x86/kaslr: Fix the vaddr_end mess

vaddr_end for KASLR is only documented in the KASLR code itself and is
adjusted depending on config options. So it's not surprising that a change
of the memory layout causes KASLR to have the wrong vaddr_end. This can map
arbitrary stuff into other areas causing hard to understand problems.

Remove the whole ifdef magic and define the start of the cpu_entry_area to
be the end of the KASLR vaddr range.

Add documentation to that effect.

Fixes: 92a0f81 ("x86/cpu_entry_area: Move it out of the fixmap")
Reported-by: Benjamin Gilbert <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Benjamin Gilbert <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: stable <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Garnier <[email protected]>,
Cc: Alexander Kuleshov <[email protected]>
Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1801041320360.1771@nanos

Documentation/x86/x86_64/mm.txt

@@ -12,6 +12,7 @@ ffffea0000000000 - ffffeaffffffffff (=40 bits) virtual memory map (1TB)
 ... unused hole ...
 ffffec0000000000 - fffffbffffffffff (=44 bits) kasan shadow memory (16TB)
 ... unused hole ...
+				    vaddr_end for KASLR
 fffffe0000000000 - fffffe7fffffffff (=39 bits) cpu_entry_area mapping
 fffffe8000000000 - fffffeffffffffff (=39 bits) LDT remap for PTI
 ffffff0000000000 - ffffff7fffffffff (=39 bits) %esp fixup stacks
@@ -37,6 +38,7 @@ ffd4000000000000 - ffd5ffffffffffff (=49 bits) virtual memory map (512TB)
 ... unused hole ...
 ffdf000000000000 - fffffc0000000000 (=53 bits) kasan shadow memory (8PB)
 ... unused hole ...
+				    vaddr_end for KASLR
 fffffe0000000000 - fffffe7fffffffff (=39 bits) cpu_entry_area mapping
 ... unused hole ...
 ffffff0000000000 - ffffff7fffffffff (=39 bits) %esp fixup stacks
@@ -71,3 +73,7 @@ during EFI runtime calls.
 Note that if CONFIG_RANDOMIZE_MEMORY is enabled, the direct mapping of all
 physical memory, vmalloc/ioremap space and virtual memory map are randomized.
 Their order is preserved but their base will be offset early at boot time.
+
+Be very careful vs. KASLR when changing anything here. The KASLR address
+range must not overlap with anything except the KASAN shadow area, which is
+correct as KASAN disables KASLR.

arch/x86/include/asm/pgtable_64_types.h

@@ -75,7 +75,13 @@ typedef struct { pteval_t pte; } pte_t;
 #define PGDIR_SIZE	(_AC(1, UL) << PGDIR_SHIFT)
 #define PGDIR_MASK	(~(PGDIR_SIZE - 1))
 
-/* See Documentation/x86/x86_64/mm.txt for a description of the memory map. */
+/*
+ * See Documentation/x86/x86_64/mm.txt for a description of the memory map.
+ *
+ * Be very careful vs. KASLR when changing anything here. The KASLR address
+ * range must not overlap with anything except the KASAN shadow area, which
+ * is correct as KASAN disables KASLR.
+ */
 #define MAXMEM			_AC(__AC(1, UL) << MAX_PHYSMEM_BITS, UL)
 
 #ifdef CONFIG_X86_5LEVEL

arch/x86/mm/kaslr.c

@@ -34,25 +34,14 @@
 #define TB_SHIFT 40
 
 /*
- * Virtual address start and end range for randomization. The end changes base
- * on configuration to have the highest amount of space for randomization.
- * It increases the possible random position for each randomized region.
+ * Virtual address start and end range for randomization.
  *
- * You need to add an if/def entry if you introduce a new memory region
- * compatible with KASLR. Your entry must be in logical order with memory
- * layout. For example, ESPFIX is before EFI because its virtual address is
- * before. You also need to add a BUILD_BUG_ON() in kernel_randomize_memory() to
- * ensure that this order is correct and won't be changed.
+ * The end address could depend on more configuration options to make the
+ * highest amount of space for randomization available, but that's too hard
+ * to keep straight and caused issues already.
  */
 static const unsigned long vaddr_start = __PAGE_OFFSET_BASE;
-
-#if defined(CONFIG_X86_ESPFIX64)
-static const unsigned long vaddr_end = ESPFIX_BASE_ADDR;
-#elif defined(CONFIG_EFI)
-static const unsigned long vaddr_end = EFI_VA_END;
-#else
-static const unsigned long vaddr_end = __START_KERNEL_map;
-#endif
+static const unsigned long vaddr_end = CPU_ENTRY_AREA_BASE;
 
 /* Default values */
 unsigned long page_offset_base = __PAGE_OFFSET_BASE;
@@ -101,15 +90,12 @@ void __init kernel_randomize_memory(void)
 	unsigned long remain_entropy;
 
 	/*
-	 * All these BUILD_BUG_ON checks ensures the memory layout is
-	 * consistent with the vaddr_start/vaddr_end variables.
+	 * These BUILD_BUG_ON checks ensure the memory layout is consistent
+	 * with the vaddr_start/vaddr_end variables. These checks are very
+	 * limited....
 	 */
 	BUILD_BUG_ON(vaddr_start >= vaddr_end);
-	BUILD_BUG_ON(IS_ENABLED(CONFIG_X86_ESPFIX64) &&
-		     vaddr_end >= EFI_VA_END);
-	BUILD_BUG_ON((IS_ENABLED(CONFIG_X86_ESPFIX64) ||
-		      IS_ENABLED(CONFIG_EFI)) &&
-		     vaddr_end >= __START_KERNEL_map);
+	BUILD_BUG_ON(vaddr_end != CPU_ENTRY_AREA_BASE);
 	BUILD_BUG_ON(vaddr_end > __START_KERNEL_map);
 
 	if (!kaslr_memory_enabled())