Re: [PATCH 1/2] x86/kprobes: Fix kprobes instruction boudary check with CONFIG_RETHUNK

Peter Zijlstra
2022-09-07T14:28:54Z

On Wed, Sep 07, 2022 at 11:14:50PM +0900, Masami Hiramatsu wrote:
> On Wed, 7 Sep 2022 15:05:13 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
>
> > On Wed, Sep 07, 2022 at 10:02:41AM +0200, Peter Zijlstra wrote:
> >
> > > struct queue q;
> > >
> > > start = paddr - offset;
> > > end = start + size;
> > > push(&q, paddr - offset);
> > >
> > > while (start = pop(&q)) {
> > > for_each_insn(&insn, start, end, buf) {
> > > if (insn.kaddr == paddr)
> > > return 1;
> > >
> > > target = insn_get_branch_addr(&insn);
> > > if (target)
> > > push(&q, target);
> > >
> > > if (dead_end_insn(&insn))
> > > break;
> > > }
> > > }
> >
> > There is the very rare case of intra-function-calls; but I *think*
> > they're all in noinstr/nokprobe code anyway.
> >
> > For instance we have RSB stuffing code like:
> >
> > .rept 16
> > call 1f;
> > int3
> > 1:
> > .endr
> > add $(BITS_PER_LONG/8) * 16, %_ASM_SP
> >
> > And the proposed will be horribly confused by that. But like said; it
> > should also never try and untangle it.
>
> Yeah, but I guess if we break the decoding (internal) loop when we
> hit an INT3, it maybe possible to be handled?

If you make insn_get_branch_addr() return the target of CALL
instructions when this target is between function start and end, it
should work I think.

But like said; this construct is rare and all instances I can remember
should not be kprobes to begin with. These are all 'fun' things like
retpoline stubs and the the above RSB stuff loop.