linux-kernel - Re: x86/fpu/xsave: protection key test failures

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Message-Id: <6bc401e7-6e34-4fbc-8ce3-a99be7966528@www.fastmail.com>
Date:   Wed, 26 May 2021 14:17:13 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Babu Moger" <babu.moger@....com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org
Cc:     "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, dave.hansen@...ux.intel.com,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, shuah@...nel.org,
        jroedel@...e.de, "Uros Bizjak" <ubizjak@...il.com>,
        viro@...iv.linux.org.uk, "Petteri Aimonen" <jpa@....mail.kapsi.fi>,
        "Fenghua Yu" <fenghua.yu@...el.com>,
        "Kan Liang" <kan.liang@...ux.intel.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Mike Rapoport" <rppt@...nel.org>,
        "Fan Yang" <Fan_Yang@...u.edu.cn>, anshuman.khandual@....com,
        "Benjamin Thiel" <b.thiel@...teo.de>,
        "Juergen Gross" <jgross@...e.com>, keescook@...omium.org,
        "Sean Christopherson" <seanjc@...gle.com>, mh@...ndium.org,
        sashal@...nel.org, krisman@...labora.com,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>, 0x7f454c46@...il.com,
        jhubbard@...dia.com, sandipan@...ux.ibm.com, ziy@...dia.com,
        kirill.shutemov@...ux.intel.com, suxingxing@...ngson.cn,
        harish@...ux.ibm.com, rong.a.chen@...el.com, linuxram@...ibm.com,
        bauerman@...ux.ibm.com, dave.kleikamp@...cle.com
Subject: Re: x86/fpu/xsave: protection key test failures



On Wed, May 26, 2021, at 2:14 PM, Babu Moger wrote:
> 
> 
> On 5/25/21 7:36 PM, Andy Lutomirski wrote:
> > On 5/25/21 2:37 PM, Babu Moger wrote:
> > 
> >> Investigation so far.
> >> 1. The test fails on AMD(Milan) systems. Test appears to pass on Intel
> >> systems. Tested on old Intel system available here.
> > 
> > Ten cents says that you have discovered that AMD returns a synthesized
> > value for XINUSE[PKRU] and that write_pkru() is buggy.
> > 
> > Any volunteers to fix the if (!pk) case in write_pkru() and see if the
> > problem goes away?  (Or at least to print something for the if (!pk)
> > case and seee if it gets printed?)
> 
> I inserted this patch to test this path.
> 
> 
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index b1099f2d9800..a0bca917f864 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -151,6 +160,8 @@ static inline void write_pkru(u32 pkru)
>         fpregs_lock();
>         if (pk)
>                 pk->pkru = pkru;
> +       else
> +               printk_once("%s read_pkru 0x%x\n",__FUNCTION__, read_pkru());
>         __write_pkru(pkru);
>         fpregs_unlock();
>  }
> 
> 
> Yes. The pk value is sometimes NULL and the value in the pkru register is
> 0 at that time. Here is the dmesg output.
> 
> [    1.700889] write_pkru read_pkru 0x0
> 

Sweet, maybe I get my hypothetical dime back!   Both bugs are real.

Dave and I (mostly Dave — I’m supplying the snark) are working on an omnibus patch to fix this whole mess.

Thanks,
Andy