linux-kernel - [PATCH 1/3] x86/boot: Fix page table access in 5-level to 4-level paging transition

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Message-ID: <20251022220755.1026144-2-usamaarif642@gmail.com>
Date: Wed, 22 Oct 2025 23:06:25 +0100
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 1/3] x86/boot: 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 entry 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:
- read_cr3_pa(): Uses CR3_ADDR_MASK properly clearing SME encryption bit
  and extracting only the physical address portion.
- mask pgd value with PTE_PFN_MASK instead of PAGE_MASK, accounting for
  flags above physical address (_PAGE_BIT_NOPTISHADOW in particular).

Fixes: e9d0e6330eb8 ("x86/boot/compressed/64: Prepare new top-level page table for trampoline")
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>
---
 arch/x86/boot/compressed/pgtable_64.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
index bdd26050dff77..a56449938b7ec 100644
--- a/arch/x86/boot/compressed/pgtable_64.c
+++ b/arch/x86/boot/compressed/pgtable_64.c
@@ -170,7 +170,8 @@ asmlinkage void configure_5level_paging(struct boot_params *bp, void *pgtable)
 		 */
 		*trampoline_32bit = __native_read_cr3() | _PAGE_TABLE_NOENC;
 	} else {
-		unsigned long src;
+		u64 *new_cr3;
+		pgd_t *pgdp;
 
 		/*
 		 * For 5- to 4-level paging transition, copy page table pointed
@@ -180,8 +181,9 @@ asmlinkage void configure_5level_paging(struct boot_params *bp, void *pgtable)
 		 * We cannot just point to the page table from trampoline as it
 		 * may be above 4G.
 		 */
-		src = *(unsigned long *)__native_read_cr3() & PAGE_MASK;
-		memcpy(trampoline_32bit, (void *)src, PAGE_SIZE);
+		pgdp = (pgd_t *)read_cr3_pa();
+		new_cr3 = (u64 *)(pgd_val(pgdp[0]) & PTE_PFN_MASK);
+		memcpy(trampoline_32bit, new_cr3, PAGE_SIZE);
 	}
 
 	toggle_la57(trampoline_32bit);
-- 
2.47.3