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

[Thomas HellstrÃm (VMware)]

On 9/5/19 5:59 PM, Dave Hansen wrote:
> On 9/5/19 8:21 AM, Thomas HellstrÃm (VMware) wrote:
> > > >  Â #define pgprot_modify pgprot_modify
> > > >  Â static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t
> > > > newprot)
> > > >  Â {
> > > > -ÂÂÂ pgprotval_t preservebits = pgprot_val(oldprot) & _PAGE_CHG_MASK;
> > > > -ÂÂÂ pgprotval_t addbits = pgprot_val(newprot);
> > > > +ÂÂÂ pgprotval_t preservebits = pgprot_val(oldprot) &
> > > > +ÂÂÂÂÂÂÂ (_PAGE_CHG_MASK | sme_me_mask);
> > > > +ÂÂÂ pgprotval_t addbits = pgprot_val(newprot) & ~sme_me_mask;
> > > >  ÂÂÂÂÂ return __pgprot(preservebits | addbits);
> > > >  Â }
> > > _PAGE_CHG_MASK is claiming similar functionality about preserving bits
> > > when changing PTEs:
> ...
> > > > #define _PAGE_CHG_MASKÂ (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT
> > > > |ÂÂÂÂÂÂÂÂ \
> > > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ _PAGE_SPECIAL | _PAGE_ACCESSED |
> > > > _PAGE_DIRTY | \
> > > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ _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.
> I think there's a built-in assumption that 'newprot' won't have any of
> the _PAGE_CHG_MASK bits set.  That makes sense because there are no
> protection bits in the mask.  But, the code certainly doesn't enforce that.
>
> Are you seeing 'sme_me_mask' bits set in 'newprot'?

Yes. AFAIK it's only one bit, and typically always set.

/Thomas