[PATCH v2 03/12] x86/alternatives: Disable LASS when patching kernel alternatives

From: Alexander Shishkin
Date: Tue May 30 2023 - 07:44:23 EST

From: Sohil Mehta <sohil.mehta@xxxxxxxxx>

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

Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
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 f615e0cb6d93..eac6a5406d39 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1526,16 +1526,24 @@ static inline void unuse_temporary_mm(temp_mm_state_t prev_state)
__ro_after_init struct mm_struct *poking_mm;
__ro_after_init unsigned long poking_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);
+ stac();
+ __inline_memcpy(dst, src, len);
+ clac();

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

- memset(dst, c, len);
+ stac();
+ __inline_memset(dst, c, len);
+ clac();

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