Re: [PATCH 0/3] Resolve problems with kexec identity mapping

[Steve Wahl]

Gentle ping.  Can someone give me some feedback, please?  

Thanks,

Steve Wahl, HPE.

On Thu, May 23, 2024 at 10:54:33AM +0800, Dave Young wrote:
> Cc kexec list as well.
> 
> On Thu, 23 May 2024 at 10:52, Dave Young <dyoung@xxxxxxxxxx> wrote:
> >
> > Add Tao in the cc list.
> >
> > On Tue, 21 May 2024 at 02:37, Steve Wahl <steve.wahl@xxxxxxx> wrote:
> > >
> > > Although there was a previous fix to avoid early kernel access to the
> > > EFI config table on Intel systems, the problem can still exist on AMD
> > > systems that support SEV (Secure Encrypted Virtualization).  The
> > > command line option "nogbpages" brings this bug to the surface.  And
> > > this is what caused the regression with my earlier patch that
> > > attempted to reduce the use of gbpages.  This patch series fixes that
> > > problem and restores my earlier patch.
> > >
> > > The following 2 commits caused the EFI config table, and the CC_BLOB
> > > entry in that table, to be accessed when enabling SEV at kernel
> > > startup.
> > >
> > >     commit ec1c66af3a30 ("x86/compressed/64: Detect/setup SEV/SME features
> > >                           earlier during boot")
> > >     commit c01fce9cef84 ("x86/compressed: Add SEV-SNP feature
> > >                           detection/setup")
> > >
> > > These accesses happen before the new kernel establishes its own
> > > identity map, and before establishing a routine to handle page faults.
> > > But the areas referenced are not explicitly added to the kexec
> > > identity map.
> > >
> > > This goes unnoticed when these areas happen to be placed close enough
> > > to others areas that are explicitly added to the identity map, but
> > > that is not always the case.
> > >
> > > Under certain conditions, for example Intel Atom processors that don't
> > > support 1GB pages, it was found that these areas don't end up mapped,
> > > and the SEV initialization code causes an unrecoverable page fault,
> > > and the kexec fails.
> > >
> > > Tau Liu had offered a patch to put the config table into the kexec
> > > identity map to avoid this problem:
> > >
> > > https://lore.kernel.org/all/20230601072043.24439-1-ltao@xxxxxxxxxx/
> > >
> > > But the community chose instead to avoid referencing this memory on
> > > non-AMD systems where the problem was reported.
> > >
> > >     commit bee6cf1a80b5 ("x86/sev: Do not try to parse for the CC blob
> > >                           on non-AMD hardware")
> > >
> > > I later wanted to make a different change to kexec identity map
> > > creation, and had this patch accepted:
> > >
> > >     commit d794734c9bbf ("x86/mm/ident_map: Use gbpages only where full GB page should be mapped.")
> > >
> > > but it quickly needed to be reverted because of problems on AMD systems.
> > >
> > > The reported regression problems on AMD systems were due to the above
> > > mentioned references to the EFI config table.  In fact, on the same
> > > systems, the "nogbpages" command line option breaks kexec as well.
> > >
> > > So I resubmit Tau Liu's original patch that maps the EFI config
> > > table, add an additional patch by me that ensures that the CC blob is
> > > also mapped (if present), and also resubmit my earlier patch to use
> > > gpbages only when a full GB of space is requested to be mapped.
> > >
> > > I do not advocate for removing the earlier, non-AMD fix.  With kexec,
> > > two different kernel versions can be in play, and the earlier fix
> > > still covers non-AMD systems when the kexec'd-from kernel doesn't have
> > > these patches applied.
> > >
> > > All three of the people who reported regression with my earlier patch
> > > have retested with this patch series and found it to work where my
> > > single patch previously did not.  With current kernels, all fail to
> > > kexec when "nogbpages" is on the command line, but all succeed with
> > > "nogbpages" after the series is applied.
> > >
> > > Tao Liu (1):
> > >   x86/kexec: Add EFI config table identity mapping for kexec kernel
> > >
> > > Steve Wahl (2):
> > >   x86/kexec: Add EFI Confidential Computing blob to kexec identity
> > >     mapping.
> > >   x86/mm/ident_map: Use gbpages only where full GB page should be
> > >     mapped.
> > >
> > >  arch/x86/kernel/machine_kexec_64.c | 82 ++++++++++++++++++++++++++++--
> > >  arch/x86/mm/ident_map.c            | 23 +++++++--
> > >  2 files changed, 95 insertions(+), 10 deletions(-)
> > >
> > > --
> > > 2.26.2
> > >
>