linux-kernel - Re: [PATCH] x86/tdx: Fix crash on kexec with CONFIG

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Message-ID: <npjx2z3adipvsxngnsoj6hsgk7rxta6ojdomm4tcd42maakuuz@rij273zia5ek>
Date: Wed, 28 Aug 2024 09:21:51 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: "Kalra, Ashish" <ashish.kalra@....com>
Cc: Tom Lendacky <thomas.lendacky@....com>, 
	"Maciej W. Rozycki" <macro@...am.me.uk>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
	Kevin Loughlin <kevinloughlin@...gle.com>, linux-kernel@...r.kernel.org, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCH] x86/tdx: Fix crash on kexec with CONFIG_EISA

On Tue, Aug 27, 2024 at 05:15:56PM -0500, Kalra, Ashish wrote:
> Hello Kirill,
> 
> On 8/26/2024 10:52 AM, Tom Lendacky wrote:
> > On 8/26/24 07:25, Kirill A. Shutemov wrote:
> >> On Sat, Aug 24, 2024 at 11:29:39PM +0100, Maciej W. Rozycki wrote:
> >>> On Thu, 22 Aug 2024, Kirill A. Shutemov wrote:
> >>>
> >>>> This issue causes real problems:
> >>>>
> >>>> 1. If the kernel is compiled with EISA support, it will attempt to probe
> >>>>    EISA by reading 4 bytes from the 0x0FFFD9 address (see eisa_bus_probe()).
> >>>>    The kernel treats this read as MMIO and accesses this memory via
> >>>>    shared mapping as we do for MMIO.
> >>>>
> >>>>    KVM converts memory to shared upon such access.
> >>>>
> >>>> 2. The same memory range (0xF0000-0x100000) is scanned to look for the MP
> >>>>    table (see mpparse_find_mptable()). However, this is not MMIO and it
> >>>>    is accessed via private mapping.
> >>>>
> >>>>    This will cause a crash if the memory is not private.
> >>>>
> >>>> During normal boot, the kernel scans for SMP information before probing
> >>>> for EISA, and it boots fine. However, the memory becomes shared and causes
> >>>> issues on kexec when the second kernel attempts to scan for SMP information.
> >>>  ISTM that `eisa_bus_probe' has to be updated to `memremap' analogously to 
> >>> `mpparse_find_mptable', complementing changes such as commit f7750a795687 
> >>> ("x86, mpparse, x86/acpi, x86/PCI, x86/dmi, SFI: Use memremap() for RAM 
> >>> mappings") or commit 5997efb96756 ("x86/boot: Use memremap() to map the 
> >>> MPF and MPC data").  Both just access BIOS memory.
> >>>
> >>>  Can you please try and verify if my proposed change at: 
> >>> <https://lore.kernel.org/r/alpine.DEB.2.21.2408242025210.30766@angie.orcam.me.uk> 
> >>> has fixed the problem for you?
> >> I like the direction your patch took. I hate sprinkling
> >> X86_FEATURE_TDX_GUEST checks over the kernel.
> >>
> >> Unfortunately, it is not enough to fix the issue. memremap() in this case
> >> will still boil down to ioremap() that would set shared bit:
> >>
> >> memremap()
> >>   arch_memremap_wb()
> >>     ioremap_cache()
> >>       __ioremap_caller(.encrytped = false)
> >>
> >> I think arch_memremap_wb() should be mapped ioremap_encrypted() in x86
> >> case. See the patch below.
> >>
> >> It seems to be working fine on TDX, but I am not sure about SEV.
> >>
> >> Tom, any comments?
> > I haven't dug through the code that thoroughly, but I don't think making
> > arch_memremap_wb() be ioremap_encrypted() will work for SME, where some
> > data, e.g. setup data, is unencrypted and needs to be mapped shared.
> >
> > Let me add @Ashish to the thread and have him investigate this since he
> > has been working on the kexec support under SNP. Can someone provide the
> > specific kernel options that need to be in place?
> 
> As Tom asked for, please provide the specific kernel options to test
> with this configuration.

It is not about testing a specific configuration. The question is if it
safe for memremap() to map all WB memory as encrypted by default.

Looks like it is safe for TDX, but I am not sure about SME/SEV.

Maybe we want a specific flag to make memremap() map WB memory as
decrypted/shared. Make everything encrypted by default seems like a sane
default.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov