efi/libstub: 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

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 to additionally mask metadata out
  of CR3 (like SME or LAM bits). All remaining bits are real address bits or
  reserved and must be 0.

- mask pgd value with PTE_PFN_MASK instead of PAGE_MASK, accounting for flags
  above bit 51 (_PAGE_BIT_NOPTISHADOW in particular). Bits below 51, but above
  the max physical address are reserved and must be 0.

Fixes: cb1c9e02b0c1 ("x86/efistub: Perform 4/5 level paging switch from the stub")
Reported-by: Michael van der Westhuizen <rmikey@meta.com>
Reported-by: Tobias Fleig <tfleig@meta.com>
Co-developed-by: Kiryl Shutsemau <kas@kernel.org>
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Signed-off-by: Usama Arif <usamaarif642@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://patch.msgid.link/20251103141002.2280812-3-usamaarif642@gmail.com