linux-kernel - [PATCH v2 0/2] x86: Fix kexec 5-level to 4-level paging transition

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Message-ID: <20251028105637.769470-1-usamaarif642@gmail.com>
Date: Tue, 28 Oct 2025 10:55:55 +0000
From: Usama Arif <usamaarif642@...il.com>
To: dwmw@...zon.co.uk,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	ardb@...nel.org,
	hpa@...or.com
Cc: 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,
	Usama Arif <usamaarif642@...il.com>,
	Michael van der Westhuizen <rmikey@...a.com>,
	Tobias Fleig <tfleig@...a.com>
Subject: [PATCH v2 0/2] x86: Fix kexec 5-level to 4-level paging transition

This series addresses critical bugs in the kexec path when transitioning
from a kernel using 5-level page tables to one using 4-level page tables.

The root cause is improper handling of PGD entry value during the page level
transition. Specifically PGD entry value is masked with PAGE_MASK instead of
PTE_PFN_MASK, failing to account for high-order software bits like
_PAGE_BIT_NOPTISHADOW (bit 58).

When bit 58 (_PAGE_BIT_NOPTISHADOW) is set in the source kernel, the target
4-level kernel doesn't recognize it and fails to mask it properly, leading
to kexec failure.

Patch 1: Fixes the x86 boot compressed code path by replacing direct CR3
dereferencing with read_cr3_pa() and using PTE_PFN_MASK instead
of PAGE_MASK.

Patch 2: Applies the same fix to the EFI stub code path. (Done in a
separate patch as Fixes tag is different).

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>

The patches are based on aaa9c3550b60d6259d6ea8b1175ade8d1242444e (next-20251022)

v1 -> v2:
- Remove patch 3 from v1 to fix kexec for source kernel in 5-level to 4-level
  transition where the 4 level kernel doesnt have patch 1 and 2 (Dave Hansen)
- Add include for asm/pgtable.h to fix build for x86_64-allnoconfig (kernel test bot)
- Use native_read_cr3_pa and for both paths (Ard Biesheuvel)

Usama Arif (2):
  x86/boot: Fix page table access in 5-level to 4-level paging
    transition
  efi/libstub: Fix page table access in 5-level to 4-level paging
    transition

 arch/x86/boot/compressed/pgtable_64.c   | 11 +++++++----
 drivers/firmware/efi/libstub/x86-5lvl.c |  4 ++--
 2 files changed, 9 insertions(+), 6 deletions(-)

-- 
2.47.3