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



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