Re: [PATCH v3] Support ESRT in Xen dom0

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Sep 22, 2022 at 05:05:43PM +0200, Ard Biesheuvel wrote:
> On Thu, 22 Sept 2022 at 16:56, Demi Marie Obenour
> <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Thu, Sep 22, 2022 at 08:12:14AM +0200, Jan Beulich wrote:
> > > On 22.09.2022 03:09, Demi Marie Obenour wrote:
> > > > On Wed, Sep 21, 2022 at 10:34:04PM +0200, Jan Beulich wrote:
> > > >> On 20.09.2022 18:09, Ard Biesheuvel wrote:
> > > >>> On Tue, 20 Sept 2022 at 17:54, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> > > >>>>
> > > >>>> On 20.09.2022 17:36, Ard Biesheuvel wrote:
> > > >>>>> On Mon, 19 Sept 2022 at 21:33, Demi Marie Obenour
> > > >>>>> <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > >>>>>>
> > > >>>>>> fwupd requires access to the EFI System Resource Table (ESRT) to
> > > >>>>>> discover which firmware can be updated by the OS.  Currently, 
> > > >>>>>> Linux does
> > > >>>>>> not expose the ESRT when running as a Xen dom0.  Therefore, it is 
> > > >>>>>> not
> > > >>>>>> possible to use fwupd in a Xen dom0, which is a serious problem 
> > > >>>>>> for e.g.
> > > >>>>>> Qubes OS.
> > > >>>>>>
> > > >>>>>> Before Xen 4.16, this was not fixable due to hypervisor 
> > > >>>>>> limitations.
> > > >>>>>> The UEFI specification requires the ESRT to be in 
> > > >>>>>> EfiBootServicesData
> > > >>>>>> memory, which Xen will use for whatever purposes it likes.  
> > > >>>>>> Therefore,
> > > >>>>>> Linux cannot safely access the ESRT, as Xen may have overwritten 
> > > >>>>>> it.
> > > >>>>>>
> > > >>>>>> Starting with Xen 4.17, Xen checks if the ESRT is in 
> > > >>>>>> EfiBootServicesData
> > > >>>>>> or EfiRuntimeServicesData memory.  If the ESRT is in 
> > > >>>>>> EfiBootServicesData
> > > >>>>>> memory, Xen allocates some memory of type EfiRuntimeServicesData, 
> > > >>>>>> copies
> > > >>>>>> the ESRT to it, and finally replaces the ESRT pointer with a 
> > > >>>>>> pointer to
> > > >>>>>> the copy.  Since Xen will not clobber EfiRuntimeServicesData 
> > > >>>>>> memory,
> > > >>>>>> this ensures that the ESRT can safely be accessed by the OS.  It 
> > > >>>>>> is safe
> > > >>>>>> to access the ESRT under Xen if, and only if, it is in memory of 
> > > >>>>>> type
> > > >>>>>> EfiRuntimeServicesData.
> > > >>>>>>
> > > >>>>>
> > > >>>>> Thanks for the elaborate explanation. This is really helpful.
> > > >>>>>
> > > >>>>> So here, you are explaining that the only way for Xen to prevent
> > > >>>>> itself from potentially clobbering the ESRT is by creating a
> > > >>>>> completely new allocation?
> > > >>>>
> > > >>>> There are surely other ways, e.g. preserving BootServices* regions
> > > >>>> alongside RuntimeServices* ones. But as the maintainer of the EFI
> > > >>>> code in Xen I don't view this as a reasonable approach.
> > > >>>
> > > >>> Why not?
> > > >>
> > > >> Because it's against the intentions the EFI has (or at least had)
> > > >> for this memory type. Much more than EfiAcpiReclaimMemory this
> > > >> type is intended for use as ordinary RAM post-boot.
> > > >
> > > > What about giving that memory to dom0?  dom0’s balloon driver will give
> > > > anything dom0 doesn’t wind up using back to Xen.
> > >
> > > While perhaps in principle possible, this would require special casing
> > > in Xen. Except for the memory the initrd comes in, we don't directly
> > > hand memory to Dom0. Instead everything goes through the page allocator
> > > first. Plus if we really were convinced boot services memory needed
> > > retaining, then it would also need retaining across kexec (and hence
> > > shouldn't be left to Dom0 to decide what to do with it).
> >
> > So how should dom0 handle the various EFI tables other than the ESRT?
> > Right now most uses of these tables in Linux are not guarded by any
> > checks for efi_enabled(EFI_MEMMAP) or similar.  If some of them are in
> > EfiBootServicesData memory, they might be corrupted before Linux gets
> > them.
> 
> Yes, this is an annoying oversight of the EFI design: the config
> tables are <guid, address> tuples, and the size of the table is
> specific to each table type. So without knowing the GUID, there is no
> way you can reserve the right size.
> 
> Perhaps you could implement something like a hypercall in
> efi_arch_mem_reserve(), which is called by the EFI code to reserve
> regions that are in boot services memory but need to remain reserved?
> This is used for all config tables that it knows or cares about.

On versions of Xen that support spawning multiple domains at boot
(instead of just dom0) this will be racy.  What about refusing to use
tables in EfiBootServicesData when running under Xen unless a hypercall
indicates that Xen has reserved all EfiBootServicesData memory?  Where
could such a check be placed?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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