[SRU Focal 6/6] x86/mm: Do not shuffle CPU entry areas without KASLR

[Cengiz Can]

From: Michal Koutný <mkoutny at suse.com>

The commit 97e3d26b5e5f ("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 3f148f331814 ("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: 97e3d26b5e5f ("x86/mm: Randomize per-cpu entry area")
Signed-off-by: Michal Koutný <mkoutny at suse.com>
Signed-off-by: Dave Hansen <dave.hansen at linux.intel.com>
Cc: stable at vger.kernel.org
Link: https://lore.kernel.org/all/20230306193144.24605-1-mkoutny%40suse.com
(cherry picked from commit a3f547addcaa10df5a226526bc9e2d9a94542344)
CVE-2023-0597
[cengizcan: fixes the fix commit]
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
 arch/x86/mm/cpu_entry_area.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/mm/cpu_entry_area.c b/arch/x86/mm/cpu_entry_area.c
index b6a4219565f1..c385e3183a88 100644
--- a/arch/x86/mm/cpu_entry_area.c
+++ b/arch/x86/mm/cpu_entry_area.c
@@ -11,6 +11,7 @@
 #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);
 
@@ -30,6 +31,12 @@ 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) */
-- 
2.39.2