Re: [tip:x86/asm] x86/asm/entry/64: Migrate error and IRQ exit work to C and remove old assembly code

Frederic Weisbecker
2015-08-12T13:32:22Z

On Tue, Aug 11, 2015 at 04:33:05PM -0700, Andy Lutomirski wrote:
> On Tue, Aug 11, 2015 at 4:22 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> >
> > On Tue, Aug 11, 2015 at 03:51:26PM -0700, Andy Lutomirski wrote:
> >> On Tue, Aug 11, 2015 at 3:38 PM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> >> >
> >> > This makes me very nervous as well!
> >> >
> >> > It means that instead of using the context tracking save/restore model that we had
> >> > with exception_enter/exception_exit(), now we rely on the CS register.
> >> >
> >> > I don't think we can do that because our "context tracking" is a soft tracking whereas
> >> > CS is hard tracking and both are not atomically synchronized together.
> >> >
> >> > Imagine this situation: we are running in userspace. Context tracking knows it, everything
> >> > is fine. Now we do a syscall, we enter in kernel entry code but we trigger an exception
> >> > (DEBUG for example) before we got a chance to call user_exit(), which means that the context
> >> > tracking code still thinks we are in userspace, so we look at CS from the exception entry code
> >> > and it says the exception happened in the kernel. Hence we don't call user_exit() before calling
> >> > the exception handler. There is the bug because the exception handler may use RCU which still
> >> > thinks we run in userspace.
> >>
> >> #DB doesn't go through this patch -- it uses the paranoid entry path
> >> and ist_enter. But I see your point. I think that, if we have a
> >> problem like this in practice, then we should fix it.
> >
> > Whatever hack we do to prevent from exceptions happening in between real kernel entry
> > to tracked kernel entry is going to be far less robust than relying strictly on soft
> > context tracking.
> >
>
> Why?
>
> Any exception that doesn't leave the context tracking state exactly
> the way it found it is buggy. That means that we need to make sure
> that context tracking itself is safe wrt exceptions and that we need
> to make sure that any exception that can happen early in entry is
> itself safe.

Right, and doing it the way we did previously was safe wrt. that.

Can't we have exceptions slow path just like the way we do it in syscalls?

Then the exception slow path would just do:

if TIF_NOHZ
ctx = exception_enter()
exception_handler()
if TIF_NOHZ
exception_exit(ctx)

Right now we are calling unconditionally the context tracking code, which is
not good.

>
> The latter is annoying, but the entry code needs to deal with it
> anyway. For example, any exception early in NMI is currently really
> bad. Non-IST exceptions very early in SYSCALL are fatal.
> Non-paranoid exceptions outside swapgs are fatal. Etc.

Sure but that doesn't mean I'm happy with introducing new fragile path
like those. Especially as we have a way to fix without more overhead.

>
> > The resulting bugs are rare and very hard to reproduce and diagnose.
>
> That's why I stuck assertions all over the place. I know of exactly
> one case that will trip the assertion, and it's a false positive and I
> plan on fixing it soon.
>
> >
> >>
> >> But the old code had the same issue. If we got an exception (the most
> >> likely one is probably a vmalloc fault) during user_exit and we then
> >> hit exception_enter, the result would probably be bad.
> >
> > We have a recursion protection in context tracking that should protect against
> > exceptions triggering in the middle of half-set states.
>
> I sure hope so. It would be nice to mark it with with nokprobes, etc
> if needed, too.

Sure.

> > Also as long as there is at least one instruction between entry to the kernel
> > and context tracking noting it, there is a risk for an exception. Hence entry
> > code will never be atomic enough to avoid this kind of bugs.
>
> By that argument, we're doomed. Non-IST exceptions outside swapgs are fatal.

Does that concern only error_entry() exceptions?

> >
> > Heh if only we had something like local_exception_save()!
>
> What would that mean?
>
> Exceptions aren't magic asynchronous things. They happen only when
> you do something that can trigger an exception.

Sure but, did you really never wish to have such an API? :-p