@@ -387,6 +387,13 @@ 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
@@ -517,6 +524,26 @@ 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
@@ -116,6 +116,7 @@ config X86
select HAVE_ARCH_MMAP_RND_COMPAT_BITS if MMU && COMPAT
select HAVE_ARCH_COMPAT_MMAP_BASES if MMU && COMPAT
select HAVE_ARCH_SECCOMP_FILTER
+ select HAVE_ARCH_STACKLEAK
select HAVE_ARCH_TRACEHOOK
select HAVE_ARCH_TRANSPARENT_HUGEPAGE
select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if X86_64
@@ -76,6 +76,66 @@
#endif
.endm
+.macro erase_kstack
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ call erase_kstack
+#endif
+.endm
+
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+/* For the detailed comments, see erase_kstack in entry_64.S */
+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
+
+1:
+ mov %edi, %ecx
+ and $THREAD_SIZE_asm - 1, %ecx
+ shr $2, %ecx
+ repne scasl
+ jecxz 2f
+
+ cmp $STACKLEAK_POISON_CHECK_DEPTH / 4, %ecx
+ jc 2f
+
+ mov $STACKLEAK_POISON_CHECK_DEPTH / 4, %ecx
+ repe scasl
+ jecxz 2f
+ jne 1b
+
+2:
+ cld
+ or $2 * 4, %edi
+ mov %esp, %ecx
+ sub %edi, %ecx
+
+ cmp $THREAD_SIZE_asm, %ecx
+ jb 3f
+ ud2
+
+3:
+ shr $2, %ecx
+ rep stosl
+
+ 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
*
@@ -286,6 +346,7 @@ 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 */
@@ -446,6 +507,8 @@ 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 */
@@ -532,6 +595,8 @@ ENTRY(entry_INT80_32)
call do_int80_syscall_32
.Lsyscall_32_done:
+ erase_kstack
+
restore_all:
TRACE_IRQS_IRET
.Lrestore_all_notrace:
@@ -65,6 +65,112 @@ 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
@@ -411,6 +517,8 @@ 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
@@ -748,6 +856,7 @@ 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
@@ -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 */
@@ -496,6 +496,10 @@ 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 */
@@ -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);
@@ -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));
@@ -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);
@@ -327,4 +327,10 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
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 */
The STACKLEAK feature erases the kernel stack before returning from syscalls. That reduces the information which kernel stack leak bugs can reveal and blocks some uninitialized stack variable attacks. Moreover, STACKLEAK provides runtime checks for kernel stack overflow detection. This commit introduces the architecture-specific code filling the used part of the kernel stack with a poison value before returning to the userspace. Full STACKLEAK feature also contains the gcc plugin which comes in a separate commit. The STACKLEAK feature is ported from grsecurity/PaX. More information at: https://grsecurity.net/ https://pax.grsecurity.net/ This code is modified from Brad Spengler/PaX Team's code in the last public patch of grsecurity/PaX based on our understanding of the code. Changes or omissions from the original code are ours and don't reflect the original grsecurity/PaX code. Signed-off-by: Alexander Popov <alex.popov@linux.com> --- arch/Kconfig | 27 ++++++++++ arch/x86/Kconfig | 1 + arch/x86/entry/entry_32.S | 65 +++++++++++++++++++++++ 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, 241 insertions(+)