linux-kernel - Re: [PATCH v2 2/2] efi/libstub: Fix page table access in 5-level to 4-level paging transition

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Message-ID: <CAMj1kXEwfYqTG2R8m8+t=DLFsBY3wxSm1b70DyK2gu_TF-L4wQ@mail.gmail.com>
Date: Fri, 31 Oct 2025 15:43:25 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Usama Arif <usamaarif642@...il.com>, dwmw@...zon.co.uk, tglx@...utronix.de, 
	mingo@...hat.com, dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org, 
	apopple@...dia.com, thuth@...hat.com, nik.borisov@...e.com, kas@...nel.org, 
	linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org, kernel-team@...a.com, 
	Michael van der Westhuizen <rmikey@...a.com>, Tobias Fleig <tfleig@...a.com>
Subject: Re: [PATCH v2 2/2] efi/libstub: Fix page table access in 5-level to
 4-level paging transition

On Fri, 31 Oct 2025 at 15:40, Borislav Petkov <bp@...en8.de> wrote:
>
> On Tue, Oct 28, 2025 at 10:55:57AM +0000, Usama Arif wrote:
> > When transitioning from 5-level to 4-level paging, the existing code
> > incorrectly accesses page table entries by directly dereferencing CR3
> > and applying PAGE_MASK. This approach has several issues:
> >
> > - __native_read_cr3() returns the raw CR3 register value, which on
> >   x86_64 includes not just the physical address but also flags Bits
> >   above the physical address width of the system (i.e. above
> >   __PHYSICAL_MASK_SHIFT) are also not masked.
> > - The pgd value is masked by PAGE_SIZE which doesn't take into account
> >   the higher bits such as _PAGE_BIT_NOPTISHADOW.
> >
> > Replace this with proper accessor functions:
> > - native_read_cr3_pa(): Uses CR3_ADDR_MASK properly clearing SME encryption
> >   bit and extracting only the physical address portion.
> > - mask pgd value with PTE_PFN_MASK instead of PAGE_MASK, accounting for
> >   flags above physical address (_PAGE_BIT_NOPTISHADOW in particular).
> >
> > Fixes: cb1c9e02b0c1 ("x86/efistub: Perform 4/5 level paging switch from the stub")
> > Co-developed-by: Kiryl Shutsemau <kas@...nel.org>
> > Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
> > Signed-off-by: Usama Arif <usamaarif642@...il.com>
> > Reported-by: Michael van der Westhuizen <rmikey@...a.com>
> > Reported-by: Tobias Fleig <tfleig@...a.com>
> > ---
> >  drivers/firmware/efi/libstub/x86-5lvl.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
>
> allmodconfig build:
>
> ld: error: unplaced orphan section `__bug_table' from `drivers/firmware/efi/libstub/x86-5lvl.stub.o'
> ld: error: unplaced orphan section `.altinstructions' from `drivers/firmware/efi/libstub/x86-5lvl.stub.o'
> ld: error: unplaced orphan section `.altinstr_replacement' from `drivers/firmware/efi/libstub/x86-5lvl.stub.o'
> ld: Unexpected GOT/PLT entries detected!
> ld: Unexpected run-time procedure linkages detected!
> ld: Unexpected run-time relocations (.rela) detected!
> ld: drivers/firmware/efi/libstub/x86-5lvl.stub.o: in function `efi_5level_switch':
> x86-5lvl.c:(.text+0x13e): undefined reference to `pv_ops'
> ld: x86-5lvl.c:(.text+0x14d): undefined reference to `pv_ops'
> ld: drivers/firmware/efi/libstub/x86-5lvl.stub.o:(.altinstr_replacement+0x1): undefined reference to `BUG_func'
> ld: arch/x86/boot/compressed/vmlinux: hidden symbol `pv_ops' isn't defined
> ld: final link failed: bad value
> make[3]: *** [arch/x86/boot/compressed/Makefile:116: arch/x86/boot/compressed/vmlinux] Error 1
> make[2]: *** [arch/x86/boot/Makefile:96: arch/x86/boot/compressed/vmlinux] Error 2
> make[1]: *** [arch/x86/Makefile:308: bzImage] Error 2
> make: *** [Makefile:248: __sub-make] Error 2
>

This code should be using native_pgd_val() not pgd_val().