Re: [PATCH] efi: discover ESRT table on Xen PV too

Marek Marczykowski-Górecki
2020-08-18T15:00:40Z

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On Tue, Aug 18, 2020 at 02:47:10PM +0200, Roger Pau Monné wrote:
> On Tue, Aug 18, 2020 at 02:01:35PM +0200, Marek Marczykowski-Górecki wrote:
> > Do you mean PV dom0 should receive full EFI memory map? Jan already
> > objected this as it would be a layering violation.
>
> dom0 is already capable of getting the native e820 memory map using
> XENMEM_machine_memory_map, I'm not sure why allowing to return the
> memory map in EFI form would be any different (or a layering
> violation in the PV dom0 case).
>
> Do you have a reference to that thread? I certainly missed it.

See this thread: http://markmail.org/message/nrrvuau5whebksy2

> For PVH dom0 we could consider something different, since in that case
> there's a guest memory map which could likely be returned in EFI
> format and with the EFI regions if required.
>
> > > > Skip this part on Xen PV (let Xen do the right thing if it deems
> > > > necessary) and use ESRT table normally.
> > >
> > > Maybe it would be better to introduce a new hypercall (or add a
> > > parameter to XENMEM_machine_memory_map) in order to be able to fetch
> > > the EFI memory map?
> > >
> > > That should allow a PV dom0 to check the ESRT is correct and thus not
> > > diverge from bate metal.
> >
> > Note the EFI memory map there is used not just to check things, but to
> > actually modify it to reserve the region. I think that's rather Xen
> > responsibility, not dom0. See the comment from Ard.
>
> But that would modify Linux copy of the memory map, which is fine? My
> understanding of EFI is limited, but I don't think such changes are
> feed back into EFI, so Linux is completely free to do whatever it
> wants with it's copy of the EFI memory map.

Yes, but the thing is to make sure Xen doesn't use that memory, not only
dom0. See below.

> > > > + if (efi_enabled(EFI_MEMMAP)) {
> > > > + rc = efi_mem_desc_lookup(efi.esrt, &md);
> > > > + if (rc < 0 ||
> > > > + (!(md.attribute & EFI_MEMORY_RUNTIME) &&
> > > > + md.type != EFI_BOOT_SERVICES_DATA &&
> > > > + md.type != EFI_RUNTIME_SERVICES_DATA)) {
> > > > + pr_warn("ESRT header is not in the memory map.\n");
> > > > + return;
> > > > + }
> > >
> > > Here you blindly trust the data in the ESRT in the PV case, without
> > > checking it matches the regions on the memory map, which could lead to
> > > errors if ESRT turns to be wrong.
> >
> > I don't think checking merely if ESRT lives somewhere in
> > EFI_{BOOT,RUNTIME}_SERVICES_DATA area guarantees its correctness.
> >
> > On the other hand, this seems to be done to prevent overwriting that
> > memory with something else (see that in case of EFI_BOOT_SERVICES_DATA
> > it is later marked as reserved. I think it should be rather done by Xen,
> > not dom0.
>
> Forcing Xen to do all those checks seems quite a tedious work, and in
> fact the memory map that dom0 has might be more complete than the one
> Xen is able to construct, as Xen doesn't have an AML parser so it's
> not able to get all the possible info from ACPI.

Let me draw the picture from the beginning.

EFI memory map contains various memory regions. Some of them are marked
as not needed after ExitBootServices() call (done in Xen before
launching dom0). This includes EFI_BOOT_SERVICES_DATA and
EFI_BOOT_SERVICES_CODE.

EFI SystemTable contains pointers to various ConfigurationTables -
physical addresses (at least in this case). Xen does interpret some of
them, but not ESRT. Xen pass the whole (address of) SystemTable to Linux
dom0 (at least in PV case). Xen doesn't do anything about tables it
doesn't understand.

Now, the code in Linux takes the (ESRT) table address early and checks
the memory map for it. We have 3 cases:
- it points at area marked as neither EFI_*_SERVICES_DATA, nor with
EFI_MEMORY_RUNTIME attribute -> Linux refuse to use it
- it points to EFI_RUNTIME_SERVICES_DATA or with EFI_MEMORY_RUNTIME
attribute - Linux uses the table; memory map already says the area
belongs to EFI and the OS should not use it for something else
- it points to EFI_BOOT_SERVICES_DATA - Linux mark the area as reserved
to not release it after calling ExitBootServices()

The problematic is the third case - at the time when Linux dom0 is run,
ExitBootServices() was already called and EFI_BOOT_SERVICES_* memory was
already released. It could be already used for something else (for
example Xen could overwrite it while loading dom0).

Note the problematic case should be the most common - UEFI specification
says "The ESRT shall be stored in memory of type EfiBootServicesData"
(chapter 22.3 of UEFI Spec v2.6).

For this reason, to use ESRT in dom0, Xen should do something about it
before ExitBootServices() call. While analyzing all the EFI tables is
probably not a viable option, it can do some simple action:
- retains all the EFI_BOOT_SERVICES_* areas - there is already code
for that, controlled with /mapbs boot switch (to xen.efi, would need
another option for multiboot2+efi)
- have a list of tables to retain - since Xen already do analyze some
of the ConfigurationTables, it can also have a list of those to
preserve even if they live in EFI_BOOT_SERVICES_DATA. In this case,
while Xen doesn't need to parse the whole table, it need to parse it's
header to get the table size - to reserve that memory and not reuse
it after ExitBootServices().

I think the second solution is slightly more elegant.

--
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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