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

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Date:   Wed, 22 Jul 2020 11:48:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        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

On Tue, Jul 21, 2020 at 10:27:09PM -0700, Ira Weiny 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...)
> ...
> 
> The ref count ends up completely messed up after this sequence...  and the PKRS
> register gets out of sync as well.  This is starting to make some sense of how
> I was getting what seemed like random crashes before.
> 
> Unfortunately I don't understand the idt entry/exit code well enough to see
> clearly what is going on.  Is there any reason idtentry_exit() is not called
> after idtentry_enter()?  Perhaps some NMI/MCE or 'not normal' exception code
> path I'm not seeing?  In my searches I see a corresponding exit for every
> enter.  But MCE is pretty hard to follow.
> 
> Also is there any chance that the process could be getting scheduled and that
> is causing an issue?

Ooh, I think I see the problem. We also use idtentry_enter() for #PF,
and #PF can schedule, exactly as you observed. Argh!

This then means you need to go frob something in
arch/x86/include/asm/idtentry.h, which is somewhat unfortunate.

Thomas, would it make sense to add a type parameter to
idtentry_{enter,exit}() ?