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

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Date:   Thu, 23 Jul 2020 21:53:20 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     ira.weiny@...el.com, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Fenghua Yu <fenghua.yu@...el.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions

Ira,

ira.weiny@...el.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