[PATCH 4.9 42/61] parisc: Fix kernel panic in mem_init()

[Greg Kroah-Hartman]

From: Helge Deller <deller@xxxxxx>

commit bf71bc16e02162388808949b179d59d0b571b965 upstream.

The Debian kernel v5.6 triggers this kernel panic:

 Kernel panic - not syncing: Bad Address (null pointer deref?)
 Bad Address (null pointer deref?): Code=26 (Data memory access rights trap) at addr 0000000000000000
 CPU: 0 PID: 0 Comm: swapper Not tainted 5.6.0-2-parisc64 #1 Debian 5.6.14-1
  IAOQ[0]: mem_init+0xb0/0x150
  IAOQ[1]: mem_init+0xb4/0x150
  RP(r2): start_kernel+0x6c8/0x1190
 Backtrace:
  [<0000000040101ab4>] start_kernel+0x6c8/0x1190
  [<0000000040108574>] start_parisc+0x158/0x1b8

on a HP-PARISC rp3440 machine with this memory layout:
 Memory Ranges:
  0) Start 0x0000000000000000 End 0x000000003fffffff Size   1024 MB
  1) Start 0x0000004040000000 End 0x00000040ffdfffff Size   3070 MB

Fix the crash by avoiding virt_to_page() and similar functions in
mem_init() until the memory zones have been fully set up.

Signed-off-by: Helge Deller <deller@xxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v5.0+
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>


---
 arch/parisc/mm/init.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/parisc/mm/init.c
+++ b/arch/parisc/mm/init.c
@@ -604,7 +604,7 @@ void __init mem_init(void)
 			> BITS_PER_LONG);
 
 	high_memory = __va((max_pfn << PAGE_SHIFT));
-	set_max_mapnr(page_to_pfn(virt_to_page(high_memory - 1)) + 1);
+	set_max_mapnr(max_low_pfn);
 	free_all_bootmem();
 
 #ifdef CONFIG_PA11