[RFC PATCH v3 1/4] arm64: Introduce stack trace reliability checks in the unwinder

[madvenka@linux.microsoft.com]

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

The unwinder should check for the presence of various features and
conditions that can render the stack trace unreliable and mark the
the stack trace as unreliable for the benefit of the caller.

Introduce the first reliability check - If a return PC encountered in a
stack trace is not a valid kernel text address, the stack trace is
considered unreliable. It could be some generated code. Mark the stack
trace unreliable.

Other reliability checks will be added in the future.

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
 arch/arm64/include/asm/stacktrace.h |  4 ++++
 arch/arm64/kernel/stacktrace.c      | 19 ++++++++++++++++---
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index eb29b1fe8255..f1eab6b029f7 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -49,6 +49,8 @@ struct stack_info {
  *
  * @graph:       When FUNCTION_GRAPH_TRACER is selected, holds the index of a
  *               replacement lr value in the ftrace graph stack.
+ *
+ * @reliable:	Is this stack frame reliable?
  */
 struct stackframe {
 	unsigned long fp;
@@ -59,6 +61,7 @@ struct stackframe {
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	int graph;
 #endif
+	bool reliable;
 };
 
 extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame);
@@ -169,6 +172,7 @@ static inline void start_backtrace(struct stackframe *frame,
 	bitmap_zero(frame->stacks_done, __NR_STACK_TYPES);
 	frame->prev_fp = 0;
 	frame->prev_type = STACK_TYPE_UNKNOWN;
+	frame->reliable = true;
 }
 
 #endif	/* __ASM_STACKTRACE_H */
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index d55bdfb7789c..c21a1bca28f3 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -44,6 +44,8 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 	unsigned long fp = frame->fp;
 	struct stack_info info;
 
+	frame->reliable = true;
+
 	/* Terminal record; nothing to unwind */
 	if (!fp)
 		return -ENOENT;
@@ -86,12 +88,24 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 	 */
 	frame->fp = READ_ONCE_NOCHECK(*(unsigned long *)(fp));
 	frame->pc = READ_ONCE_NOCHECK(*(unsigned long *)(fp + 8));
+	frame->pc = ptrauth_strip_insn_pac(frame->pc);
 	frame->prev_fp = fp;
 	frame->prev_type = info.type;
 
+	/*
+	 * First, make sure that the return address is a proper kernel text
+	 * address. A NULL or invalid return address probably means there's
+	 * some generated code which __kernel_text_address() doesn't know
+	 * about. Mark the stack trace as not reliable.
+	 */
+	if (!__kernel_text_address(frame->pc)) {
+		frame->reliable = false;
+		return 0;
+	}
+
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	if (tsk->ret_stack &&
-		(ptrauth_strip_insn_pac(frame->pc) == (unsigned long)return_to_handler)) {
+		frame->pc == (unsigned long)return_to_handler) {
 		struct ftrace_ret_stack *ret_stack;
 		/*
 		 * This is a case where function graph tracer has
@@ -103,11 +117,10 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 		if (WARN_ON_ONCE(!ret_stack))
 			return -EINVAL;
 		frame->pc = ret_stack->ret;
+		frame->pc = ptrauth_strip_insn_pac(frame->pc);
 	}
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
 
-	frame->pc = ptrauth_strip_insn_pac(frame->pc);
-
 	return 0;
 }
 NOKPROBE_SYMBOL(unwind_frame);
-- 
2.25.1