linux-kernel - Re: [RFC PATCH 1/2] x86: Don't let pgprot_modify() change the page encryption bit

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Date:   Thu, 5 Sep 2019 17:21:24 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, pv-drivers@...are.com
Cc:     Thomas Hellstrom <thellstrom@...are.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christian König <christian.koenig@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [RFC PATCH 1/2] x86: Don't let pgprot_modify() change the page
 encryption bit

On 9/5/19 4:15 PM, Dave Hansen wrote:
> Hi Thomas,
>
> Thanks for the second batch of patches!  These look much improved on all
> fronts.

Yes, although the TTM functionality isn't in yet. Hopefully we won't 
have to bother you with those though, since this assumes TTM will be 
using the dma API.

> On 9/5/19 3:35 AM, Thomas Hellström (VMware) wrote:
>> -/* mprotect needs to preserve PAT bits when updating vm_page_prot */
>> +/*
>> + * mprotect needs to preserve PAT and encryption bits when updating
>> + * vm_page_prot
>> + */
>>   #define pgprot_modify pgprot_modify
>>   static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
>>   {
>> -	pgprotval_t preservebits = pgprot_val(oldprot) & _PAGE_CHG_MASK;
>> -	pgprotval_t addbits = pgprot_val(newprot);
>> +	pgprotval_t preservebits = pgprot_val(oldprot) &
>> +		(_PAGE_CHG_MASK | sme_me_mask);
>> +	pgprotval_t addbits = pgprot_val(newprot) & ~sme_me_mask;
>>   	return __pgprot(preservebits | addbits);
>>   }
> _PAGE_CHG_MASK is claiming similar functionality about preserving bits
> when changing PTEs:
>
>> /*
>>   * Set of bits not changed in pte_modify.  The pte's
>>   * protection key is treated like _PAGE_RW, for
>>   * instance, and is *not* included in this mask since
>>   * pte_modify() does modify it.
>>   */
>> #define _PAGE_CHG_MASK  (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |         \
>>                           _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY | \
>>                           _PAGE_SOFT_DIRTY | _PAGE_DEVMAP)
> This makes me wonder if we should be including sme_me_mask in
> _PAGE_CHG_MASK (logically).

I was thinking the same. But what confuses me is that addbits isn't 
masked with ~_PAGE_CHG_MASK, which is needed for sme_me_mask, since the 
problem otherwise is typically that the encryption bit is incorrectly 
set in addbits. I wonder whether it's an optimization or intentional.

/Thomas