[PATCH] arm64: avoid race condition issue in dump_backtrace

[Ji.Zhang]

On Wed, 2018-03-28 at 11:12 +0100, Mark Rutland wrote:
> On Wed, Mar 28, 2018 at 05:33:32PM +0800, Ji.Zhang wrote:
> 
> I'm very much not keen on this.
> 
> I think that if we're going to do this, the only sane way to do it is to
> have unwind_frame() verify the current fp against the previous one, and
> verify that we have some strict nesting of stacks. Generally, that means
> we can go:
> 
>   overflow -> irq -> task
> 
> ... though I'm not sure what to do about the SDEI stack vs the overflow
> stack.
Actually I have had the fp check in unwind_frame(), but since I use the
in_entry_text() to determine if stack spans, and I did not want to
include traps.h in stacktrace.c, so I move the check out to
dump_backtrace.
Anyway, It seems that the key point is how should we verify that there
are some nesting of stacks. Since in unwind_frame() we already have the
previous fp and current fp, could we assume that if these two fps are
NOT belong to the same stack, there should be stack spans (no matter
task->irq, or irq->overflow, etc), and we can do the tricky to bypass
the fp check.The sample of the prosal just like:

diff --git a/arch/arm64/include/asm/stacktrace.h
b/arch/arm64/include/asm/stacktrace.h
index 902f9ed..fc2bf4d 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -92,4 +92,18 @@ static inline bool on_accessible_stack(struct
task_struct *tsk, unsigned long sp
        return false;
 }

+static inline bool on_same_stack(struct task_struct *tsk, unsigned long
sp1, unsigned sp2)
+{
+       if (on_task_stack(tsk, sp1) && on_task_stack(tsk, sp2))
+               return true;
+       if (on_irq_stack(sp1) && on_irq_stack(sp2))
+               return true;
+       if (on_overflow_stack(sp1) && on_overflow_stack(sp2))
+               return true;
+       if (on_sdei_stack(sp1) && on_sdei_stack(sp2))
+               return true;
+
+       return false;
+}
+
 #endif /* __ASM_STACKTRACE_H */
diff --git a/arch/arm64/kernel/stacktrace.c
b/arch/arm64/kernel/stacktrace.c
index d5718a0..4907a67 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -56,6 +56,13 @@ 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));

+       if (!on_same_stack(fp, frame->fp))
+               fp = frame->fp + 0x8;
+       if (fp <= frame->fp) {
+               pr_notice("fp invalid, stop unwind\n");
+               return -EINVAL;
+       }
+
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
        if (tsk->ret_stack &&
                        (frame->pc == (unsigned long)return_to_handler))
{

Could this work?

Best Regards,
Ji