linux-kernel - [PATCH v7 09/15] x86/mm: LAM compatible non-canonical definition

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Message-ID: <cb345787e860e604efb4faf7fd6b9d5ceb80ab26.1765386422.git.m.wieczorretman@pm.me>
Date: Wed, 10 Dec 2025 17:29:47 +0000
From: Maciej Wieczor-Retman <m.wieczorretman@...me>
To: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Cc: m.wieczorretman@...me, Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, Alexander Potapenko <glider@...gle.com>, linux-kernel@...r.kernel.org
Subject: [PATCH v7 09/15] x86/mm: LAM compatible non-canonical definition

From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>

For an address to be canonical it has to have its top bits equal to each
other. The number of bits depends on the paging level and whether
they're supposed to be ones or zeroes depends on whether the address
points to kernel or user space.

With Linear Address Masking (LAM) enabled, the definition of linear
address canonicality is modified. Not all of the previously required
bits need to be equal, only the first and last from the previously equal
bitmask. So for example a 5-level paging kernel address needs to have
bits [63] and [56] set.

Change the canonical checking function to use bit masks instead of bit
shifts.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
Acked-by: Alexander Potapenko <glider@...gle.com>
---
Changelog v7:
- Add Alexander's acked-by tag.
- Add parentheses around vaddr_bits as suggested by checkpatch.
- Apply the bitmasks to the __canonical_address() function which is used
  in kvm code.

Changelog v6:
- Use bitmasks to check both kernel and userspace addresses in the
  __is_canonical_address() (Dave Hansen and Samuel Holland).

Changelog v4:
- Add patch to the series.

 arch/x86/include/asm/page.h | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
index bcf5cad3da36..b7940fa49e64 100644
--- a/arch/x86/include/asm/page.h
+++ b/arch/x86/include/asm/page.h
@@ -82,9 +82,22 @@ static __always_inline void *pfn_to_kaddr(unsigned long pfn)
 	return __va(pfn << PAGE_SHIFT);
 }

+#ifdef CONFIG_KASAN_SW_TAGS
+#define CANONICAL_MASK(vaddr_bits) (BIT_ULL(63) | BIT_ULL((vaddr_bits) - 1))
+#else
+#define CANONICAL_MASK(vaddr_bits) GENMASK_ULL(63, vaddr_bits)
+#endif
+
+/*
+ * To make an address canonical either set or clear the bits defined by the
+ * CANONICAL_MASK(). Clear the bits for userspace addresses if the top address
+ * bit is a zero. Set the bits for kernel addresses if the top address bit is a
+ * one.
+ */
 static __always_inline u64 __canonical_address(u64 vaddr, u8 vaddr_bits)
 {
-	return ((s64)vaddr << (64 - vaddr_bits)) >> (64 - vaddr_bits);
+	return (vaddr & BIT_ULL(63)) ? vaddr | CANONICAL_MASK(vaddr_bits) :
+				       vaddr & ~CANONICAL_MASK(vaddr_bits);
 }

 static __always_inline u64 __is_canonical_address(u64 vaddr, u8 vaddr_bits)
-- 
2.52.0