x86/mm: Fix overlap of i386 CPU_ENTRY_AREA with FIX_BTMAP

Since commit 92a0f81d ("x86/cpu_entry_area: Move it out of the
fixmap"), i386's CPU_ENTRY_AREA has been mapped to the memory area just
below FIXADDR_START. But already immediately before FIXADDR_START is the
FIX_BTMAP area, which means that early_ioremap can collide with the entry
area.

It's especially bad on PAE where FIX_BTMAP_BEGIN gets aligned to exactly
match CPU_ENTRY_AREA_BASE, so the first early_ioremap slot clobbers the
IDT and causes interrupts during early boot to reset the system.

The overlap wasn't a problem before the CPU entry area was introduced,
as the fixmap has classically been preceded by the pkmap or vmalloc
areas, neither of which is used until early_ioremap is out of the
picture.

Relocate CPU_ENTRY_AREA to below FIX_BTMAP, not just below the permanent
fixmap area.

Fixes: commit 92a0f81d ("x86/cpu_entry_area: Move it out of the fixmap")
Signed-off-by: William Grant <william.grant@canonical.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/7041d181-a019-e8b9-4e4e-48215f841e2c@canonical.com

arch/x86/include/asm/fixmap.h

@@ -137,8 +137,10 @@ enum fixed_addresses {
 
 extern void reserve_top_address(unsigned long reserve);
 
-#define FIXADDR_SIZE	(__end_of_permanent_fixed_addresses << PAGE_SHIFT)
-#define FIXADDR_START	(FIXADDR_TOP - FIXADDR_SIZE)
+#define FIXADDR_SIZE		(__end_of_permanent_fixed_addresses << PAGE_SHIFT)
+#define FIXADDR_START		(FIXADDR_TOP - FIXADDR_SIZE)
+#define FIXADDR_TOT_SIZE	(__end_of_fixed_addresses << PAGE_SHIFT)
+#define FIXADDR_TOT_START	(FIXADDR_TOP - FIXADDR_TOT_SIZE)
 
 extern int fixmaps_set;
 

arch/x86/include/asm/pgtable_32_types.h

@@ -44,8 +44,9 @@ extern bool __vmalloc_start_set; /* set once high_memory is set */
  */
 #define CPU_ENTRY_AREA_PAGES	(NR_CPUS * 40)
 
-#define CPU_ENTRY_AREA_BASE				\
-	((FIXADDR_START - PAGE_SIZE * (CPU_ENTRY_AREA_PAGES + 1)) & PMD_MASK)
+#define CPU_ENTRY_AREA_BASE						\
+	((FIXADDR_TOT_START - PAGE_SIZE * (CPU_ENTRY_AREA_PAGES + 1))   \
+	 & PMD_MASK)
 
 #define PKMAP_BASE		\
 	((CPU_ENTRY_AREA_BASE - PAGE_SIZE) & PMD_MASK)