Re: [PATCH v3 1/2] kretprobe: produce sane stack traces

[Aleksa Sarai <asarai@suse.de>]

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On 2018-11-09, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> On Thu, 8 Nov 2018 08:44:37 -0600
> Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> 
> > On Thu, Nov 08, 2018 at 07:04:48PM +1100, Aleksa Sarai wrote:
> > > On 2018-11-08, Aleksa Sarai <cyphar@cyphar.com> wrote:
> > > > I will attach what I have at the moment to hopefully explain what the
> > > > issue I've found is (re-using the kretprobe architecture but with the
> > > > shadow-stack idea).
> > > 
> > > Here is the patch I have at the moment (it works, except for the
> > > question I have about how to handle the top-level pt_regs -- I've marked
> > > that code with XXX).
> > > 
> > > -- 
> > > Aleksa Sarai
> > > Senior Software Engineer (Containers)
> > > SUSE Linux GmbH
> > > <https://www.cyphar.com/>
> > > 
> > > --8<---------------------------------------------------------------------
> > > 
> > > Since the return address is modified by kretprobe, the various unwinders
> > > can produce invalid and confusing stack traces. ftrace mostly solved
> > > this problem by teaching each unwinder how to find the original return
> > > address for stack trace purposes. This same technique can be applied to
> > > kretprobes by simply adding a pointer to where the return address was
> > > replaced in the stack, and then looking up the relevant
> > > kretprobe_instance when a stack trace is requested.
> > > 
> > > [WIP: This is currently broken because the *first entry* will not be
> > >       overwritten since it looks like the stack pointer is different
> > >       when we are provided pt_regs. All other addresses are correctly
> > >       handled.]
> > 
> > When you see this problem, what does regs->ip point to?  If it's
> > pointing to generated code, then we don't _currently_ have a way of
> > dealing with that.  If it's pointing to a real function, we can fix that
> > with unwind hints.
> 
> As I replied, If the stackdump is called from kretprobe event, regs->ip
> always points trampoline function. Otherwise (maybe from kprobe event,
> or panic, BUG etc.) it always be the address which the event occurs.
> 
> So fixing regs->ip is correct.

The problem is that the pointer to the *return address* is wrong
(kernel_stack_pointer() gives you a different result than the function
entry), it's not that regs->ip is wrong. And I'm sure that it's "wrong"
because it's not possible for "regs->ip == kretprobe_trampoline" unless
you are in a stack frame that has been modified by the kretprobe core.

I will take a closer look at this over the weekend -- I posted the patch
to try to help explain what the underlying issue I was trying to solve
with this patch series is (and why I don't think the ftrace changes
proposed in the thread will completely fix them).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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