[PATCH v3 0/2] x86: Fix kexec 5-level to 4-level paging transition

[Usama Arif]

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@xxxxxxxxxx>
Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
Reported-by: Michael van der Westhuizen <rmikey@xxxxxxxx>
Reported-by: Tobias Fleig <tfleig@xxxxxxxx>

The patches are based on aaa9c3550b60d6259d6ea8b1175ade8d1242444e (next-20251022)

v2 -> v3:
- Use native_pgd_val instead of pgd_val to fix broken build with allmodconfig.
  I wanted to keep the code between pgtable_64.c and x86-5lvl.c consistent
  so changed it in both patches
  (Borislav Petkov and Ard Biesheuvel)
- Commit message improvements (Dave Hansen)

v1 -> v2:
- Remove patch 3 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