linux-kernel - Re: [PATCH v3 3/3] x86/hyperv: Make encrypted/decrypted changes safe for load_unaligned

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]

Message-ID: <9f0c0b3d-6021-466d-8ee0-375f66c5006a@linux.intel.com>
Date: Mon, 8 Jan 2024 11:24:06 -0800
From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
To: Michael Kelley <mhklinux@...look.com>,
 "tglx@...utronix.de" <tglx@...utronix.de>,
 "mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
 "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
 "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
 "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
 "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
 "wei.liu@...nel.org" <wei.liu@...nel.org>,
 "decui@...rosoft.com" <decui@...rosoft.com>,
 "luto@...nel.org" <luto@...nel.org>,
 "peterz@...radead.org" <peterz@...radead.org>,
 "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
 "urezki@...il.com" <urezki@...il.com>, "hch@...radead.org"
 <hch@...radead.org>, "lstoakes@...il.com" <lstoakes@...il.com>,
 "thomas.lendacky@....com" <thomas.lendacky@....com>,
 "ardb@...nel.org" <ardb@...nel.org>, "jroedel@...e.de" <jroedel@...e.de>,
 "seanjc@...gle.com" <seanjc@...gle.com>,
 "rick.p.edgecombe@...el.com" <rick.p.edgecombe@...el.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
 "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v3 3/3] x86/hyperv: Make encrypted/decrypted changes safe
 for load_unaligned_zeropad()



On 1/8/2024 11:13 AM, Michael Kelley wrote:
> From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> Sent: Monday, January 8, 2024 10:37 AM
>>
>> On 1/5/2024 10:30 AM, mhkelley58@...il.com wrote:
>>> From: Michael Kelley <mhklinux@...look.com>
>>>
>>> In a CoCo VM, when transitioning memory from encrypted to decrypted, or
>>> vice versa, the caller of set_memory_encrypted() or set_memory_decrypted()
>>> is responsible for ensuring the memory isn't in use and isn't referenced
>>> while the transition is in progress.  The transition has multiple steps,
>>> and the memory is in an inconsistent state until all steps are complete.
>>> A reference while the state is inconsistent could result in an exception
>>> that can't be cleanly fixed up.
>>>
>>> However, the kernel load_unaligned_zeropad() mechanism could cause a stray
>>> reference that can't be prevented by the caller of set_memory_encrypted()
>>> or set_memory_decrypted(), so there's specific code to handle this case.
>>> But a CoCo VM running on Hyper-V may be configured to run with a paravisor,
>>> with the #VC or #VE exception routed to the paravisor. There's no
>>> architectural way to forward the exceptions back to the guest kernel, and
>>> in such a case, the load_unaligned_zeropad() specific code doesn't work.
>>>
>>> To avoid this problem, mark pages as "not present" while a transition
>>> is in progress. If load_unaligned_zeropad() causes a stray reference, a
>>> normal page fault is generated instead of #VC or #VE, and the
>>> page-fault-based fixup handlers for load_unaligned_zeropad() resolve the
>>> reference. When the encrypted/decrypted transition is complete, mark the
>>> pages as "present" again.
>>
>> Change looks good to me. But I am wondering why are adding it part of
>> prepare and finish callbacks instead of directly in set_memory_encrypted() function.
>>
> 
> The prepare/finish callbacks are different for TDX, SEV-SNP, and
> Hyper-V CoCo guests running with a paravisor -- so there are three sets
> of callbacks.  As described in the cover letter, I've given up on using this
> scheme for the TDX and SEV-SNP cases, because of the difficulty with
> the SEV-SNP callbacks needing a valid virtual address (whereas TDX and
> Hyper-V paravisor need only a physical address).  So it seems like the
> callbacks specific to the Hyper-V paravisor are the natural place for the
> code.  That leaves the TDX and SEV-SNP code paths unchanged, which
> was my intent.
> 

Got it. Thanks for clarifying it.

> Or maybe I'm not understanding your comment?  If that's the case,
> please elaborate.
> 
> Michael
> 
>> Reviewed-by: Kuppuswamy Sathyanarayanan
>> <sathyanarayanan.kuppuswamy@...ux.intel.com>
>>

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer