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

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Message-ID: <87ft9ivv3y.fsf@nanos.tec.linutronix.de>
Date:   Thu, 23 Jul 2020 22:23:29 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Fenghua Yu <fenghua.yu@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Weiny Ira <ira.weiny@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list\:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "open list\:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions

Dave Hansen <dave.hansen@...el.com> writes:

> On 7/23/20 10:08 AM, Andy Lutomirski wrote:
>> Suppose some kernel code (a syscall or kernel thread) changes PKRS
>> then takes a page fault. The page fault handler needs a fresh PKRS.
>> Then the page fault handler (say a VMA’s .fault handler) changes
>> PKRS.  The we get an interrupt. The interrupt *also* needs a fresh
>> PKRS and the page fault value needs to be saved somewhere.
>> 
>> So we have more than one saved value per thread, and thread_struct
>> isn’t going to solve this problem.
>
> Taking a step back...  This is all true only if we decide that we want
> protection keys to provide protection during exceptions and interrupts.
>  Right now, the code supports nesting:
>
> 	kmap(foo);
> 		kmap(bar);
> 		kunmap(bar);
> 	kunmap(foo);
>
> with a reference count.  So, the nested kmap() will see the count
> elevated and do nothing.

Hopefully with a big fat warning if the nested map requires a different
key than the outer one.