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

nobuta.keiya
2021-11-04T12:47:09Z

Hi Madhavan,

> -----Original Message-----
> From: madvenka@linux.microsoft.com <madvenka@linux.microsoft.com>
> Sent: Friday, October 15, 2021 11:59 AM
> To: mark.rutland@arm.com; broonie@kernel.org; jpoimboe@redhat.com; ardb@kernel.org; Nobuta, Keiya/信田 圭哉
> <nobuta.keiya@fujitsu.com>; sjitindarsingh@gmail.com; catalin.marinas@arm.com; will@kernel.org; jmorris@namei.org;
> linux-arm-kernel@lists.infradead.org; live-patching@vger.kernel.org; linux-kernel@vger.kernel.org;
> madvenka@linux.microsoft.com
> Subject: [PATCH v10 10/11] arm64: Introduce stack trace reliability checks in the unwinder
>
> 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);
> }

Isn't it necessary to check "final frame" before unwind_check_reliability()?
The frame at this point is unwound frame, so may be last frame.

Or if move unwind_check_reliability() into unwind(), I think unwind() can
be twins as below:

~~~~~~~~
unwind(...) {
<...>
for (unwind_start(...); unwind_continue(...); unwind_next(...))
unwind_check_reliability(&frame);
}

unwind_reliable(...) {
<...>
for (unwind_start(...); unwind_continue(...); unwind_next(...)) {
unwind_check_reliability(&frame);
if (!frame.reliable)
break;
}

return (frame.reliable && !frame.failed);
}
~~~~~~~~

Thanks,
Keiya

>
> 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