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

[Dave Young]

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
>