[PATCH v12 07/10] arm64: Introduce stack trace reliability checks in the unwinder

madvenka
2022-01-03T16:56:21Z

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 in unwind().

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.

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
arch/arm64/include/asm/stacktrace.h | 3 +++
arch/arm64/kernel/stacktrace.c | 29 +++++++++++++++++++++++++++--
2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index 9d1fddc26586..47d4be69799a 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -63,6 +63,8 @@ struct stack_info {
* @final_fp Pointer to the final frame.
*
* @failed: Unwind failed.
+ *
+ * @reliable: Stack trace is reliable.
*/
struct unwind_state {
unsigned long fp;
@@ -79,6 +81,7 @@ struct unwind_state {
void *cookie;
unsigned long final_fp;
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 af0949f028c9..54c3396a65c3 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -18,6 +18,25 @@
#include <asm/stack_pointer.h>
#include <asm/stacktrace.h>

+/*
+ * Check the stack frame for conditions that make further unwinding unreliable.
+ */
+static void unwind_check_reliability(struct unwind_state *state)
+{
+ if (state->fp == state->final_fp) {
+ /* Final frame; no more unwind, no need to check reliability */
+ return;
+ }
+
+ /*
+ * 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(state->pc))
+ state->reliable = false;
+}
+
/*
* AArch64 PCS assigns the frame pointer to x29.
*
@@ -64,6 +83,8 @@ static void unwind_init_common(struct unwind_state *state,

/* Stack trace terminates here. */
state->final_fp = (unsigned long)task_pt_regs(task)->stackframe;
+
+ state->reliable = true;
}

/*
@@ -202,10 +223,14 @@ static void notrace unwind_next(struct unwind_state *state)
}
NOKPROBE_SYMBOL(unwind_next);

-static void notrace unwind(struct unwind_state *state)
+static bool notrace unwind(struct unwind_state *state)
{
- while (unwind_continue(state))
+ unwind_check_reliability(state);
+ while (unwind_continue(state)) {
unwind_next(state);
+ unwind_check_reliability(state);
+ }
+ return !state->failed && state->reliable;
}
NOKPROBE_SYMBOL(unwind);

--
2.25.1