Re: [Xen-devel] [V3 PATCH 7/9] x86/hvm: pkeys, add pkeys support for guest_walk_tables

["Han, Huaitong" <huaitong.han@xxxxxxxxx>]

On Thu, 2015-12-10 at 18:19 +0000, George Dunlap wrote:
> On 07/12/15 09:16, Huaitong Han wrote:
> > +{
> > +    void *xsave_addr;
> > +    unsigned int pkru = 0;
> > +    bool_t pkru_ad, pkru_wd;
> > +
> > +    bool_t uf = !!(pfec & PFEC_user_mode);
> > +    bool_t wf = !!(pfec & PFEC_write_access);
> > +    bool_t ff = !!(pfec & PFEC_insn_fetch);
> > +    bool_t rsvdf = !!(pfec & PFEC_reserved_bit);
> > +    bool_t pkuf  = !!(pfec & PFEC_prot_key);
> 
> So I'm just wondering out loud here -- is there actually any
> situation
> in which we would want guest_walk_tables to act differently than the
> real hardware?
> 
> That is, is there actually any situation where, pku is enabled, the
> vcpu
> is in long mode, PFEC_write_access and/or PFEC_page_present is set,
> and
> the pkey is non-zero, that we want guest_walk_tables() to only check
> the
> write-protect bit for the pte, and not also check the pkru?
> 
> Because if not, it seems like it would be much more robust to simply
> *always* check for pkru_ad if PFEC_page_present is set, and for
> pkru_wd
> if PFEC_write_access is set.
> Then in patch 8, you wouldn't need to go around all the __hvm_copy
> functions adding in PFEC_prot; instead, you'd just need to add
> PFEC_insn_fetch to the "fetch" (as is already done for SMEP and NX),
> and
> you'd be done.
See reply email from Feng discussed with me.

> > +
> > +    if ( !cpu_has_xsave || !pkuf || is_pv_vcpu(vcpu) )
> > +        return 0;
> > +
> > +    /* PKRU dom0 is always zero */
> 
> "dom0" has a very specific meaning in Xen.  I think this would be
> better
> written "pkey 0 always has full access".
> 
> > +    if ( likely(!pte_pkeys) )
> > +        return 0;
> > +
> > +    /* Update vcpu xsave area */
> > +    fpu_xsave(vcpu);
> 
> Is there a reason you're calling fpu_xsave() directly here, rather
> than
> just calling vcpu_save_fpu()?  That saves you actually doing the
> xsave
> if the fpu hasn't been modified since the last time you read it.
use fpu_xsave instead of fpu_xsave because Jan's comment:
Which is bogus by itself: That function isn't meant to be used for
purposes like the one you have, e.g. due to its side effect of
clearing ->fpu_dirtied. You really ought to be using a lower level
function here (and I don't think the corresponding struct vcpu
should get altered in any way). --Jan

And I can add 
    if ( !vcpu->fpu_dirtied )    
before fpu_xsave(vcpu);

> > +    xsave_addr = get_xsave_addr(vcpu->arch.xsave_area,
> > fls64(XSTATE_PKRU)-1);
> > +    if ( !!xsave_addr )
> > +        memcpy(&pkru, xsave_addr, sizeof(pkru));
> 
> There's no need for the !! here.  But in any case, isn't there a
> better
> function for reading the xsave state than manually calculating the
> address and doing a memcpy?
RDPKRU is disabled by hypervisor CR4 because PV mode must disable
CR4.PKE, getting PKRU value only depends on xsave.
> > +
> > +    if ( unlikely(pkru) )
> > +    {
> > +        /*
> > +         * PKU:  additional mechanism by which the paging controls
> > +         * access to user-mode addresses based on the value in the
> > +         * PKRU register. A fault is considered as a PKU violation
> > if all
> > +         * of the following conditions are ture:
> > +         * 1.CR4_PKE=1.
> > +         * 2.EFER_LMA=1.
> > +         * 3.page is present with no reserved bit violations.
> > +         * 4.the access is not an instruction fetch.
> > +         * 5.the access is to a user page.
> > +         * 6.PKRU.AD=1
> > +         *       or The access is a data write and PKRU.WD=1
> > +         *            and either CR0.WP=1 or it is a user access.
> > +         */
> > +        pkru_ad = read_pkru_ad(pkru, pte_pkeys);
> > +        pkru_wd = read_pkru_wd(pkru, pte_pkeys);
> > +        if ( hvm_pku_enabled(vcpu) && hvm_long_mode_enabled(vcpu)
> > &&
> > +            !rsvdf && !ff && (pkru_ad ||
> > +            (pkru_wd && wf && (hvm_wp_enabled(vcpu) || uf))))
> > +            return 1;
> 
> This statement here is really difficult to read.  Why don't you put
> the
> checks which don't depend on the pkru up before you read it?  e.g.,
> hvm_pku_enabled(), hvm_long_mode_enabled(), rsvdf, ff, &c?
> 
>  -George
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