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

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Date:   Fri, 24 Jul 2020 10:29:23 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Jul 24, 2020, at 10:23 AM, Ira Weiny <ira.weiny@...el.com> wrote:
> 
> On Thu, Jul 23, 2020 at 10:15:17PM +0200, Thomas Gleixner wrote:
>> Thomas Gleixner <tglx@...utronix.de> writes:
>> 
>>> Ira Weiny <ira.weiny@...el.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@...el.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?

Only the ones that use idtentry_enter() instead of, say, nmi_enter().

> 
> Ira