linux-kernel - Re: [PATCHv3 2/2] x86/mm: Make memremap(MEMREMAP

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Message-ID: <2nc5silj5wbj6kz5tcsutgcjx6wviobhem4z24x4ya2r4q4ra5@5rixeg2wo7c3>
Date: Tue, 14 Jan 2025 16:44:04 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: 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>, Andy Lutomirski <luto@...nel.org>, 
	Albert Ou <aou@...s.berkeley.edu>, Alexei Starovoitov <ast@...nel.org>, 
	Andrea Parri <parri.andrea@...il.com>, Arnd Bergmann <arnd@...db.de>, 
	Daniel Borkmann <daniel@...earbox.net>, Eric Chan <ericchancf@...gle.com>, 
	Jason Gunthorpe <jgg@...pe.ca>, Kai Huang <kai.huang@...el.com>, 
	Kefeng Wang <wangkefeng.wang@...wei.com>, Kent Overstreet <kent.overstreet@...ux.dev>, 
	Palmer Dabbelt <palmer@...osinc.com>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Russell King <linux@...linux.org.uk>, Samuel Holland <samuel.holland@...ive.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Yuntao Wang <ytcoode@...il.com>, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	stable@...r.kernel.org, Ashish Kalra <ashish.kalra@....com>, 
	"Maciej W. Rozycki" <macro@...am.me.uk>
Subject: Re: [PATCHv3 2/2] x86/mm: Make memremap(MEMREMAP_WB) map memory as
 encrypted by default

On Tue, Jan 14, 2025 at 08:33:39AM -0600, Tom Lendacky wrote:
> On 1/14/25 01:27, Kirill A. Shutemov wrote:
> > On Mon, Jan 13, 2025 at 02:47:56PM -0600, Tom Lendacky wrote:
> >> On 1/13/25 07:14, Kirill A. Shutemov wrote:
> >>> Currently memremap(MEMREMAP_WB) can produce decrypted/shared mapping:
> >>>
> >>> memremap(MEMREMAP_WB)
> >>>   arch_memremap_wb()
> >>>     ioremap_cache()
> >>>       __ioremap_caller(.encrytped = false)
> >>>
> >>> In such cases, the IORES_MAP_ENCRYPTED flag on the memory will determine
> >>> if the resulting mapping is encrypted or decrypted.
> >>>
> >>> Creating a decrypted mapping without explicit request from the caller is
> >>> risky:
> >>>
> >>>   - It can inadvertently expose the guest's data and compromise the
> >>>     guest.
> >>>
> >>>   - Accessing private memory via shared/decrypted mapping on TDX will
> >>>     either trigger implicit conversion to shared or #VE (depending on
> >>>     VMM implementation).
> >>>
> >>>     Implicit conversion is destructive: subsequent access to the same
> >>>     memory via private mapping will trigger a hard-to-debug #VE crash.
> >>>
> >>> The kernel already provides a way to request decrypted mapping
> >>> explicitly via the MEMREMAP_DEC flag.
> >>>
> >>> Modify memremap(MEMREMAP_WB) to produce encrypted/private mapping by
> >>> default unless MEMREMAP_DEC is specified.
> >>>
> >>> Fix the crash due to #VE on kexec in TDX guests if CONFIG_EISA is enabled.
> >>
> >> This patch causes my bare-metal system to crash during boot when using
> >> mem_encrypt=on:
> >>
> >> [    2.392934] efi: memattr: Entry type should be RuntimeServiceCode/Data
> >> [    2.393731] efi: memattr: ! 0x214c42f01f1162a-0xee70ac7bd1a9c629 [type=2028324321|attr=0x6590648fa4209879]
> > 
> > Could you try if this helps?
> > 
> > diff --git a/drivers/firmware/efi/memattr.c b/drivers/firmware/efi/memattr.c
> > index c38b1a335590..b5051dcb7c1d 100644
> > --- a/drivers/firmware/efi/memattr.c
> > +++ b/drivers/firmware/efi/memattr.c
> > @@ -160,7 +160,7 @@ int __init efi_memattr_apply_permissions(struct mm_struct *mm,
> >  	if (WARN_ON(!efi_enabled(EFI_MEMMAP)))
> >  		return 0;
> >  
> > -	tbl = memremap(efi_mem_attr_table, tbl_size, MEMREMAP_WB);
> > +	tbl = memremap(efi_mem_attr_table, tbl_size, MEMREMAP_WB | MEMREMAP_DEC);
> 
> Well that would work for SME where EFI tables/data are not encrypted,
> but will break for SEV where EFI tables/data are encrypted.

Hm. Why would it break for SEV? It brings the situation back to what it
was before the patch.

Note that that __ioremap_caller() would still check io_desc.flags before
mapping it as decrypted.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov