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

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Message-ID: <gnxxp3bzk7cc5eidwvqvqb5hz2ojgjdadujpthweufxms2gjsc@ibcbkjst3pzf>
Date: Thu, 29 Aug 2024 15:53:29 +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 Wed, Aug 28, 2024 at 02:43:23PM -0500, Kalra, Ashish wrote:
> Hello Kirill,
> 
> On 8/28/2024 1:21 AM, Kirill A. Shutemov wrote:
> > 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.
> 
> For SEV it may make sense, but for SME we don't want memremap() to map
> all WB memory as encrypted by default.

Could you elaborate why?

I mean if it is specific memory ranges that can be identified as such we
could make exception for them.

> >
> > 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.
> 
> What are MEMREMAP_ENC, MEMREMAP_DEC flags being used for currently ?

Good question.

I see zero use of MEMREMAP_ENC in the kernel. And MEMREMAP_DEC only used
for setup data which I believe AMD thing.

If it is the only memory that must be decrypted, I guess we can make it
work with encrypted by default.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov