x86/mm: Do not shuffle CPU entry areas without KASLR

The commit 97e3d26 ("x86/mm: Randomize per-cpu entry area") fixed
an omission of KASLR on CPU entry areas. It doesn't take into account
KASLR switches though, which may result in unintended non-determinism
when a user wants to avoid it (e.g. debugging, benchmarking).

Generate only a single combination of CPU entry areas offsets -- the
linear array that existed prior randomization when KASLR is turned off.

Since we have 3f148f3 ("x86/kasan: Map shadow for percpu pages on
demand") and followups, we can use the more relaxed guard
kasrl_enabled() (in contrast to kaslr_memory_enabled()).

Fixes: 97e3d26 ("x86/mm: Randomize per-cpu entry area")
Signed-off-by: Michal Koutný <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/all/20230306193144.24605-1-mkoutny%40suse.com

arch/x86/mm/cpu_entry_area.c

@@ -10,6 +10,7 @@
 #include <asm/fixmap.h>
 #include <asm/desc.h>
 #include <asm/kasan.h>
+#include <asm/setup.h>
 
 static DEFINE_PER_CPU_PAGE_ALIGNED(struct entry_stack_page, entry_stack_storage);
 
@@ -29,6 +30,12 @@ static __init void init_cea_offsets(void)
 	unsigned int max_cea;
 	unsigned int i, j;
 
+	if (!kaslr_enabled()) {
+		for_each_possible_cpu(i)
+			per_cpu(_cea_offset, i) = i;
+		return;
+	}
+
 	max_cea = (CPU_ENTRY_AREA_MAP_SIZE - PAGE_SIZE) / CPU_ENTRY_AREA_SIZE;
 
 	/* O(sodding terrible) */