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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 15 Jul 2021 13:14:35 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     brijesh.singh@....com, 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 7/15/21 12:51 PM, Sean Christopherson wrote:
> On Thu, Jul 15, 2021, Brijesh Singh wrote:
>>
>> On 7/14/21 5:25 PM, Sean Christopherson wrote:
>>>> @@ -2375,6 +2375,12 @@ int rmpupdate(struct page *page, struct rmpupdate *val)
>>>>    	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
>>>>    		return -ENXIO;
>>>> +	ret = set_memory_4k((unsigned long)page_to_virt(page), 1);
>>>
>>> IIUC, this shatters the direct map for page that's assigned to an SNP guest, and
>>> the large pages are never recovered?
>>>
>>> I believe a better approach would be to do something similar to memfd_secret[*],
>>> which encountered a similar problem with the direct map.  Instead of forcing the
>>> direct map to be forever 4k, unmap the direct map when making a page guest private,
>>> and restore the direct map when it's made shared (or freed).
>>>
>>> I thought memfd_secret had also solved the problem of restoring large pages in
>>> the direct map, but at a glance I can't tell if that's actually implemented
>>> anywhere.  But, even if it's not currently implemented, I think it makes sense
>>> to mimic the memfd_secret approach so that both features can benefit if large
>>> page preservation/restoration is ever added.
>>>
>>
>> thanks for the memfd_secrets pointer. At the lowest level it shares the
>> same logic to split the physmap. We both end up calling to
>> change_page_attrs_set_clr() which split the page and updates the page
>> table attributes.
>>
>> Given this, I believe in future if the change_page_attrs_set_clr() is
>> enhanced to track the splitting of the pages and restore it later then it
>> should work transparently.
> 
> But something actually needs to initiate the restore.  If the RMPUDATE path just
> force 4k pages then there will never be a restore.  And zapping the direct map
> for private pages is a good thing, e.g. prevents the kernel from reading garbage,
> which IIUC isn't enforced by the RMP?
> 

Yes, something need to initiated the restore. Since the restore support 
is not present today so its difficult to say how it will be look. I am 
thinking that restore thread may use some kind of notifier to check with 
the caller whether its safe to restore the page ranges. In case of the 
SEV-SNP, the SNP registered notifier will reject if the guest is running.

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).

thanks