[PATCH] s390/livepatch: Implement reliable stack tracing for the consistency model

[Miroslav Benes]

The livepatch consistency model requires reliable stack tracing
architecture support in order to work properly. In order to achieve
this, two main issues have to be solved. First, reliable and consistent
call chain backtracing has to be ensured. Second, the unwinder needs to
be able to detect stack corruptions and return errors.

The "zSeries ELF Application Binary Interface Supplement" says:

  "The stack pointer points to the first word of the lowest allocated
  stack frame. If the "back chain" is implemented this word will point to
  the previously allocated stack frame (towards higher addresses), except
  for the first stack frame, which shall have a back chain of zero (NULL).
  The stack shall grow downwards, in other words towards lower addresses."

"back chain" is optional. GCC option -mbackchain enables it. Quoting
Martin Schwidefsky [1]:

  "The compiler is called with the -mbackchain option, all normal C
  function will store the backchain in the function prologue. All
  functions written in assembler code should do the same, if you find one
  that does not we should fix that. The end result is that a task that
  *voluntarily* called schedule() should have a proper backchain at all
  times.

  Dependent on the use case this may or may not be enough. Asynchronous
  interrupts may stop the CPU at the beginning of a function, if kernel
  preemption is enabled we can end up with a broken backchain.  The
  production kernels for IBM Z are all compiled *without* kernel
  preemption. So yes, we might get away without the objtool support.

  On a side-note, we do have a line item to implement the ORC unwinder for
  the kernel, that includes the objtool support. Once we have that we can
  drop the -mbackchain option for the kernel build. That gives us a nice
  little performance benefit. I hope that the change from backchain to the
  ORC unwinder will not be too hard to implement in the livepatch tools."

Thus, the call chain backtracing should be currently ensured and objtool
should not be necessary for livepatch purposes.

Regarding the second issue, stack corruptions and non-reliable states
have to be recognized by the unwinder. Mainly it means to detect
preemption or page faults, the end of the task stack must be reached,
return addresses must be valid text addresses and hacks like function
graph tracing and kretprobes must be properly detected.

Unwinding a running task's stack is not a problem, because there is a
livepatch requirement that every checked task is blocked, except for the
current task. Due to that, the implementation can be much simpler
compared to the existing non-reliable infrastructure. We can consider a
task's kernel/thread stack only and skip the other stacks.

Idle tasks are a bit special. Their final back chains point to no_dat
stacks. See for reference CALL_ON_STACK() in smp_start_secondary()
callback used in __cpu_up(). The unwinding is stopped there and it is
not considered to be a stack corruption.

Signed-off-by: Miroslav Benes <mbenes@xxxxxxx>
---
- based on Linus' master
- passes livepatch kselftests
- passes tests from https://github.com/lpechacek/qa_test_klp, which
  stress the consistency model and the unwinder a bit more

 arch/s390/Kconfig                  |  1 +
 arch/s390/include/asm/stacktrace.h |  5 ++
 arch/s390/include/asm/unwind.h     | 19 ++++++
 arch/s390/kernel/dumpstack.c       | 28 +++++++++
 arch/s390/kernel/stacktrace.c      | 78 +++++++++++++++++++++++++
 arch/s390/kernel/unwind_bc.c       | 93 ++++++++++++++++++++++++++++++
 6 files changed, 224 insertions(+)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index fdb4246265a5..ea73e555063d 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -170,6 +170,7 @@ config S390
 	select HAVE_PERF_EVENTS
 	select HAVE_RCU_TABLE_FREE
 	select HAVE_REGS_AND_STACK_ACCESS_API
+	select HAVE_RELIABLE_STACKTRACE
 	select HAVE_RSEQ
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_VIRT_CPU_ACCOUNTING
diff --git a/arch/s390/include/asm/stacktrace.h b/arch/s390/include/asm/stacktrace.h
index 0ae4bbf7779c..2b5c913c408f 100644
--- a/arch/s390/include/asm/stacktrace.h
+++ b/arch/s390/include/asm/stacktrace.h
@@ -23,6 +23,11 @@ const char *stack_type_name(enum stack_type type);
 int get_stack_info(unsigned long sp, struct task_struct *task,
 		   struct stack_info *info, unsigned long *visit_mask);
 
+#ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
+int get_stack_info_reliable(unsigned long sp, struct task_struct *task,
+			    struct stack_info *info);
+#endif
+
 static inline bool on_stack(struct stack_info *info,
 			    unsigned long addr, size_t len)
 {
diff --git a/arch/s390/include/asm/unwind.h b/arch/s390/include/asm/unwind.h
index d827b5b9a32c..1cc96c54169c 100644
--- a/arch/s390/include/asm/unwind.h
+++ b/arch/s390/include/asm/unwind.h
@@ -45,6 +45,25 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 bool unwind_next_frame(struct unwind_state *state);
 unsigned long unwind_get_return_address(struct unwind_state *state);
 
+#ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
+void __unwind_start_reliable(struct unwind_state *state,
+			     struct task_struct *task, unsigned long sp);
+bool unwind_next_frame_reliable(struct unwind_state *state);
+
+static inline void unwind_start_reliable(struct unwind_state *state,
+					 struct task_struct *task)
+{
+	unsigned long sp;
+
+	if (task == current)
+		sp = current_stack_pointer();
+	else
+		sp = task->thread.ksp;
+
+	__unwind_start_reliable(state, task, sp);
+}
+#endif
+
 static inline bool unwind_done(struct unwind_state *state)
 {
 	return state->stack_info.type == STACK_TYPE_UNKNOWN;
diff --git a/arch/s390/kernel/dumpstack.c b/arch/s390/kernel/dumpstack.c
index ac06c3949ab3..b21ef2a766ff 100644
--- a/arch/s390/kernel/dumpstack.c
+++ b/arch/s390/kernel/dumpstack.c
@@ -127,6 +127,34 @@ int get_stack_info(unsigned long sp, struct task_struct *task,
 	return -EINVAL;
 }
 
+#ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
+int get_stack_info_reliable(unsigned long sp, struct task_struct *task,
+			    struct stack_info *info)
+{
+	if (!sp)
+		goto error;
+
+	/* Sanity check: ABI requires SP to be aligned 8 bytes. */
+	if (sp & 0x7)
+		goto error;
+
+	if (!task)
+		goto error;
+
+	/*
+	 * The unwinding should not start on nodat_stack, async_stack or
+	 * restart_stack. The task is either current or must be inactive.
+	 */
+	if (!in_task_stack(sp, task, info))
+		goto error;
+
+	return 0;
+error:
+	info->type = STACK_TYPE_UNKNOWN;
+	return -EINVAL;
+}
+#endif
+
 void show_stack(struct task_struct *task, unsigned long *stack)
 {
 	struct unwind_state state;
diff --git a/arch/s390/kernel/stacktrace.c b/arch/s390/kernel/stacktrace.c
index f6a620f854e1..7d774a325163 100644
--- a/arch/s390/kernel/stacktrace.c
+++ b/arch/s390/kernel/stacktrace.c
@@ -13,6 +13,7 @@
 #include <linux/export.h>
 #include <asm/stacktrace.h>
 #include <asm/unwind.h>
+#include <asm/kprobes.h>
 
 void save_stack_trace(struct stack_trace *trace)
 {
@@ -60,3 +61,80 @@ void save_stack_trace_regs(struct pt_regs *regs, struct stack_trace *trace)
 	}
 }
 EXPORT_SYMBOL_GPL(save_stack_trace_regs);
+
+#ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
+/*
+ * This function returns an error if it detects any unreliable features of the
+ * stack.  Otherwise it guarantees that the stack trace is reliable.
+ *
+ * If the task is not 'current', the caller *must* ensure the task is inactive.
+ */
+static __always_inline int
+__save_stack_trace_tsk_reliable(struct task_struct *tsk,
+				struct stack_trace *trace)
+{
+	struct unwind_state state;
+
+	for (unwind_start_reliable(&state, tsk);
+	     !unwind_done(&state) && !unwind_error(&state);
+	     unwind_next_frame_reliable(&state)) {
+
+		if (!__kernel_text_address(state.ip))
+			return -EINVAL;
+
+#ifdef CONFIG_KPROBES
+		/*
+		 * Mark stacktraces with kretprobed functions on them
+		 * as unreliable.
+		 */
+		if (state.ip == (unsigned long)kretprobe_trampoline)
+			return -EINVAL;
+#endif
+
+		if (trace->nr_entries >= trace->max_entries)
+			return -E2BIG;
+
+		if (!trace->skip)
+			trace->entries[trace->nr_entries++] = state.ip;
+		else
+			trace->skip--;
+	}
+
+	/* Check for stack corruption */
+	if (unwind_error(&state))
+		return -EINVAL;
+
+	/* Store kernel_thread_starter, null for swapper/0 */
+	if (tsk->flags & (PF_KTHREAD | PF_IDLE)) {
+		if (trace->nr_entries >= trace->max_entries)
+			return -E2BIG;
+
+		if (!trace->skip)
+			trace->entries[trace->nr_entries++] =
+				state.regs->psw.addr;
+		else
+			trace->skip--;
+	}
+
+	return 0;
+}
+
+int save_stack_trace_tsk_reliable(struct task_struct *tsk,
+				  struct stack_trace *trace)
+{
+	int ret;
+
+	/*
+	 * If the task doesn't have a stack (e.g., a zombie), the stack is
+	 * "reliably" empty.
+	 */
+	if (!try_get_task_stack(tsk))
+		return 0;
+
+	ret = __save_stack_trace_tsk_reliable(tsk, trace);
+
+	put_task_stack(tsk);
+
+	return ret;
+}
+#endif
diff --git a/arch/s390/kernel/unwind_bc.c b/arch/s390/kernel/unwind_bc.c
index 3ce8a0808059..ada3a8538961 100644
--- a/arch/s390/kernel/unwind_bc.c
+++ b/arch/s390/kernel/unwind_bc.c
@@ -153,3 +153,96 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 	state->reliable = reliable;
 }
 EXPORT_SYMBOL_GPL(__unwind_start);
+
+#ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
+void __unwind_start_reliable(struct unwind_state *state,
+			     struct task_struct *task, unsigned long sp)
+{
+	struct stack_info *info = &state->stack_info;
+	struct stack_frame *sf;
+	unsigned long ip;
+
+	memset(state, 0, sizeof(*state));
+	state->task = task;
+
+	/* Get current stack pointer and initialize stack info */
+	if (get_stack_info_reliable(sp, task, info) ||
+	    !on_stack(info, sp, sizeof(struct stack_frame))) {
+		/* Something is wrong with the stack pointer */
+		info->type = STACK_TYPE_UNKNOWN;
+		state->error = true;
+		return;
+	}
+
+	/* Get the instruction pointer from the stack frame */
+	sf = (struct stack_frame *) sp;
+	ip = READ_ONCE_NOCHECK(sf->gprs[8]);
+
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+	/* Decode any ftrace redirection */
+	if (ip == (unsigned long) return_to_handler)
+		ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
+					   ip, NULL);
+#endif
+
+	/* Update unwind state */
+	state->sp = sp;
+	state->ip = ip;
+}
+
+bool unwind_next_frame_reliable(struct unwind_state *state)
+{
+	struct stack_info *info = &state->stack_info;
+	struct stack_frame *sf;
+	struct pt_regs *regs;
+	unsigned long sp, ip;
+
+	sf = (struct stack_frame *) state->sp;
+	sp = READ_ONCE_NOCHECK(sf->back_chain);
+	/*
+	 * Idle tasks are special. The final back-chain points to nodat_stack.
+	 * See CALL_ON_STACK() in smp_start_secondary() callback used in
+	 * __cpu_up(). We just accept it, go to else branch and look for
+	 * pt_regs.
+	 */
+	if (likely(sp && !(is_idle_task(state->task) &&
+			   outside_of_stack(state, sp)))) {
+		/* Non-zero back-chain points to the previous frame */
+		if (unlikely(outside_of_stack(state, sp)))
+			goto out_err;
+
+		sf = (struct stack_frame *) sp;
+		ip = READ_ONCE_NOCHECK(sf->gprs[8]);
+	} else {
+		/* No back-chain, look for a pt_regs structure */
+		sp = state->sp + STACK_FRAME_OVERHEAD;
+		regs = (struct pt_regs *) sp;
+		if ((unsigned long)regs != info->end - sizeof(struct pt_regs))
+			goto out_err;
+		if (!(state->task->flags & (PF_KTHREAD | PF_IDLE)) &&
+		     !user_mode(regs))
+			goto out_err;
+
+		state->regs = regs;
+		goto out_stop;
+	}
+
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+	/* Decode any ftrace redirection */
+	if (ip == (unsigned long) return_to_handler)
+		ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
+					   ip, (void *) sp);
+#endif
+
+	/* Update unwind state */
+	state->sp = sp;
+	state->ip = ip;
+	return true;
+
+out_err:
+	state->error = true;
+out_stop:
+	state->stack_info.type = STACK_TYPE_UNKNOWN;
+	return false;
+}
+#endif
-- 
2.22.0