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

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Date:   Wed, 26 May 2021 16:14:34 -0500
From:   Babu Moger <babu.moger@....com>
To:     Andy Lutomirski <luto@...nel.org>, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, dave.hansen@...ux.intel.com, peterz@...radead.org,
        shuah@...nel.org, jroedel@...e.de, ubizjak@...il.com,
        viro@...iv.linux.org.uk, jpa@....mail.kapsi.fi,
        fenghua.yu@...el.com, kan.liang@...ux.intel.com,
        akpm@...ux-foundation.org, rppt@...nel.org, Fan_Yang@...u.edu.cn,
        anshuman.khandual@....com, b.thiel@...teo.de, jgross@...e.com,
        keescook@...omium.org, seanjc@...gle.com, mh@...ndium.org,
        sashal@...nel.org, krisman@...labora.com, 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 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