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x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

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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>
This commit is contained in:
Alexander Popov 2018-08-17 01:16:58 +03:00 committed by Kees Cook
parent 57361846b5
commit afaef01c00
13 changed files with 157 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
Documentation/x86/x86_64/mm.txt
View File

@ -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

7
arch/Kconfig
View File

@ -419,6 +419,13 @@ config SECCOMP_FILTER
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

1
arch/x86/Kconfig
View File

@ -127,6 +127,7 @@ config X86
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

14
arch/x86/entry/calling.h
View File

@ -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.

7
arch/x86/entry/entry_32.S
View File

@ -46,6 +46,8 @@
#include <asm/frame.h>
#include <asm/nospec-branch.h>
#include "calling.h"
.section .entry.text, "ax"
/*
@ -711,6 +713,7 @@ ENTRY(ret_from_fork)
/* 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 @@ ENTRY(entry_SYSENTER_32)
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 @@ ENTRY(entry_INT80_32)
call do_int80_syscall_32
.Lsyscall_32_done:
STACKLEAK_ERASE
restore_all:
TRACE_IRQS_IRET
SWITCH_TO_ENTRY_STACK

3
arch/x86/entry/entry_64.S
View File

@ -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.
*/
STACKLEAK_ERASE_NOCLOBBER
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
popq %rdi
@ -688,6 +690,7 @@ 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.
*/
STACKLEAK_ERASE_NOCLOBBER
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi

5
arch/x86/entry/entry_64_compat.S
View File

@ -261,6 +261,11 @@ GLOBAL(entry_SYSCALL_compat_after_hwframe)
/* 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 */

4
include/linux/sched.h
View File

@ -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.

26
include/linux/stackleak.h Normal file
View File

@ -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

4
kernel/Makefile
View File

@ -117,6 +117,10 @@ obj-$(CONFIG_HAS_IOMEM) += iomem.o
obj-$(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

3
kernel/fork.c
View File

@ -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)) {

62
kernel/stackleak.c Normal file
View File

@ -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;
}

19
scripts/gcc-plugins/Kconfig
View File

@ -139,4 +139,23 @@ 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.
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