Re: [PATCH v2 3/7] x86: stop calling fixup_exception() from kprobe_fault_handler()

[Jann Horn]

On Mon, Aug 27, 2018 at 9:02 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> On Mon, Aug 27, 2018 at 11:56 AM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> > This removes the call into exception fixup that was added in
> > commit c28f896634f2 ("[PATCH] kprobes: fix broken fault handling for
> > x86_64").
> >
> > On X86, kprobe_fault_handler() is called from two places:
> > do_general_protection() (for #GP) and kprobes_fault() (for #PF).
> > In both paths, the fixup_exception() call in the kprobe fault handler is
> > redundant.
> >
> > For #GP, fixup_exception() is called immediately before
> > kprobe_fault_handler() is invoked - if someone wanted to fix up our #GP,
> > they've already done so, no need to try again. (This assumes that the
> > kprobe's fault handler isn't going to do something crazy like changing RIP
> > so that it suddenly points to an instruction that does userspace access.)
>
> This needs review by someone who understands kprobes better than I do.
> What happens if someone puts a kprobe on a uaccess instruction and the
> uaccess subsequently faults?

Ugh, good point. I'll admit to not having thought about that properly.

I think that's the "if (unlikely(regs->ip == (unsigned
long)cur->ainsn.insn))" branch in kprobe_fault_handler(), which I'm
not touching.

For #PF, both without and with my patch, stuff should get fixed up by
the normal pagefault handler, since the fixup happens after the kprobe
handler has fiddled with the exception state.

For #GP, we're already past the fixup call, and I think both without
and with my patch, nothing will catch it - so I think that's a bug,
but I don't think it's one I'm introducing.

> > For #PF on a kernel address from kernel space, after the kprobe fault
> > handler has run, we'll go into no_context(), which calls fixup_exception().
> >
> > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> > ---
> >  arch/x86/kernel/kprobes/core.c | 7 -------
> >  1 file changed, 7 deletions(-)
> >
> > diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> > index 467ac22691b0..7315ac202aad 100644
> > --- a/arch/x86/kernel/kprobes/core.c
> > +++ b/arch/x86/kernel/kprobes/core.c
> > @@ -1021,13 +1021,6 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
> >                 if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
> >                         return 1;
> >
> > -               /*
> > -                * In case the user-specified fault handler returned
> > -                * zero, try to fix up.
> > -                */
> > -               if (fixup_exception(regs, trapnr))
> > -                       return 1;
> > -
> >                 /* fixup routine could not handle it. */
> >         }
> >
> > --
> > 2.19.0.rc0.228.g281dcd1b4d0-goog
> >