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

[Ard Biesheuvel]

On Tue, 28 Oct 2025 at 11:57, Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>
> 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)
>
> 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
>

Reviewed-by: Ard Biesheuvel <ardb@xxxxxxxxxx>