[PATCH] x86: disable non-instrumented version of copy_page when KMSAN is enabled

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

I found that commit afb2d666d025 ("zsmalloc: use copy_page for full page
copy") caused KMSAN warning. We need to fallback to instrumented version
when KMSAN is enabled.

  [   50.030627][ T2974] BUG: KMSAN: use-after-free in obj_malloc+0x6cc/0x7b0

  [   50.165956][ T2974] Uninit was stored to memory at:
  [   50.170819][ T2974]  obj_malloc+0x70a/0x7b0

  [   50.328931][ T2974] Uninit was created at:
  [   50.341845][ T2974]  free_unref_page_prepare+0x130/0xfc0

Since the destination page likely already holds previously written value
(i.e. KMSAN considers that the page was already initialized), whether to
globally enforce an instrumented version when KMSAN is enabled might be
questionable.

But since finding why KMSAN considers that value is not initialized is
difficult (developers tend to choose optimized version without knowing
KMSAN), let's choose human-friendly version. That is, since
arch/x86/include/asm/page_32.h implements copy_page() using memcpy(), let
arch/x86/include/asm/page_64.h implement copy_page() using memcpy() when
KMSAN is enabled.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 arch/x86/include/asm/page_64.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
index cc6b8e087192..f13bba3a9dab 100644
--- a/arch/x86/include/asm/page_64.h
+++ b/arch/x86/include/asm/page_64.h
@@ -58,7 +58,16 @@ static inline void clear_page(void *page)
 			   : "cc", "memory", "rax", "rcx");
 }
 
+#ifdef CONFIG_KMSAN
+/* Use of non-instrumented assembly version confuses KMSAN. */
+void *memcpy(void *to, const void *from, __kernel_size_t len);
+static inline void copy_page(void *to, void *from)
+{
+	memcpy(to, from, PAGE_SIZE);
+}
+#else
 void copy_page(void *to, void *from);
+#endif
 
 #ifdef CONFIG_X86_5LEVEL
 /*
-- 
2.34.1