Re: kprobes vs __ex_table[]

[Peter Zijlstra]

On Fri, Feb 24, 2017 at 10:04:51AM +0900, Masami Hiramatsu wrote:
> On Thu, 23 Feb 2017 19:30:02 +0100
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > Hi Masami,
> > 
> > I just wondered what would happen if I put a probe on an instruction
> > that was listed in __ex_table[] or __bug_table[].
> 
> Ah, thanks for reporting, I know __ex_table issue and fixed, but
> I didn't care about __bug_table.
> 
> > And it looks like it will happily do that. It will then run the
> > instruction out-of-line, and when said instruction traps, the
> > instruction address will not match the one listed in either __ex_table[]
> > or __bug_table[] and badness will happen.
> 
> For the __ex_table[], at least on x86, kprobes already handles it in
> kprobe_fault_handler, which restore regs->ip to original place when
> a pagefault happens on singlestepping.

Ah, but that is only #PF, we also use __ex_table on other faults/traps,
like #GP which would need help in do_general_protection(), and I have a
patch (that might not ever go anywhere) that uses it on #UD (but for all
I know we already use #UD to probe existence of instructions).

In any case, we call fixup_exception() from pretty much all exception
vectors, not only #PF.

But see below.

> > If kprobes does indeed not check this, we should probably fix it, if it
> > does do check this, could you point me to it?
> 
> Yeah, for BUG() case, as far as I can see, there is no check about that.

So I've a patch that extends __bug_table[] to WARN (like many other
architectures already have).

http://lkml.kernel.org/r/20170223132813.GB6515@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

> So, there are 2 ways to fix it up, one is to just reject to put kprobes on
> UD2, another is fixup trap address as we did for exceptions_table.
> I think latter is better because if there is a divide error happening
> on single-step, anyway we should fixup the address...

Right.

So I like the fixup idea, just not sure the current
kprobe_fault_handler() is sufficient or correct.

It looks like it rewrites regs->ip, which would make return from fault
return to the wrong place, no?

Would it not be better to do the fixup in fixup_exception()/fixup_bug()?
Because then we cover all callers, not just #PF.

One more complication with __ex_table and optimized kprobes is that we
need to be careful not to clobber __ex_table[].fixup. It would be very
bad if the optimized probe were to clobber the address we let the fixup
return to -- or that needs fixups too, _after_ running
__ex_table[].handler().