x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls
The STACKLEAK feature (initially developed by PaX Team) has the following
benefits:
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.
This commit introduces the code filling the used part of the kernel
stack with a poison value before returning to 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.
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".
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>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
diff --git a/Documentation/x86/x86_64/mm.txt b/Documentation/x86/x86_64/mm.txt
index 5432a96d3..600bc2a 100644
--- a/Documentation/x86/x86_64/mm.txt
+++ b/Documentation/x86/x86_64/mm.txt
@@ -24,6 +24,7 @@
[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 @@
[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 6801123..ee79ff5 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -419,6 +419,13 @@
See Documentation/userspace-api/seccomp_filter.rst 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_STACKPROTECTOR
bool
help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1a0be02..662cb2c 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -127,6 +127,7 @@
select HAVE_ARCH_PREL32_RELOCATIONS
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..20d0885 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 STACKLEAK_ERASE_NOCLOBBER
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ PUSH_AND_CLEAR_REGS
+ call stackleak_erase
+ POP_REGS
+#endif
+.endm
+
#endif /* CONFIG_X86_64 */
+.macro STACKLEAK_ERASE
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ call stackleak_erase
+#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 2767c62..dfb975b 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"
/*
@@ -711,6 +713,7 @@
/* When we fork, we trace the syscall return in the child, too. */
movl %esp, %eax
call syscall_return_slowpath
+ STACKLEAK_ERASE
jmp restore_all
/* kernel thread */
@@ -885,6 +888,8 @@
ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
"jmp .Lsyscall_32_done", X86_FEATURE_XENPV
+ STACKLEAK_ERASE
+
/* Opportunistic SYSEXIT */
TRACE_IRQS_ON /* User mode traces as IRQs on. */
@@ -996,6 +1001,8 @@
call do_int80_syscall_32
.Lsyscall_32_done:
+ STACKLEAK_ERASE
+
restore_all:
TRACE_IRQS_IRET
SWITCH_TO_ENTRY_STACK
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 957dfb6..a5dd280 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -329,6 +329,8 @@
* We are on the trampoline stack. All regs except RDI are live.
* We can do future final exit work right here.
*/
+ STACKLEAK_ERASE_NOCLOBBER
+
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
popq %rdi
@@ -688,6 +690,7 @@
* We are on the trampoline stack. All regs except RDI are live.
* We can do future final exit work right here.
*/
+ STACKLEAK_ERASE_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 7d0df78..8eaf895 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -261,6 +261,11 @@
/* Opportunistic SYSRET */
sysret32_from_system_call:
+ /*
+ * We are not going to return to userspace from the trampoline
+ * stack. So let's erase the thread stack right now.
+ */
+ STACKLEAK_ERASE
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/include/linux/sched.h b/include/linux/sched.h
index 977cb57..c1a23ac 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1192,6 +1192,10 @@ struct task_struct {
void *security;
#endif
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ unsigned long lowest_stack;
+#endif
+
/*
* New fields for task_struct should be added above here, so that
* they are included in the randomized portion of task_struct.
diff --git a/include/linux/stackleak.h b/include/linux/stackleak.h
new file mode 100644
index 0000000..628c2b9
--- /dev/null
+++ b/include/linux/stackleak.h
@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_STACKLEAK_H
+#define _LINUX_STACKLEAK_H
+
+#include <linux/sched.h>
+#include <linux/sched/task_stack.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_SEARCH_DEPTH 128
+
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+#include <asm/stacktrace.h>
+
+static inline void stackleak_task_init(struct task_struct *t)
+{
+ t->lowest_stack = (unsigned long)end_of_stack(t) + sizeof(unsigned long);
+}
+#else /* !CONFIG_GCC_PLUGIN_STACKLEAK */
+static inline void stackleak_task_init(struct task_struct *t) { }
+#endif
+
+#endif
diff --git a/kernel/Makefile b/kernel/Makefile
index 7a63d56..7343b3a 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -117,6 +117,10 @@
obj-$(CONFIG_ZONE_DEVICE) += memremap.o
obj-$(CONFIG_RSEQ) += rseq.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/fork.c b/kernel/fork.c
index d896e9c..47911e4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -91,6 +91,7 @@
#include <linux/kcov.h>
#include <linux/livepatch.h>
#include <linux/thread_info.h>
+#include <linux/stackleak.h>
#include <asm/pgtable.h>
#include <asm/pgalloc.h>
@@ -1880,6 +1881,8 @@ static __latent_entropy struct task_struct *copy_process(
if (retval)
goto bad_fork_cleanup_io;
+ stackleak_task_init(p);
+
if (pid != &init_struct_pid) {
pid = alloc_pid(p->nsproxy->pid_ns_for_children);
if (IS_ERR(pid)) {
diff --git a/kernel/stackleak.c b/kernel/stackleak.c
new file mode 100644
index 0000000..deba0d8
--- /dev/null
+++ b/kernel/stackleak.c
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This code fills the used part of the kernel stack with a poison value
+ * before returning to userspace. It's 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.
+ */
+
+#include <linux/stackleak.h>
+
+asmlinkage void stackleak_erase(void)
+{
+ /* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
+ unsigned long kstack_ptr = current->lowest_stack;
+ unsigned long boundary = (unsigned long)end_of_stack(current);
+ unsigned int poison_count = 0;
+ const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
+
+ /* Check that 'lowest_stack' value is sane */
+ if (unlikely(kstack_ptr - boundary >= THREAD_SIZE))
+ kstack_ptr = boundary;
+
+ /* Search for the poison value in the kernel stack */
+ while (kstack_ptr > boundary && poison_count <= depth) {
+ if (*(unsigned long *)kstack_ptr == STACKLEAK_POISON)
+ poison_count++;
+ else
+ poison_count = 0;
+
+ kstack_ptr -= 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=y).
+ */
+ if (kstack_ptr == boundary)
+ kstack_ptr += sizeof(unsigned long);
+
+ /*
+ * Now write the poison value to the kernel stack. Start from
+ * 'kstack_ptr' 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();
+
+ while (kstack_ptr < boundary) {
+ *(unsigned long *)kstack_ptr = STACKLEAK_POISON;
+ kstack_ptr += sizeof(unsigned long);
+ }
+
+ /* Reset the 'lowest_stack' value for the next syscall */
+ current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;
+}
+
diff --git a/scripts/gcc-plugins/Kconfig b/scripts/gcc-plugins/Kconfig
index cb0c889..977b84e 100644
--- a/scripts/gcc-plugins/Kconfig
+++ b/scripts/gcc-plugins/Kconfig
@@ -139,4 +139,23 @@
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.
+
+ 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/
+
endif
