[PATCH v2 0/1] Accept unaccepted kexec segments' destination addresses

[Ashish Kalra]

> On Thu, Feb 13, 2025 at 07:55:15AM -0800, Dave Hansen wrote:
>> On 1/13/25 06:59, Eric W. Biederman wrote:
>> ...
>> > I have a new objection.  I believe ``unaccepted memory'' and especially
>> > lazily initialized ``unaccepted memory'' is an information leak that
>> > could defeat the purpose of encrypted memory.  For that reason I have
>> > Cc'd the security list.  I don't know who to CC to get expertise on this
>> > issue, and the security list folks should.
>> > 
>> > Unless I am misunderstanding things the big idea with encrypted
>> > memory is that the hypervisor won't be able to figure out what you
>> > are doing, because it can't read your memory.
>> 
>> At a super high level, you are right. Accepting memory tells the
>> hypervisor that the guest is _allocating_ memory. It even tells the host
>> what the guest physical address of the memory is. But that's far below
>> the standard we've usually exercised in the kernel for rejecting on
>> security concerns.
>> 
>> Did anyone on the security list raise any issues here? I've asked them
>> about a few things in the past and usually I've thought that no news is
>> good news.
>> 
>> > My concern is that by making the ``acceptance'' of memory lazy, that
>> > there is a fairly strong indication of the function of different parts
>> > of memory.  I expect that signal is strong enough to defeat whatever
>> > elements of memory address randomization that we implement in the
>> > kernel.
>> 
>> In the end, the information that the hypervisor gets is that the guest
>> allocated _some_ page within a 4MB physical region and the time. It gets
>> that signal once per boot for each region. It will mostly see a pattern
>> of acceptance going top-down from high to low physical addresses.
>> 
>> The hypervisor never learns anything about KASLR. The fact that the
>> physical allocation patterns are predictable (with or without memory
>> acceptance) is one of the reasons KASLR is in place.
>> 
>> I don't think memory acceptance has any real impact on "memory address
>> randomization". This is especially true because it's a once-per-boot
>> signal, not a continuous thing that can be leveraged. 4MB is also
>> awfully coarse.
>> 
>> > So not only does it appear to me that implementation of ``accepting''
>> > memory has a stupidly slow implementation, somewhat enshrined by a bad
>> > page at a time ACPI standard, but it appears to me that lazily
>> > ``accepting'' that memory probably defeats the purpose of having
>> > encrypted memory.
>> 
>> Memory acceptance is pitifully slow. But it's slow because it
>> fundamentally requires getting guest memory into a known state before
>> guest use. You either have slow memory acceptance as a thing or you have
>> slow guest boot.
>> 
>> Are there any other CoCo systems that don't have to zero memory like TDX
>> does? On the x86 side, we have SGX the various flavors of SEV. They all,
>> as far as I know, require some kind of slow "conversion" process when
>> pages change security domains.
>>
>> > I think the actual solution is to remove all code except for the
>> > "accept_memory=eager" code paths.  AKA delete the "accept_memory=lazy"
>> > code.  At that point there are no more changes that need to be made to
>> > kexec.
>> 
>> That was my first instinct too: lazy acceptance is too complicated to
>> live and must die.
>> 
>> It sounds like you're advocating for the "slow guest boot" option.
>> Kirill, can you remind us how fast a guest boots to the shell for
>> modestly-sized (say 256GB) memory with "accept_memory=eager" versus
>> "accept_memory=lazy"? IIRC, it was a pretty remarkable difference.

>I only have 128GB machine readily available and posted some number on
>other thread[1]:

>  On single vCPU it takes about a minute to accept 90GiB of memory.

>  It improves a bit with number of vCPUs. It is 40 seconds with 4 vCPU, but
>  it doesn't scale past that in my setup.

>I've mentioned it before in other thread:

>[1] https://lore.kernel.org/all/ihzvi5pwn5hrn4ky2ehjqztjxoixaiaby4igmeihqfehy2vrii@tsg6j5qvmyrm

We essentially rely on lazy acceptance support for reducing SNP guest boot time.

Here are some performance numbers for SNP guests which i have here after discussing with
Michael Roth (who is also CCed here): 

Just did quick boot of a 128GB SNP guest with accept_memory=lazy guest kernel parameter
and that took 22s to boot, and with accept_memory=eager it takes 3 minutes and 47s, so it 
is a remarkable difference.

Thanks,
Ashish

>-- 
>  Kiryl Shutsemau / Kirill A. Shutemov