erase_kstack(void)
+{
+	/*
+	 * It would be nice not to have p and boundary on the stack.
+	 * Setting the register specifier for them is the best we can do.
+	 */
+	register unsigned long p = current->thread.lowest_stack.val;
+	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.
+	 * We assume that the stack pointer doesn't change when we write poison.
+	 */
+	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.val = current_top_of_stack() - 256;
+}
+
-- 
2.7.4

[PATCH v12 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 blocks kernel stack depth overflow caused by alloca (aka
Stack Clash attack).

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            |  31 +++
 kernel/stackleak.c                     |  30 +++
 scripts/Makefile.gcc-plugins           |   3 +
 scripts/gcc-plugins/stackleak_plugin.c | 474 +++++++++++++++++++++++++++++++++
 5 files changed, 552 insertions(+)
 create mode 100644 scripts/gcc-plugins/stackleak_plugin.c

diff --git a/arch/Kconfig b/arch/Kconfig
index a4d7794..e9d3841 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
 	  returning from system calls. 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 18fa9d7..83a5a90 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -422,3 +422,34 @@ 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;
+
+	if (size >= stack_left) {
+		/*
+		 * Kernel stack depth overflow is detected, let's report that.
+		 * If CONFIG_VMAP_STACK is enabled, we can safely use BUG().
+		 * If CONFIG_VMAP_STACK is disabled, BUG() handling can corrupt
+		 * the neighbour memory. CONFIG_SCHED_STACK_END_CHECK calls
+		 * panic() in a similar situation, so let's do the same if that
+		 * option is on. Otherwise just use BUG() and hope for the best.
+		 */
+#if !defined(CONFIG_VMAP_STACK) && defined(CONFIG_SCHED_STACK_END_CHECK)
+		panic("alloca over the kernel stack boundary\n");
+#else
+		BUG();
+#endif
+	}
+}
+EXPORT_SYMBOL(check_alloca);
+#endif
diff --git a/kernel/stackleak.c b/kernel/stackleak.c
index 2ebfbaa..27565db 100644
--- a/kernel/stackleak.c
+++ b/kernel/stackleak.c
@@ -15,6 +15,7 @@
 #include <linux/bug.h>
 #include <linux/sched.h>
 #include <linux/stackleak.h>
+#include <linux/sched/task_stack.h>
 #include <asm/linkage.h>
 
 asmlinkage void erase_kstack(void)
@@ -70,3 +71,32 @@ asmlinkage void erase_kstack(void)
 	current->thread.lowest_stack.val = current_top_of_stack() - 256;
 }
 
+void __used track_stack(void)
+{
+	/*
+	 * N.B. erase_kstack() 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.val &&
+	    sp >= (unsigned long)task_stack_page(current) +
+					sizeof(unsigned long)) {
+		current->thread.lowest_stack.val = sp;
+	}
+}
+EXPORT_SYMBOL(track_stack);
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..9df3302
--- /dev/null
+++ b/scripts/gcc-plugins/stackleak_plugin.c
@@ -0,0 +1,474 @@
+/*
+ * 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) == PARALLEL)
+			body = XVECEXP(body, 0, 0);
+
+		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 v12 4/6] lkdtm: Add a test for STACKLEAK

[Alexander Popov]

Introduce lkdtm tests for the STACKLEAK feature.

First, all of them check that the current task stack is properly erased
(filled with STACKLEAK_POISON).

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

STACKLEAK_BIG_ALLOCA and STACKLEAK_RECURSION_WITH_ALLOCA test that
alloca calls which overflow the kernel stack hit BUG()/panic() in
check_alloca().

Signed-off-by: Alexander Popov <alex.popov@linux.com>
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
---
 drivers/misc/lkdtm/Makefile    |   2 +
 drivers/misc/lkdtm/core.c      |   3 +
 drivers/misc/lkdtm/lkdtm.h     |   5 ++
 drivers/misc/lkdtm/stackleak.c | 147 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 157 insertions(+)
 create mode 100644 drivers/misc/lkdtm/stackleak.c

diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
index 3370a41..951c984 100644
--- a/drivers/misc/lkdtm/Makefile
+++ b/drivers/misc/lkdtm/Makefile
@@ -8,7 +8,9 @@ lkdtm-$(CONFIG_LKDTM)		+= perms.o
 lkdtm-$(CONFIG_LKDTM)		+= refcount.o
 lkdtm-$(CONFIG_LKDTM)		+= rodata_objcopy.o
 lkdtm-$(CONFIG_LKDTM)		+= usercopy.o
+lkdtm-$(CONFIG_LKDTM)		+= stackleak.o
 
+KASAN_SANITIZE_stackleak.o	:= n
 KCOV_INSTRUMENT_rodata.o	:= n
 
 OBJCOPYFLAGS :=
diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index 2154d1b..9d0324a 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -183,6 +183,9 @@ static const struct crashtype crashtypes[] = {
 	CRASHTYPE(USERCOPY_STACK_FRAME_FROM),
 	CRASHTYPE(USERCOPY_STACK_BEYOND),
 	CRASHTYPE(USERCOPY_KERNEL),
+	CRASHTYPE(STACKLEAK_BIG_ALLOCA),
+	CRASHTYPE(STACKLEAK_DEEP_RECURSION),
+	CRASHTYPE(STACKLEAK_RECURSION_WITH_ALLOCA),
 };
 
 
diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index 9e513dc..865a6c3 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -83,4 +83,9 @@ void lkdtm_USERCOPY_STACK_FRAME_FROM(void);
 void lkdtm_USERCOPY_STACK_BEYOND(void);
 void lkdtm_USERCOPY_KERNEL(void);
 
+/* lkdtm_stackleak.c */
+void lkdtm_STACKLEAK_BIG_ALLOCA(void);
+void lkdtm_STACKLEAK_DEEP_RECURSION(void);
+void lkdtm_STACKLEAK_RECURSION_WITH_ALLOCA(void);
+
 #endif
diff --git a/drivers/misc/lkdtm/stackleak.c b/drivers/misc/lkdtm/stackleak.c
new file mode 100644
index 0000000..8422e8d
--- /dev/null
+++ b/drivers/misc/lkdtm/stackleak.c
@@ -0,0 +1,147 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This code tests several aspects of the STACKLEAK feature:
+ *  - the current task stack is properly erased (filled with STACKLEAK_POISON);
+ *  - exhausting the current task stack with deep recursion is detected by
+ *     CONFIG_VMAP_STACK (which is implied by CONFIG_GCC_PLUGIN_STACKLEAK);
+ *  - alloca calls which overflow the kernel stack hit BUG()/panic() in
+ *     check_alloca().
+ *
+ * Authors:
+ *   Tycho Andersen <tycho@tycho.ws>
+ *   Alexander Popov <alex.popov@linux.com>
+ */
+
+#include "lkdtm.h"
+#include <linux/sched.h>
+#include <linux/compiler.h>
+
+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_BIG_ALLOCA(void)
+{
+	if (!stack_is_erased())
+		return;
+
+	pr_info("try a small alloca of 16 bytes...\n");
+	do_alloca(16);
+	pr_info("small alloca is successful\n");
+
+	pr_info("try alloca over the thread stack boundary...\n");
+	do_alloca(THREAD_SIZE);
+	pr_err("FAIL: alloca over the thread stack boundary is not detected\n");
+}
+
+static noinline unsigned long recursion(unsigned long prev_sp, bool with_alloca)
+{
+	char buf[400];
+	unsigned long sp = (unsigned long)&sp;
+
+	snprintf(buf, sizeof(buf), "testing deep recursion...\n");
+
+	if (with_alloca)
+		do_alloca(400);
+
+	if (prev_sp < sp + THREAD_SIZE)
+		sp = recursion(prev_sp, with_alloca);
+
+	return sp;
+}
+
+void lkdtm_STACKLEAK_DEEP_RECURSION(void)
+{
+	unsigned long sp = (unsigned long)&sp;
+
+	if (!stack_is_erased())
+		return;
+
+	/*
+	 * Overflow the thread stack using 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 overflow the thread stack using deep recursion...\n");
+	pr_err("FAIL: stack depth overflow (%lu bytes) is not detected\n",
+							sp - recursion(sp, 0));
+}
+
+void lkdtm_STACKLEAK_RECURSION_WITH_ALLOCA(void)
+{
+	unsigned long sp = (unsigned long)&sp;
+
+	if (!stack_is_erased())
+		return;
+
+	/*
+	 * Overflow the thread stack using deep recursion with alloca.
+	 * It should hit BUG()/panic() in check_alloca().
+	 */
+	pr_info("try to overflow the thread stack using recursion & alloca\n");
+	recursion(sp, 1);
+	pr_err("FAIL: stack depth overflow is not detected\n");
+}
-- 
2.7.4

[PATCH v12 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/kernel/process_32.c |  3 +++
 arch/x86/kernel/process_64.c |  3 +++
 fs/proc/base.c               | 18 ++++++++++++++++++
 include/linux/stackleak.h    |  3 +++
 kernel/stackleak.c           |  4 ++++
 6 files changed, 43 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index e9d3841..4ebe06f 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/kernel/process_32.c b/arch/x86/kernel/process_32.c
index a19ea44..3f5f6b8 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.val = (unsigned long)end_of_stack(p) +
 						sizeof(unsigned long);
+# ifdef CONFIG_STACKLEAK_METRICS
+	p->thread.lowest_stack.prev_val = p->thread.lowest_stack.val;
+# 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 f7412c6..a2d1af8 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -298,6 +298,9 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
 #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
 	p->thread.lowest_stack.val = (unsigned long)end_of_stack(p) +
 						sizeof(unsigned long);
+# ifdef CONFIG_STACKLEAK_METRICS
+	p->thread.lowest_stack.prev_val = p->thread.lowest_stack.val;
+# endif
 #endif
 
 	savesegment(gs, p->thread.gsindex);
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 1b2ede6..a4d37a2 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2924,6 +2924,21 @@ 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.lowest_stack.prev_val & (THREAD_SIZE - 1));
+	unsigned long depth = THREAD_SIZE -
+		(task->thread.lowest_stack.val & (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
  */
@@ -3025,6 +3040,9 @@ 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)
diff --git a/include/linux/stackleak.h b/include/linux/stackleak.h
index 458c73b..73f18be 100644
--- a/include/linux/stackleak.h
+++ b/include/linux/stackleak.h
@@ -13,6 +13,9 @@
 struct lowest_stack {
 #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
 	unsigned long		val;
+# ifdef CONFIG_STACKLEAK_METRICS
+	unsigned long		prev_val;
+# endif
 #endif
 };
 
diff --git a/kernel/stackleak.c b/kernel/stackleak.c
index 27565db..e49d694 100644
--- a/kernel/stackleak.c
+++ b/kernel/stackleak.c
@@ -50,6 +50,10 @@ asmlinkage void erase_kstack(void)
 	if (p == boundary)
 		p += sizeof(unsigned long);
 
+#ifdef CONFIG_STACKLEAK_METRICS
+	current->thread.lowest_stack.prev_val = 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.
-- 
2.7.4

[PATCH v12 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 v12 0/6] Introduce the STACKLEAK feature and a test for it

[Kees Cook]

On Wed, May 16, 2018 at 9:28 AM, Alexander Popov <alex.popov@linux.com> wrote:
> This is the 12th version of the patch series introducing STACKLEAK to the
> mainline kernel for x86. Some code is made common for easier porting to
> arm64 (will be done by Laura Abbott).

Thanks for the update! I've pushed this for 0-day testing here:
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=kspp/gcc-plugin/stackleak-v12

I'll let that churn and push this to -next in a day or so. :) And then
hopefully we can add the arm64 bits too. :)

Thanks!

-Kees

-- 
Kees Cook
Pixel Security

Re: [PATCH v12 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Ingo Molnar]

* Alexander Popov <alex.popov@linux.com> wrote:

> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -329,8 +329,22 @@ For 32-bit we have the following conventions - kernel is built with
>  
>  #endif
>  
> +.macro ERASE_KSTACK_NOCLOBBER
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +	PUSH_AND_CLEAR_REGS
> +	call erase_kstack
> +	POP_REGS
> +#endif
> +.endm
> +
>  #endif /* CONFIG_X86_64 */
>  
> +.macro ERASE_KSTACK
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +	call erase_kstack
> +#endif
> +.endm

Please use a well-organized, common, visually easy to ignore namespace.

For example:

	STACKLEAK_ERASE_NOCLOBBER

> @@ -298,6 +300,7 @@ 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

Ditto:

	STACKLEAK_ERASE

etc.

> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -32,6 +32,7 @@ struct vm86;
>  #include <linux/err.h>
>  #include <linux/irqflags.h>
>  #include <linux/mem_encrypt.h>
> +#include <linux/stackleak.h>

>  	mm_segment_t		addr_limit;
>  
> +	struct lowest_stack	lowest_stack;

This too should be something more organized and more opaque, like:

	struct stackleak_info	stackleak_info;

And the field name should not be a meaningless 'val', but 'lowest_stack'.

I.e. "p->stackleak_info.lowest_stack", which is so much more informative ...

> --- 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.val = (unsigned long)end_of_stack(p) +
> +						sizeof(unsigned long);
> +#endif

This should use an inline helper:

	stackleak_task_init(p);

> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +	p->thread.lowest_stack.val = (unsigned long)end_of_stack(p) +
> +						sizeof(unsigned long);
> +#endif

Beyond the lower visual impact this duplication will be removed by the inline 
helper as well.

> +++ b/kernel/stackleak.c
> @@ -0,0 +1,72 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * This code fills the used part of the kernel stack with a poison value
> + * before returning to the userspace. It's a part of the STACKLEAK feature
> + * ported from grsecurity/PaX.
> + *
> + * Author: Alexander Popov <alex.popov@linux.com>
> + *
> + * STACKLEAK reduces the information which kernel stack leak bugs can
> + * reveal and blocks some uninitialized stack variable attacks. Moreover,
> + * STACKLEAK blocks stack depth overflow caused by alloca (aka Stack Clash
> + * attack).
> + */

s/alloca
 /alloca()

> +#include <linux/bug.h>
> +#include <linux/sched.h>
> +#include <linux/stackleak.h>
> +#include <asm/linkage.h>

Yeah, so since processor.h includes stackleak.h I strongly doubt the stackleak.h 
inclusion is necessary here. Please review every header inclusion line and remove 
the unnecessary ones.

> +
> +asmlinkage void erase_kstack(void)

This too should be in the stackleak_*() namespace.

> +{
> +	/*
> +	 * It would be nice not to have p and boundary on the stack.
> +	 * Setting the register specifier for them is the best we can do.
> +	 */
> +	register unsigned long p = current->thread.lowest_stack.val;
> +	register unsigned long boundary = p & ~(THREAD_SIZE - 1);

Does the 'register' keyword actually have any effect on the generated code?

> +	unsigned long poison = 0;
> +	const unsigned long check_depth = STACKLEAK_POISON_CHECK_DEPTH /
> +							sizeof(unsigned long);

Please don't break lines in such an ugly fashion!

Also, 'poison' is a very weird name for something that looks like an index.

Plus since it's bound by "check_depth" is the 'unsigned long' justified,
or could it be 32-bit?

> +
> +	/*
> +	 * 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);
> +	}

This comment would be so much easier to read if the initialization was done right 
before the first use, i.e.:

	/*
	 * Let's search for the poison value in the stack.
	 * Start from the lowest_stack and go to the bottom:
	 */

	p = current->thread.lowest_stack.val;
	boundary = p & ~(THREAD_SIZE - 1);

	while (p > boundary && poison <= check_depth) {
		if (*(unsigned long *)p == STACKLEAK_POISON)
			poison++;
		else
			poison = 0;
	...

> +
> +	/*
> +	 * 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);

Please put types into quotes where it's ambigous. I first read this sentence as 
"One long ..." and went "wtf". It's a totally unnecessary disruption of the 
reading flow.

> +	/*
> +	 * So let's write the poison value to the kernel stack.
> +	 * Start from the address in p and move up till the new boundary.
> +	 * We assume that the stack pointer doesn't change when we write poison.
> +	 */

Here too 'p' is easier to read.

But 'p' is a very weird name: in the kernel it's usually some sort of process 
pointer. Please rename it to something more descriptive, such as "kstack_ptr" or 
so.

> +	if (on_thread_stack())
> +		boundary = current_stack_pointer;
> +	else
> +		boundary = current_top_of_stack();
> +
> +	BUG_ON(boundary - p >= THREAD_SIZE);

Please make this:

	if ( WARN_ON_ONCE())
		return;

... or so, so that if this code is buggy we get actual useful user reports, not 
just "my machine froze, help!"...

> +	/* Reset the lowest_stack value for the next syscall */
> +	current->thread.lowest_stack.val = current_top_of_stack() - 256;

Magic, unexplained '256' literal.

Thanks,

	Ingo

Re: [PATCH v12 0/6] Introduce the STACKLEAK feature and a test for it

[Ingo Molnar]

* Kees Cook <keescook@chromium.org> wrote:

> On Wed, May 16, 2018 at 9:28 AM, Alexander Popov <alex.popov@linux.com> wrote:
> > This is the 12th version of the patch series introducing STACKLEAK to the
> > mainline kernel for x86. Some code is made common for easier porting to
> > arm64 (will be done by Laura Abbott).
> 
> Thanks for the update! I've pushed this for 0-day testing here:
> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=kspp/gcc-plugin/stackleak-v12

Please don't commit this to any stable Git tree yet, it still needs more work.

Thanks,

	Ingo

Re: [PATCH v12 0/6] Introduce the STACKLEAK feature and a test for it

[Kees Cook]

On Thu, May 17, 2018 at 11:54 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Kees Cook <keescook@chromium.org> wrote:
>
>> On Wed, May 16, 2018 at 9:28 AM, Alexander Popov <alex.popov@linux.com> wrote:
>> > This is the 12th version of the patch series introducing STACKLEAK to the
>> > mainline kernel for x86. Some code is made common for easier porting to
>> > arm64 (will be done by Laura Abbott).
>>
>> Thanks for the update! I've pushed this for 0-day testing here:
>> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=kspp/gcc-plugin/stackleak-v12
>
> Please don't commit this to any stable Git tree yet, it still needs more work.

Sure thing and thanks for your review! I put it up on 0-day to help
with build testing -- the kspp subtrees aren't considered sha-stable.

-Kees

-- 
Kees Cook
Pixel Security

Re: [PATCH v12 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Alexander Popov]

Hello Ingo,

Thanks a lot for the review!
I agree with your points. I'll fix the series and return with v13.

There are some comments/questions below.

On 18.05.2018 09:53, Ingo Molnar wrote:
> * Alexander Popov <alex.popov@linux.com> wrote:
>> +	/*
>> +	 * It would be nice not to have p and boundary on the stack.
>> +	 * Setting the register specifier for them is the best we can do.
>> +	 */
>> +	register unsigned long p = current->thread.lowest_stack.val;
>> +	register unsigned long boundary = p & ~(THREAD_SIZE - 1);
> 
> Does the 'register' keyword actually have any effect on the generated code?

No, for gcc it doesn't give any effect for this code. I used it to show the
intention.

However, even if the compiler allocates 'p' and 'boundary' on stack, that will
not break the stack erasing.

Should I drop 'register'?

>> +	unsigned long poison = 0;
>> +	const unsigned long check_depth = STACKLEAK_POISON_CHECK_DEPTH /
>> +							sizeof(unsigned long);
> 
> Please don't break lines in such an ugly fashion!

Ok. I'll make it look like that:

	const unsigned long check_depth =
			STACKLEAK_POISON_CHECK_DEPTH / sizeof(unsigned long);


> Also, 'poison' is a very weird name for something that looks like an index.
> 
> Plus since it's bound by "check_depth" is the 'unsigned long' justified,
> or could it be 32-bit?

Thanks, I can turn both 'check_depth' and 'poison' (which I'll rename) into
'unsigned int'.

>> +	/* Reset the lowest_stack value for the next syscall */
>> +	current->thread.lowest_stack.val = current_top_of_stack() - 256;
> 
> Magic, unexplained '256' literal.

Here I'm choosing the point from which we'll start the poison search on the next
syscall in case 'lowest_stack' is not updated in track_stack().

Would you like if I use "current_top_of_stack() - THREAD_SIZE / 64" ?

Best regards,
Alexander

Re: [PATCH v12 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Alexander Popov]

Hello Ingo and Kees,

On 18.05.2018 09:53, Ingo Molnar wrote:
> * Alexander Popov <alex.popov@linux.com> wrote:
>> --- 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.val = (unsigned long)end_of_stack(p) +
>> +						sizeof(unsigned long);
>> +#endif
> 
> This should use an inline helper:
> 
> 	stackleak_task_init(p);

Right now I can't define such a helper because 'lowest_stack' is a part of
'thread_struct', but arch/x86/include/asm/processor.h and include/linux/sched.h
are independent.

I think the best solution is to move 'lowest_stack' variable to 'task_struct'.

Is there any reason to fold this variable into a 'stackleak_info' structure? I
see everybody happily uses #ifdef in 'task_struct'.

May I define STACKLEAK_POISON and STACKLEAK_POISON_CHECK_DEPTH in sched.h as well?

Looking forward to your reply.
Best regards,
Alexander

Re: [PATCH v12 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Kees Cook]

On Tue, May 22, 2018 at 3:58 AM, Alexander Popov <alex.popov@linux.com> wrote:
> Hello Ingo and Kees,
>
> On 18.05.2018 09:53, Ingo Molnar wrote:
>> * Alexander Popov <alex.popov@linux.com> wrote:
>>> --- 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.val = (unsigned long)end_of_stack(p) +
>>> +                                            sizeof(unsigned long);
>>> +#endif
>>
>> This should use an inline helper:
>>
>>       stackleak_task_init(p);
>
> Right now I can't define such a helper because 'lowest_stack' is a part of
> 'thread_struct', but arch/x86/include/asm/processor.h and include/linux/sched.h
> are independent.
>
> I think the best solution is to move 'lowest_stack' variable to 'task_struct'.

It might be possible work around this by using a macro in stackleak.h
(pardon any whitespace damage):

#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
# define stackleak_task_init(task)     do {                      \
                      task->thread.stack_leak.lowest_stack = \
                                   (unsigned long)end_of_stack(task) + \
                                   sizeof(unsigned long);                  \
               } while (0)
#else
# define stackleak_task_init(task)   do { } while (0)
#endif

> Is there any reason to fold this variable into a 'stackleak_info' structure? I
> see everybody happily uses #ifdef in 'task_struct'.

I'd rather it stay in threadinfo, but I could be convinced otherwise.

-Kees

-- 
Kees Cook
Pixel Security

Re: [PATCH v12 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Alexander Popov]

On 22.05.2018 20:20, Kees Cook wrote:
> On Tue, May 22, 2018 at 3:58 AM, Alexander Popov <alex.popov@linux.com> wrote:
>> Hello Ingo and Kees,
>>
>> On 18.05.2018 09:53, Ingo Molnar wrote:
>>> * Alexander Popov <alex.popov@linux.com> wrote:
>>>> --- 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.val = (unsigned long)end_of_stack(p) +
>>>> +                                            sizeof(unsigned long);
>>>> +#endif
>>>
>>> This should use an inline helper:
>>>
>>>       stackleak_task_init(p);
>>
>> Right now I can't define such a helper because 'lowest_stack' is a part of
>> 'thread_struct', but arch/x86/include/asm/processor.h and include/linux/sched.h
>> are independent.
>>
>> I think the best solution is to move 'lowest_stack' variable to 'task_struct'.
> 
> It might be possible work around this by using a macro in stackleak.h
> (pardon any whitespace damage):
> 
> #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> # define stackleak_task_init(task)     do {                      \
>                       task->thread.stack_leak.lowest_stack = \
>                                    (unsigned long)end_of_stack(task) + \
>                                    sizeof(unsigned long);                  \
>                } while (0)
> #else
> # define stackleak_task_init(task)   do { } while (0)
> #endif
> 
>> Is there any reason to fold this variable into a 'stackleak_info' structure? I
>> see everybody happily uses #ifdef in 'task_struct'.
> 
> I'd rather it stay in threadinfo, but I could be convinced otherwise.

Currently 'lowest_stack' is a part of 'thread_struct', which is a "CPU-specific
state of this task". But in fact 'lowest_stack' is not CPU-specific. If I move
it to 'task_struct', Laura will avoid redefining it in 'thread_struct' in
arch/arm64/include/asm/processor.h. And that will also allow me to create a
stackleak_task_init() inline helper in stackleak.h.

Best regards,
Alexander

Re: [PATCH v12 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Kees Cook]

On Tue, May 22, 2018 at 2:00 PM, Alexander Popov <alex.popov@linux.com> wrote:
> Currently 'lowest_stack' is a part of 'thread_struct', which is a "CPU-specific
> state of this task". But in fact 'lowest_stack' is not CPU-specific. If I move
> it to 'task_struct', Laura will avoid redefining it in 'thread_struct' in
> arch/arm64/include/asm/processor.h. And that will also allow me to create a
> stackleak_task_init() inline helper in stackleak.h.

Okay, give it a go! :)

-Kees

-- 
Kees Cook
Pixel Security

Re: [PATCH v12 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Alexander Popov]

On 18.05.2018 09:53, Ingo Molnar wrote:
> * Alexander Popov <alex.popov@linux.com> wrote:
>> +	if (on_thread_stack())
>> +		boundary = current_stack_pointer;
>> +	else
>> +		boundary = current_top_of_stack();
>> +
>> +	BUG_ON(boundary - p >= THREAD_SIZE);
> 
> Please make this:
> 
> 	if ( WARN_ON_ONCE())
> 		return;
> 
> ... or so, so that if this code is buggy we get actual useful user reports, not 
> just "my machine froze, help!"...

I've just double-checked that BUG() in erase_kstack() works fine when it's
called from the trampoline stack and thread stack.

But this "WARN_ON_ONCE() + return" logic would introduce an undesirable effect:
modifying 'lowest_stack' value would allow to silently skip stack erasing.

Best regards,
Alexander