linux-kernel - Re: [PATCH v3 1/2] x86/kexec: Add EFI config table identity mapping for kexec kernel

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Message-ID: <CAMj1kXEgvORTj3FT54Dq39d-tQLXDdmEPEVc=x6H8Schmdu6fg@mail.gmail.com>
Date: Wed, 17 Jul 2024 14:53:11 -0700
From: Ard Biesheuvel <ardb@...nel.org>
To: Steve Wahl <steve.wahl@....com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org, 
	Pavin Joseph <me@...injoseph.com>, Eric Hagberg <ehagberg@...il.com>, 
	Simon Horman <horms@...ge.net.au>, Eric Biederman <ebiederm@...ssion.com>, 
	Dave Young <dyoung@...hat.com>, Sarah Brofeldt <srhb@....dk>, Russ Anderson <rja@....com>, 
	Dimitri Sivanich <sivanich@....com>, Hou Wenlong <houwenlong.hwl@...group.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Baoquan He <bhe@...hat.com>, 
	Yuntao Wang <ytcoode@...il.com>, Bjorn Helgaas <bhelgaas@...gle.com>, Joerg Roedel <jroedel@...e.de>, 
	Michael Roth <michael.roth@....com>, Tao Liu <ltao@...hat.com>, kexec@...ts.infradead.org, 
	"Kalra, Ashish" <ashish.kalra@....com>, linux-efi@...r.kernel.org
Subject: Re: [PATCH v3 1/2] x86/kexec: Add EFI config table identity mapping
 for kexec kernel

On Wed, 17 Jul 2024 at 14:32, Steve Wahl <steve.wahl@....com> wrote:
>
> From: Tao Liu <ltao@...hat.com>
>
> A kexec kernel boot failure is sometimes observed on AMD CPUs due to
> an unmapped EFI config table array.  This can be seen when "nogbpages"
> is on the kernel command line, and has been observed as a full BIOS
> reboot rather than a successful kexec.
>
> This was also the cause of reported regressions attributed to Commit
> 7143c5f4cf20 ("x86/mm/ident_map: Use gbpages only where full GB page
> should be mapped.") which was subsequently reverted.
>
> To avoid this page fault, explicitly include the EFI config table
> array in the kexec identity map.
>
> Further explanation:
>
> The following 2 commits caused the EFI config table array 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")
>
> This is in the code that examines whether SEV should be enabled or
> not, so it can even affect systems that are not SEV capable.
>
> This may result in a page fault if the EFI config table array's
> address is unmapped. Since the page fault occurs before the new kernel
> establishes its own identity map and page fault routines, it is
> unrecoverable and kexec fails.
>
> Most often, this problem is not seen because the EFI config table
> array gets included in the map by the luck of being placed at a memory
> address close enough to other memory areas that *are* included in the
> map created by kexec.
>
> Both the "nogbpages" command line option and the "use gpbages only
> where full GB page should be mapped" patch greatly reduce the chance
> of being included in the map by luck, which is why the problem
> appears.
>
> Signed-off-by: Tao Liu <ltao@...hat.com>
> Signed-off-by: Steve Wahl <steve.wahl@....com>
> Tested-by: Pavin Joseph <me@...injoseph.com>
> Tested-by: Sarah Brofeldt <srhb@....dk>
> Tested-by: Eric Hagberg <ehagberg@...il.com>
> ---
>
> Version 3: Do not rename map_efi_systab to map_efi_tables, and don't add
> 'config table' to the comments, per Ard Biesheuvel request.
>
>  arch/x86/kernel/machine_kexec_64.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
>

Reviewed-by: Ard Biesheuvel <ardb@...nel.org>