Re: [RFC PATCH 1/2] x86: WARN() when uaccess helpers fault on kernel addresses

[Jann Horn]

On Thu, Aug 23, 2018 at 2:28 AM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
> On Wed, Aug 22, 2018 at 4:53 PM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> > On Tue, Aug 7, 2018 at 4:55 AM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >> > On Aug 6, 2018, at 6:22 PM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> >> > There have been multiple kernel vulnerabilities that permitted userspace to
> >> > pass completely unchecked pointers through to userspace accessors:
> >> >
> >> > - the waitid() bug - commit 96ca579a1ecc ("waitid(): Add missing
> >> >  access_ok() checks")
> >> > - the sg/bsg read/write APIs
> >> > - the infiniband read/write APIs
> >> >
> >> > These don't happen all that often, but when they do happen, it is hard to
> >> > test for them properly; and it is probably also hard to discover them with
> >> > fuzzing. Even when an unmapped kernel address is supplied to such buggy
> >> > code, it just returns -EFAULT instead of doing a proper BUG() or at least
> >> > WARN().
> >> >
> >> > This patch attempts to make such misbehaving code a bit more visible by
> >> > WARN()ing in the pagefault handler code when a userspace accessor causes
> >> > #PF on a kernel address and the current context isn't whitelisted.
> >>
> >> I like this a lot, and, in fact, I once wrote a patch to do something similar. It was before the fancy extable code, though, so it was a mess.  Here are some thoughts:
> >>
> >> - It should be three patches. One patch to add the _UA annotations, one to improve the info passes to the handlers, and one to change behavior.
> >>
> >> - You should pass the vector, the error code, and the address to the handler.
> >
> > I'm polishing the patch a bit, and I've noticed that to plumb the
> > error code and address through properly, I might need significantly
> > more code churn because of kprobes - I want to make sure I'm not going
> > down the completely wrong path here.
> >
> > I'm extending fixup_exception() to take two extra args "unsigned long
> > error_code, unsigned long fault_addr". Most callers of
> > fixup_exception() are fairly straightforward, but
> > kprobe_fault_handler() has a dozen callchains from different exception
> > handlers, and most of them are coming via notify_die().
>
> KILL IT WITH FIRE!!!!!!!!
>
> More seriously, kill kprobe_exceptions_notify() and just fold the
> contents into do_general_protection().  This notifier chain crap is
> incomprehensible.  I would love to see notify_die() removed entirely,
> and crap like this is just more reason to want it gone.

This isn't just do_general_protection() though, that's just one
example. As far as I can tell, similar stuff happens everywhere where
notify_die() is used - #DF, #BR, #BP, #MF and so on.

> > I think there's also some inconsistency between #PF and #GP in the
> > ordering of error handling:
>
> It's probably a bug.  It's also probably irrelevant, but maybe not.

Depends on what people do in their ->fault_handler hooks, I guess.
Yeah, probably doesn't matter.