efi/libstub: Fix page table access in 5-level to 4-level paging transition

[ Upstream commit 8436112 ]

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: cb1c9e0 ("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
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: uarif1

drivers/firmware/efi/libstub/x86-5lvl.c

@@ -66,7 +66,7 @@ void efi_5level_switch(void)
 	bool have_la57 = native_read_cr4() & X86_CR4_LA57;
 	bool need_toggle = want_la57 ^ have_la57;
 	u64 *pgt = (void *)la57_toggle + PAGE_SIZE;
-	u64 *cr3 = (u64 *)__native_read_cr3();
+	pgd_t *cr3 = (pgd_t *)native_read_cr3_pa();
 	u64 *new_cr3;
 
 	if (!la57_toggle || !need_toggle)
@@ -82,7 +82,7 @@ void efi_5level_switch(void)
 		new_cr3[0] = (u64)cr3 | _PAGE_TABLE_NOENC;
 	} else {
 		/* take the new root table pointer from the current entry #0 */
-		new_cr3 = (u64 *)(cr3[0] & PAGE_MASK);
+		new_cr3 = (u64 *)(native_pgd_val(cr3[0]) & PTE_PFN_MASK);
 
 		/* copy the new root table if it is not 32-bit addressable */
 		if ((u64)new_cr3 > U32_MAX)