[PATCH v10 10/11] arm64: Introduce stack trace reliability checks in the unwinder

madvenka
2021-10-15T02:59:38Z

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