linux-kernel - [PATCH 2/3] efi/libstub: Fix page table access in 5-level to 4-level paging transition

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Message-ID: <20251022220755.1026144-3-usamaarif642@gmail.com>
Date: Wed, 22 Oct 2025 23:06:26 +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 2/3] 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:
- 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: cb1c9e02b0c1 ("x86/efistub: Perform 4/5 level paging switch from the stub")
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>
---
 drivers/firmware/efi/libstub/x86-5lvl.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/libstub/x86-5lvl.c b/drivers/firmware/efi/libstub/x86-5lvl.c
index f1c5fb45d5f7c..34b72da457487 100644
--- a/drivers/firmware/efi/libstub/x86-5lvl.c
+++ b/drivers/firmware/efi/libstub/x86-5lvl.c
@@ -81,8 +81,11 @@ void efi_5level_switch(void)
 		new_cr3 = memset(pgt, 0, PAGE_SIZE);
 		new_cr3[0] = (u64)cr3 | _PAGE_TABLE_NOENC;
 	} else {
+		pgd_t *pgdp;
+
+		pgdp = (pgd_t *)read_cr3_pa();
 		/* take the new root table pointer from the current entry #0 */
-		new_cr3 = (u64 *)(cr3[0] & PAGE_MASK);
+		new_cr3 = (u64 *)(pgd_val(pgdp[0]) & PTE_PFN_MASK);
 
 		/* copy the new root table if it is not 32-bit addressable */
 		if ((u64)new_cr3 > U32_MAX)
-- 
2.47.3