linux-kernel - Re: [PATCH Part2 RFC v4 07/40] x86/sev: Split the physmap when adding the page in RMP table

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Message-ID: <YPCwPd2OzbBPs9DH@google.com>
Date:   Thu, 15 Jul 2021 22:01:33 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 07/40] x86/sev: Split the physmap when
 adding the page in RMP table

On Thu, Jul 15, 2021, Brijesh Singh wrote:
> 
> 
> On 7/15/21 1:39 PM, Sean Christopherson wrote:
> > On Thu, Jul 15, 2021, Brijesh Singh wrote:
> > > The memfd_secrets uses the set_direct_map_{invalid,default}_noflush() and it
> > > is designed to remove/add the present bit in the direct map. We can't use
> > > them, because in our case the page may get accessed by the KVM (e.g
> > > kvm_guest_write, kvm_guest_map etc).
> > 
> > But KVM should never access a guest private page, i.e. the direct map should
> > always be restored to PRESENT before KVM attempts to access the page.
> > 
> 
> Yes, KVM should *never* access the guest private pages. So, we could
> potentially enhance the RMPUPDATE() to check for the assigned and act
> accordingly.
> 
> Are you thinking something along the line of this:
> 
> int rmpupdate(struct page *page, struct rmpupdate *val)
> {
> 	...
> 	
> 	/*
> 	 * If page is getting assigned in the RMP entry then unmap
> 	 * it from the direct map before its added in the RMP table.
> 	 */
> 	if (val.assigned)
> 		set_direct_map_invalid_noflush(page_to_virt(page), 1);
> 
> 	...
> 
> 	/*
> 	 * If the page is getting unassigned then restore the mapping
> 	 * in the direct map after its removed from the RMP table.
> 	 */
> 	if (!val.assigned)
> 		set_direct_map_default_noflush(page_to_virt(page), 1);
> 	
> 	...
> }

Yep.

However, looking at the KVM usage, rmpupdate() appears to be broken.  When
handling a page state change, the guest can specify a 2mb page.  In that case,
rmpupdate() will be called once for a 2mb page, but this flow assumes a single
4kb page.  The current code works because set_memory_4k() will cause the entire
2mb page to be shattered, but it's technically wrong and switching to the above
would cause problems.