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        madvenka@linux.microsoft.com
Subject: [PATCH v10 10/11] arm64: Introduce stack trace reliability checks in the unwinder
Date:   Thu, 14 Oct 2021 21:58:46 -0500
Message-Id: <20211015025847.17694-11-madvenka@linux.microsoft.com>
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 <20211015025847.17694-1-madvenka@linux.microsoft.com>
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From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

There are some kernel features and conditions that make a stack trace
unreliable. Callers may require the unwinder to detect these cases.
E.g., livepatch.

Introduce a new function called unwind_check_reliability() that will
detect these cases and set a flag in the stack frame. Call
unwind_check_reliability() for every frame, that is, in unwind_start()
and unwind_next().

Introduce the first reliability check in unwind_check_reliability() - If
a return PC is not a valid kernel text address, consider the stack
trace unreliable. It could be some generated code. Other reliability checks
will be added in the future.

Let unwind() return a boolean to indicate if the stack trace is
reliable.

Introduce arch_stack_walk_reliable() for ARM64. This works like
arch_stack_walk() except that it returns -EINVAL if the stack trace is not
reliable.

Until all the reliability checks are in place, arch_stack_walk_reliable()
may not be used by livepatch. But it may be used by debug and test code.

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
 arch/arm64/include/asm/stacktrace.h |  3 ++
 arch/arm64/kernel/stacktrace.c      | 48 ++++++++++++++++++++++++++++-
 2 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index ba2180c7d5cd..ce0710fa3037 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -51,6 +51,8 @@ struct stack_info {
  *               replacement lr value in the ftrace graph stack.
  *
  * @failed:      Unwind failed.
+ *
+ * @reliable:    Stack trace is reliable.
  */
 struct stackframe {
 	unsigned long fp;
@@ -62,6 +64,7 @@ struct stackframe {
 	int graph;
 #endif
 	bool failed;
+	bool reliable;
 };
 
 extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index 8e9e6f38c975..142f08ae515f 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -18,6 +18,22 @@
 #include <asm/stack_pointer.h>
 #include <asm/stacktrace.h>
 
+/*
+ * Check the stack frame for conditions that make further unwinding unreliable.
+ */
+static void notrace unwind_check_reliability(struct stackframe *frame)
+{
+	/*
+	 * If the PC is not a known kernel text address, then we cannot
+	 * be sure that a subsequent unwind will be reliable, as we
+	 * don't know that the code follows our unwind requirements.
+	 */
+	if (!__kernel_text_address(frame->pc))
+		frame->reliable = false;
+}
+
+NOKPROBE_SYMBOL(unwind_check_reliability);
+
 /*
  * AArch64 PCS assigns the frame pointer to x29.
  *
@@ -55,6 +71,8 @@ static void notrace unwind_start(struct stackframe *frame, unsigned long fp,
 	frame->prev_fp = 0;
 	frame->prev_type = STACK_TYPE_UNKNOWN;
 	frame->failed = false;
+	frame->reliable = true;
+	unwind_check_reliability(frame);
 }
 
 NOKPROBE_SYMBOL(unwind_start);
@@ -138,6 +156,7 @@ static void notrace unwind_next(struct task_struct *tsk,
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
 
 	frame->pc = ptrauth_strip_insn_pac(frame->pc);
+	unwind_check_reliability(frame);
 }
 
 NOKPROBE_SYMBOL(unwind_next);
@@ -167,7 +186,7 @@ static bool notrace unwind_continue(struct task_struct *task,
 
 NOKPROBE_SYMBOL(unwind_continue);
 
-static void notrace unwind(struct task_struct *tsk,
+static bool notrace unwind(struct task_struct *tsk,
 			   unsigned long fp, unsigned long pc,
 			   bool (*fn)(void *, unsigned long),
 			   void *data)
@@ -177,6 +196,7 @@ static void notrace unwind(struct task_struct *tsk,
 	unwind_start(&frame, fp, pc);
 	while (unwind_continue(tsk, &frame, fn, data))
 		unwind_next(tsk, &frame);
+	return frame.reliable;
 }
 
 NOKPROBE_SYMBOL(unwind);
@@ -238,4 +258,30 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
 
 }
 
+/*
+ * arch_stack_walk_reliable() may not be used for livepatch until all of
+ * the reliability checks are in place in unwind_consume(). However,
+ * debug and test code can choose to use it even if all the checks are not
+ * in place.
+ */
+noinline int notrace arch_stack_walk_reliable(stack_trace_consume_fn consume_fn,
+					      void *cookie,
+					      struct task_struct *task)
+{
+	unsigned long fp, pc;
+
+	if (task == current) {
+		/* Skip arch_stack_walk_reliable() in the stack trace. */
+		fp = (unsigned long)__builtin_frame_address(1);
+		pc = (unsigned long)__builtin_return_address(0);
+	} else {
+		/* Caller guarantees that the task is not running. */
+		fp = thread_saved_fp(task);
+		pc = thread_saved_pc(task);
+	}
+	if (unwind(task, fp, pc, consume_fn, cookie))
+		return 0;
+	return -EINVAL;
+}
+
 #endif
-- 
2.25.1