[PATCHv6 03/16] x86/alternatives: Disable LASS when patching kernel alternatives

Kirill A. Shutemov
2025-06-20T13:53:42Z

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 using the stac()/clac()
instructions 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/kernel/alternative.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 9ae80fa904a2..0b9f41ed6af7 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -2447,16 +2447,24 @@ 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;

+/*
+ * poking_init() initializes the text poking address from 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