Re: [PATCH v3 3/4] Add a new hypercall to get the ESRT

[Jan Beulich <jbeulich@suse.com>]

On 29.04.2022 00:54, Demi Marie Obenour wrote:
> On Thu, Apr 28, 2022 at 08:47:49AM +0200, Jan Beulich wrote:
>> On 27.04.2022 21:08, Demi Marie Obenour wrote:
>>> On Wed, Apr 27, 2022 at 10:56:34AM +0200, Jan Beulich wrote:
>>>> On 19.04.2022 17:49, Demi Marie Obenour wrote:
>>>>> This hypercall can be used to get the ESRT from the hypervisor.  It
>>>>> returning successfully also indicates that Xen has reserved the ESRT and
>>>>> it can safely be parsed by dom0.
>>>>
>>>> I'm not convinced of the need, and I view such an addition as inconsistent
>>>> with the original intentions. The pointer comes from the config table,
>>>> which Dom0 already has access to. All a Dom0 kernel may need to know in
>>>> addition is whether the range was properly reserved. This could be achieved
>>>> by splitting the EFI memory map entry in patch 2, instead of only splitting
>>>> the E820 derivation, as then XEN_FW_EFI_MEM_INFO can be used to find out
>>>> the range's type. Another way to find out would be for Dom0 to attempt to
>>>> map this area as MMIO, after first checking that no part of the range is in
>>>> its own memory allocation. This 2nd approach may, however, not really be
>>>> suitable for PVH Dom0, I think.
>>>
>>> On further thought, I think the hypercall approach is actually better
>>> than reserving the ESRT.  I really do not want XEN_FW_EFI_MEM_INFO to
>>> return anything other than the actual firmware-provided memory
>>> information, and the current approach seems to require more and more
>>> special-casing of the ESRT, not to mention potentially wasting memory
>>> and splitting a potentially large memory region into two smaller ones.
>>> By copying the entire ESRT into memory owned by Xen, the logic becomes
>>> significantly simpler on both the Xen and dom0 sides.
>>
>> I actually did consider the option of making a private copy when you did
>> send the initial version of this, but I'm not convinced this simplifies
>> things from a kernel perspective: They'd now need to discover the table
>> by some entirely different means. In Linux at least such divergence
>> "just for Xen" hasn't been liked in the past.
>>
>> There's also the question of how to propagate the information across
>> kexec. But I guess that question exists even outside of Xen, with the
>> area living in memory which the OS is expected to recycle.
> 
> Indeed it does.  A simple rule might be, “Only trust the ESRT if it is
> in memory of type EfiRuntimeServicesData.”  That is easy to achieve by
> monkeypatching the config table as you suggested below.
> 
> I *am* worried that the config table might be mapped read-only on some
> systems, in which case the overwrite would cause a fatal page fault.  Is
> there a way for Xen to check for this?

While in boot mode, aiui page tables aren't supposed to be enforcing
access restrictions. Recall that on other architectures EFI even runs
with paging disabled; this simply is not possible for x86-64. So
portable firmware shouldn't map anything r/o. In principle the pointer
could still be in ROM; I consider this unlikely, but we could check
for that (just like we could do a page table walk to figure out
whether a r/o mapping would prevent us from updating the field).

>  It could also be undefined behavior to modify it.

That's the bigger worry I have.

>>> Is using ebmalloc() to allocate a copy of the ESRT a reasonable option?
>>
>> I'd suggest to try hard to avoid ebmalloc(). It ought to be possible to
>> make the copy before ExitBootServices(), via normal EFI allocation. If
>> replacing a pointer in the config table was okay(ish), this could even
>> be utilized to overcome the kexec problem.
> 
> What type should I use for the allocation?  EfiLoaderData looks like the
> most consistent choice, but I am not sure if memory so allocated remains
> valid when Xen hands off to the OS, so EfiRuntimeServicesData might be a
> better choice.

It definitely is. We do recycle EfiLoaderData ourselves.

>  To avoid memory leaks from repeated kexec(), this could
> be made conditional on the ESRT not being in memory of type
> EfiRuntimeServicesData to begin with.

Of course - there's no point relocating the blob when it already is
immune to recycling.

Jan