[PATCHv8 03/17] x86/alternatives: Disable LASS when patching kernel alternatives

Kirill A. Shutemov
2025-07-01T09:59:05Z

From: Sohil Mehta <sohil.mehta@intel.com>

For patching, the kernel initializes a temporary mm area in the lower
half of the address range. See commit 4fc19708b165 ("x86/alternatives:
Initialize temporary mm for patching").

Disable LASS enforcement during patching to avoid triggering a #GP
fault.

The objtool warns due to a call to a non-allowed function that exists
outside of the stac/clac guard, or references to any function with a
dynamic function pointer inside the guard. See the Objtool warnings
section #9 in the document tools/objtool/Documentation/objtool.txt.

Considering that patching is usually small, replace the memcpy and
memset functions in the text poking functions with their inline versions
respectively.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
arch/x86/include/asm/smap.h | 33 +++++++++++++++++++++++++++++++--
arch/x86/kernel/alternative.c | 14 ++++++++++++--
2 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h
index 4f84d421d1cf..d0cc24348641 100644
--- a/arch/x86/include/asm/smap.h
+++ b/arch/x86/include/asm/smap.h
@@ -23,18 +23,47 @@

#else /* __ASSEMBLER__ */

+/*
+ * The CLAC/STAC instructions toggle the enforcement of X86_FEATURE_SMAP and
+ * X86_FEATURE_LASS.
+ *
+ * SMAP enforcement is based on the _PAGE_BIT_USER bit in the page tables: the
+ * kernel is not allowed to touch pages with the bit set unless the AC bit is
+ * set.
+ *
+ * LASS enforcement is based on bit 63 of the virtual address. The kernel is
+ * not allowed to touch memory in the lower half of the virtual address space
+ * unless the AC bit is set.
+ *
+ * Use stac()/clac() when accessing userspace (_PAGE_USER) mappings,
+ * regardless of location.
+ *
+ * Use lass_stac()/lass_clac() when accessing kernel mappings (!_PAGE_USER)
+ * in the lower half of the address space.
+ *
+ * Note: a barrier is implicit in alternative().
+ */
+
static __always_inline void clac(void)
{
- /* Note: a barrier is implicit in alternative() */
alternative("", "clac", X86_FEATURE_SMAP);
}

static __always_inline void stac(void)
{
- /* Note: a barrier is implicit in alternative() */
alternative("", "stac", X86_FEATURE_SMAP);
}

+static __always_inline void lass_clac(void)
+{
+ alternative("", "clac", X86_FEATURE_LASS);
+}
+
+static __always_inline void lass_stac(void)
+{
+ alternative("", "stac", X86_FEATURE_LASS);
+}
+
static __always_inline unsigned long smap_save(void)
{
unsigned long flags;
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index ea1d984166cd..3d2bcb7682eb 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -2447,16 +2447,26 @@ void __init_or_module text_poke_early(void *addr, const void *opcode,
__ro_after_init struct mm_struct *text_poke_mm;
__ro_after_init unsigned long text_poke_mm_addr;

+/*
+ * Text poking creates and uses a mapping in the lower half of the
+ * address space. Relax LASS enforcement when accessing the poking
+ * address.
+ */
+
static void text_poke_memcpy(void *dst, const void *src, size_t len)
{
- memcpy(dst, src, len);
+ lass_stac();
+ __inline_memcpy(dst, src, len);
+ lass_clac();
}

static void text_poke_memset(void *dst, const void *src, size_t len)
{
int c = *(const int *)src;

- memset(dst, c, len);
+ lass_stac();
+ __inline_memset(dst, c, len);
+ lass_clac();
}

typedef void text_poke_f(void *dst, const void *src, size_t len);
--
2.47.2