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Message-Id: <1518804657-24905-2-git-send-email-alex.popov@linux.com>
Date: Fri, 16 Feb 2018 21:10:52 +0300
From: Alexander Popov <alex.popov@...ux.com>
To: kernel-hardening@...ts.openwall.com,
Kees Cook <keescook@...omium.org>,
PaX Team <pageexec@...email.hu>,
Brad Spengler <spender@...ecurity.net>,
Ingo Molnar <mingo@...nel.org>,
Andy Lutomirski <luto@...nel.org>,
Tycho Andersen <tycho@...ho.ws>,
Laura Abbott <labbott@...hat.com>,
Mark Rutland <mark.rutland@....com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Borislav Petkov <bp@...en8.de>,
Thomas Gleixner <tglx@...utronix.de>,
"H . Peter Anvin" <hpa@...or.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
"Dmitry V . Levin" <ldv@...linux.org>,
x86@...nel.org,
alex.popov@...ux.com
Subject: [PATCH RFC v8 1/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls
The STACKLEAK feature erases the kernel stack before returning from
syscalls. That reduces the information which kernel stack leak bugs can
reveal and blocks some uninitialized stack variable attacks. Moreover,
STACKLEAK provides runtime checks for kernel stack overflow detection.
This commit introduces the architecture-specific code filling the used
part of the kernel stack with a poison value before returning to the
userspace. Full STACKLEAK feature also contains the gcc plugin which
comes in a separate commit.
The STACKLEAK feature is ported from grsecurity/PaX. More information at:
https://grsecurity.net/
https://pax.grsecurity.net/
This code is modified from Brad Spengler/PaX Team's code in the last
public patch of grsecurity/PaX based on our understanding of the code.
Changes or omissions from the original code are ours and don't reflect
the original grsecurity/PaX code.
Signed-off-by: Alexander Popov <alex.popov@...ux.com>
---
arch/Kconfig | 27 ++++++++++
arch/x86/Kconfig | 1 +
arch/x86/entry/entry_32.S | 89 ++++++++++++++++++++++++++++++++
arch/x86/entry/entry_64.S | 109 +++++++++++++++++++++++++++++++++++++++
arch/x86/entry/entry_64_compat.S | 11 ++++
arch/x86/include/asm/processor.h | 4 ++
arch/x86/kernel/asm-offsets.c | 8 +++
arch/x86/kernel/process_32.c | 5 ++
arch/x86/kernel/process_64.c | 5 ++
include/linux/compiler.h | 6 +++
10 files changed, 265 insertions(+)
diff --git a/arch/Kconfig b/arch/Kconfig
index 76c0b54..368e2fb 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -401,6 +401,13 @@ config SECCOMP_FILTER
See Documentation/prctl/seccomp_filter.txt for details.
+config HAVE_ARCH_STACKLEAK
+ bool
+ help
+ An architecture should select this if it has the code which
+ fills the used part of the kernel stack with the STACKLEAK_POISON
+ value before returning from system calls.
+
config HAVE_GCC_PLUGINS
bool
help
@@ -531,6 +538,26 @@ config GCC_PLUGIN_RANDSTRUCT_PERFORMANCE
in structures. This reduces the performance hit of RANDSTRUCT
at the cost of weakened randomization.
+config GCC_PLUGIN_STACKLEAK
+ bool "Erase the kernel stack before returning from syscalls"
+ depends on GCC_PLUGINS
+ depends on HAVE_ARCH_STACKLEAK
+ help
+ This option makes the kernel erase the kernel stack before it
+ returns from a system call. That reduces the information which
+ kernel stack leak bugs can reveal and blocks some uninitialized
+ stack variable attacks. This option also provides runtime checks
+ for kernel stack overflow detection.
+
+ The tradeoff is the performance impact: on a single CPU system kernel
+ compilation sees a 1% slowdown, other systems and workloads may vary
+ and you are advised to test this feature on your expected workload
+ before deploying it.
+
+ This plugin was ported from grsecurity/PaX. More information at:
+ * https://grsecurity.net/
+ * https://pax.grsecurity.net/
+
config HAVE_CC_STACKPROTECTOR
bool
help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 63bf349..62748bd 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -119,6 +119,7 @@ config X86
select HAVE_ARCH_COMPAT_MMAP_BASES if MMU && COMPAT
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ARCH_THREAD_STRUCT_WHITELIST
+ select HAVE_ARCH_STACKLEAK
select HAVE_ARCH_TRACEHOOK
select HAVE_ARCH_TRANSPARENT_HUGEPAGE
select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if X86_64
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 16c2c02..4a7365a 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -77,6 +77,90 @@
#endif
.endm
+.macro erase_kstack
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ call erase_kstack
+#endif
+.endm
+
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ENTRY(erase_kstack)
+ pushl %edi
+ pushl %ecx
+ pushl %eax
+ pushl %ebp
+
+ movl PER_CPU_VAR(current_task), %ebp
+ mov TASK_lowest_stack(%ebp), %edi
+ mov $STACKLEAK_POISON, %eax
+ std
+
+ /*
+ * Let's search for the poison value in the stack.
+ * Start from the lowest_stack and go to the bottom (see std above).
+ */
+1:
+ mov %edi, %ecx
+ and $THREAD_SIZE_asm - 1, %ecx
+ shr $2, %ecx
+ repne scasl
+ jecxz 2f /* Didn't find it. Go to poisoning. */
+
+ /*
+ * Found the poison value in the stack. Go to poisoning if there is
+ * not enough space left for the poison check.
+ */
+ cmp $STACKLEAK_POISON_CHECK_DEPTH / 4, %ecx
+ jc 2f
+
+ /*
+ * Check that some further dwords contain poison. If so, the part
+ * of the stack below the address in %edi is likely to be poisoned.
+ * Otherwise we need to search deeper.
+ */
+ mov $STACKLEAK_POISON_CHECK_DEPTH / 4, %ecx
+ repe scasl
+ jecxz 2f /* Poison the upper part of the stack */
+ jne 1b /* Search deeper */
+
+2:
+ /*
+ * Prepare the counter for poisoning the kernel stack between
+ * %edi and %esp. Two dwords at the bottom of the stack are reserved
+ * and should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
+ */
+ or $2 * 4, %edi
+ cld
+ mov %esp, %ecx
+ sub %edi, %ecx
+
+ /* Check that the counter value is sane. */
+ cmp $THREAD_SIZE_asm, %ecx
+ jb 3f
+ ud2
+
+3:
+ /*
+ * So let's write the poison value to the kernel stack. Start from the
+ * address in %edi and move up (see cld above) to the address in %esp
+ * (not included, used memory).
+ */
+ shr $2, %ecx
+ rep stosl
+
+ /* Set the lowest_stack value to the top_of_stack - 128 */
+ movl PER_CPU_VAR(cpu_current_top_of_stack), %edi
+ sub $128, %edi
+ mov %edi, TASK_lowest_stack(%ebp)
+
+ popl %ebp
+ popl %eax
+ popl %ecx
+ popl %edi
+ ret
+ENDPROC(erase_kstack)
+#endif
+
/*
* User gs save/restore
*
@@ -299,6 +383,7 @@ 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 */
@@ -459,6 +544,8 @@ 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 */
@@ -545,6 +632,8 @@ 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 30c8c53..6863af8 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -66,6 +66,112 @@ END(native_usergs_sysret64)
TRACE_IRQS_FLAGS EFLAGS(%rsp)
.endm
+.macro erase_kstack
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ call erase_kstack
+#endif
+.endm
+
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ENTRY(erase_kstack)
+ pushq %rdi
+ pushq %rcx
+ pushq %rax
+ pushq %r11
+
+ mov PER_CPU_VAR(current_task), %r11
+ mov TASK_lowest_stack(%r11), %rdi
+ mov $STACKLEAK_POISON, %rax
+ std
+
+ /*
+ * Let's search for the poison value in the stack.
+ * Start from the lowest_stack and go to the bottom (see std above).
+ */
+1:
+ mov %edi, %ecx
+ and $THREAD_SIZE_asm - 1, %ecx
+ shr $3, %ecx
+ repne scasq
+ jecxz 2f /* Didn't find it. Go to poisoning. */
+
+ /*
+ * Found the poison value in the stack. Go to poisoning if there is
+ * not enough space left for the poison check.
+ */
+ cmp $STACKLEAK_POISON_CHECK_DEPTH / 8, %ecx
+ jb 2f
+
+ /*
+ * Check that some further qwords contain poison. If so, the part
+ * of the stack below the address in %rdi is likely to be poisoned.
+ * Otherwise we need to search deeper.
+ */
+ mov $STACKLEAK_POISON_CHECK_DEPTH / 8, %ecx
+ repe scasq
+ jecxz 2f /* Poison the upper part of the stack */
+ jne 1b /* Search deeper */
+
+2:
+ /*
+ * Two qwords at the bottom of the thread stack are reserved and
+ * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
+ */
+ or $2 * 8, %rdi
+
+ /*
+ * Check whether we are on the thread stack to prepare the counter
+ * for stack poisoning.
+ */
+ mov PER_CPU_VAR(cpu_current_top_of_stack), %rcx
+ sub %rsp, %rcx
+ cmp $THREAD_SIZE_asm, %rcx
+ jb 3f
+
+ /*
+ * We are not on the thread stack, so we can write poison between
+ * the address in %rdi and the stack top.
+ */
+ mov PER_CPU_VAR(cpu_current_top_of_stack), %rcx
+ sub %rdi, %rcx
+ jmp 4f
+
+3:
+ /*
+ * We are on the thread stack, so we can write poison between the
+ * address in %rdi and the address in %rsp (not included, used memory).
+ */
+ mov %rsp, %rcx
+ sub %rdi, %rcx
+
+4:
+ /* Check that the counter value is sane */
+ cmp $THREAD_SIZE_asm, %rcx
+ jb 5f
+ ud2
+
+5:
+ /*
+ * So let's write the poison value to the kernel stack. Start from the
+ * address in %rdi and move up (see cld).
+ */
+ cld
+ shr $3, %ecx
+ rep stosq
+
+ /* Set the lowest_stack value to the top_of_stack - 256 */
+ mov PER_CPU_VAR(cpu_current_top_of_stack), %rdi
+ sub $256, %rdi
+ mov %rdi, TASK_lowest_stack(%r11)
+
+ popq %r11
+ popq %rax
+ popq %rcx
+ popq %rdi
+ ret
+ENDPROC(erase_kstack)
+#endif
+
/*
* When dynamic function tracer is enabled it will add a breakpoint
* to all locations that it is about to modify, sync CPUs, update
@@ -345,6 +451,8 @@ syscall_return_via_sysret:
* We are on the trampoline stack. All regs except RDI are live.
* We can do future final exit work right here.
*/
+ erase_kstack
+
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
popq %rdi
@@ -653,6 +761,7 @@ GLOBAL(swapgs_restore_regs_and_return_to_usermode)
* We are on the trampoline stack. All regs except RDI are live.
* We can do future final exit work right here.
*/
+ erase_kstack
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 98d5358..727dde2 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -19,6 +19,12 @@
.section .entry.text, "ax"
+ .macro erase_kstack
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ call erase_kstack
+#endif
+ .endm
+
/*
* 32-bit SYSENTER entry.
*
@@ -238,6 +244,11 @@ 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 793bae7..5fd8ae1 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -494,6 +494,10 @@ struct thread_struct {
mm_segment_t addr_limit;
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ unsigned long lowest_stack;
+#endif
+
unsigned int sig_on_uaccess_err:1;
unsigned int uaccess_err:1; /* uaccess failed */
diff --git a/arch/x86/kernel/asm-offsets.c b/arch/x86/kernel/asm-offsets.c
index 76417a9..ef5d260 100644
--- a/arch/x86/kernel/asm-offsets.c
+++ b/arch/x86/kernel/asm-offsets.c
@@ -39,6 +39,9 @@ void common(void) {
BLANK();
OFFSET(TASK_TI_flags, task_struct, thread_info.flags);
OFFSET(TASK_addr_limit, task_struct, thread.addr_limit);
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ OFFSET(TASK_lowest_stack, task_struct, thread.lowest_stack);
+#endif
BLANK();
OFFSET(crypto_tfm_ctx_offset, crypto_tfm, __crt_ctx);
@@ -75,6 +78,11 @@ void common(void) {
OFFSET(PV_MMU_read_cr2, pv_mmu_ops, read_cr2);
#endif
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ BLANK();
+ DEFINE(THREAD_SIZE_asm, THREAD_SIZE);
+#endif
+
#ifdef CONFIG_XEN
BLANK();
OFFSET(XEN_vcpu_info_mask, vcpu_info, evtchn_upcall_mask);
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 5224c60..6d256ab 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -136,6 +136,11 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
p->thread.sp0 = (unsigned long) (childregs+1);
memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ p->thread.lowest_stack = (unsigned long)task_stack_page(p) +
+ 2 * sizeof(unsigned long);
+#endif
+
if (unlikely(p->flags & PF_KTHREAD)) {
/* kernel thread */
memset(childregs, 0, sizeof(struct pt_regs));
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 9eb448c..6dc55f6 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -281,6 +281,11 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
p->thread.sp = (unsigned long) fork_frame;
p->thread.io_bitmap_ptr = NULL;
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ p->thread.lowest_stack = (unsigned long)task_stack_page(p) +
+ 2 * sizeof(unsigned long);
+#endif
+
savesegment(gs, p->thread.gsindex);
p->thread.gsbase = p->thread.gsindex ? 0 : me->thread.gsbase;
savesegment(fs, p->thread.fsindex);
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index e835fc0..fe04f60 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -337,4 +337,10 @@ unsigned long read_word_at_a_time(const void *addr)
compiletime_assert(__native_word(t), \
"Need native word sized stores/loads for atomicity.")
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+/* Poison value points to the unused hole in the virtual memory map */
+# define STACKLEAK_POISON -0xBEEF
+# define STACKLEAK_POISON_CHECK_DEPTH 128
+#endif
+
#endif /* __LINUX_COMPILER_H */
--
2.7.4
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