Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions

Ira Weiny
2020-07-24T17:23:50Z

On Thu, Jul 23, 2020 at 10:15:17PM +0200, Thomas Gleixner wrote:
> Thomas Gleixner <tglx@linutronix.de> writes:
>
> > Ira Weiny <ira.weiny@intel.com> writes:
> >> On Fri, Jul 17, 2020 at 12:06:10PM +0200, Peter Zijlstra wrote:
> >>> On Fri, Jul 17, 2020 at 12:20:56AM -0700, ira.weiny@intel.com wrote:
> >> I've been really digging into this today and I'm very concerned that I'm
> >> completely missing something WRT idtentry_enter() and idtentry_exit().
> >>
> >> I've instrumented idt_{save,restore}_pkrs(), and __dev_access_{en,dis}able()
> >> with trace_printk()'s.
> >>
> >> With this debug code, I have found an instance where it seems like
> >> idtentry_enter() is called without a corresponding idtentry_exit(). This has
> >> left the thread ref counter at 0 which results in very bad things happening
> >> when __dev_access_disable() is called and the ref count goes negative.
> >>
> >> Effectively this seems to be happening:
> >>
> >> ...
> >> // ref == 0
> >> dev_access_enable() // ref += 1 ==> disable protection
> >> // exception (which one I don't know)
> >> idtentry_enter()
> >> // ref = 0
> >> _handler() // or whatever code...
> >> // *_exit() not called [at least there is no trace_printk() output]...
> >> // Regardless of trace output, the ref is left at 0
> >> dev_access_disable() // ref -= 1 ==> -1 ==> does not enable protection
> >> (Bad stuff is bound to happen now...)
> >
> > Well, if any exception which calls idtentry_enter() would return without
> > going through idtentry_exit() then lots of bad stuff would happen even
> > without your patches.
> >
> >> Also is there any chance that the process could be getting scheduled and that
> >> is causing an issue?
> >
> > Only from #PF, but after the fault has been resolved and the tasks is
> > scheduled in again then the task returns through idtentry_exit() to the
> > place where it took the fault. That's not guaranteed to be on the same
> > CPU. If schedule is not aware of the fact that the exception turned off
> > stuff then you surely get into trouble. So you really want to store it
> > in the task itself then the context switch code can actually see the
> > state and act accordingly.
>
> Actually thats nasty as well as you need a stack of PKRS values to
> handle nested exceptions. But it might be still the most reasonable
> thing to do. 7 PKRS values plus an index should be really sufficient,
> that's 32bytes total, not that bad.

I've thought about this a bit more and unless I'm wrong I think the
idtentry_state provides for that because each nested exception has it's own
idtentry_state doesn't it?

Ira