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    SubjectRe: [PATCH] arm64: avoid race condition issue in dump_backtrace
    On Wed, Mar 28, 2018 at 05:33:32PM +0800, Ji.Zhang wrote:
    > On Mon, 2018-03-26 at 12:39 +0100, Mark Rutland wrote:
    > > I think that it would be preferable to try to avoid the inifinite loop
    > > case. We could hit that by accident if we're tracing a live task.
    > >
    > > It's a little tricky to ensure that we don't loop, since we can have
    > > traces that span several stacks, e.g. overflow -> irq -> task, so we
    > > need to know where the last frame was, and we need to defnie a strict
    > > order for stack nesting.
    > Can we consider this through an easier way? According to AArch64 PCS,
    > stack should be full-descending, which means we can add validation on fp
    > by comparing the fp and previous fp, if they are equal means there is an
    > exactly loop, while if current fp is smaller than previous means the
    > uwnind is rollback, which is also unexpected. The only concern is how to
    > handle the unwind from one stack span to another (eg. overflow->irq, or
    > irq->task, etc)
    > Below diff is a proposal that we check if stack spans, and if yes, a
    > tricky is used to bypass the fp check.
    >
    > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
    > index eb2d151..760ea59 100644
    > --- a/arch/arm64/kernel/traps.c
    > +++ b/arch/arm64/kernel/traps.c
    > @@ -101,6 +101,7 @@ void dump_backtrace(struct pt_regs *regs, struct
    > task_struct *tsk)
    > {
    > struct stackframe frame;
    > int skip;
    > + unsigned long fp = 0x0;
    >
    > pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk);
    >
    > @@ -127,6 +128,20 @@ void dump_backtrace(struct pt_regs *regs, struct
    > task_struct *tsk)
    > skip = !!regs;
    > printk("Call trace:\n");
    > do {
    > + unsigned long stack;
    > + if (fp) {
    > + if (in_entry_text(frame.pc)) {
    > + stack = frame.fp - offsetof(struct
    > pt_regs, stackframe);
    > +
    > + if (on_accessible_stack(tsk, stack))
    > + fp = frame.fp + 0x8; //tricky to
    > bypass the fp check
    > + }
    > + if (fp <= frame->fp) {
    > + pr_notice("fp invalid, stop unwind\n");
    > + break;
    > + }
    > + }
    > + fp = frame.fp;

    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.

    Thanks,
    Mark.

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