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

Thomas Gleixner
2020-07-23T19:53:27Z

Ira,

ira.weiny@intel.com writes:

> ...
> // ref == 0
> dev_access_enable() // ref += 1 ==> disable protection
> irq()
> // enable protection
> // ref = 0
> _handler()
> dev_access_enable() // ref += 1 ==> disable protection
> dev_access_disable() // ref -= 1 ==> enable protection
> // WARN_ON(ref != 0)
> // disable protection
> do_pmem_thing() // all good here
> dev_access_disable() // ref -= 1 ==> 0 ==> enable protection

...

> First I'm not sure if adding this state to idtentry_state and having
> that state copied is the right way to go.

Adding the state to idtentry_state is fine at least for most interrupts
and exceptions. Emphasis on most.

#PF does not work because #PF can schedule.

> It seems like we should start passing this by reference instead of
> value. But for now this works as an RFC. Comments?

Works as in compiles, right?

static void noinstr idt_save_pkrs(idtentry_state_t state)
{
state.foo = 1;
}

How is that supposed to change the caller state? C programming basics.

> Second, I'm not 100% happy with having to save the reference count in
> the exception handler. It seems like a very ugly layering violation but
> I don't see a way around it at the moment.

That state is strict per task, right? So why do you want to store it
anywhere else that in task/thread storage. That solves your problem of
#PF scheduling nicely.

> Third, this patch has gone through a couple of revisions as I've had
> crashes which just don't make sense to me. One particular issue I've
> had is taking a MCE during memcpy_mcsafe causing my WARN_ON() to fire.
> The code path was a pmem copy and the ref count should have been
> elevated due to dev_access_enable() but why was
> idtentry_enter()->idt_save_pkrs() not called I don't know.

Because #MC does not go through idtentry_enter(). Neither do #NMI, #DB, #BP.

> Finally, it looks like the entry/exit code is being refactored into
> common code. So likely this is best handled somewhat there. Because
> this can be predicated on CONFIG_ARCH_HAS_SUPERVISOR_PKEYS and handled
> in a generic fashion. But that is a ways off I think.

The invocation of save/restore might be placed in generic code at least
for the common exception and interrupt entries.

> +static void noinstr idt_save_pkrs(idtentry_state_t state)

*state. See above.

> +#else
> +/* Define as macros to prevent conflict of inline and noinstr */
> +#define idt_save_pkrs(state)
> +#define idt_restore_pkrs(state)

Empty inlines do not need noinstr because they are optimized out. If you
want inlines in a noinstr section then use __always_inline

> /**
> * idtentry_enter - Handle state tracking on ordinary idtentries
> * @regs: Pointer to pt_regs of interrupted context
> @@ -604,6 +671,8 @@ idtentry_state_t noinstr idtentry_enter(struct pt_regs *regs)
> return ret;
> }
>
> + idt_save_pkrs(ret);

No. This really has no business to be called before the state is
established. It's not something horribly urgent and write_pkrs() is NOT
noinstr and invokes wrmsrl() which is subject to tracing.

> +
> + idt_restore_pkrs(state);

This one is placed correctly.

Thanks,

tglx