ARC: Incorrect mm reference used in vmalloc fault handler

A vmalloc fault needs to sync up PGD/PTE entry from init_mm to current
task's "active_mm".  ARC vmalloc fault handler however was using mm.

A vmalloc fault for non user task context (actually pre-userland, from
init thread's open for /dev/console) caused the handler to deref NULL mm
(for mm->pgd)

The reasons it worked so far is amazing:

1. By default (!SMP), vmalloc fault handler uses a cached value of PGD.
   In SMP that MMU register is repurposed hence need for mm pointer deref.

2. In pre-3.12 SMP kernel, the problem triggering vmalloc didn't exist in
   pre-userland code path - it was introduced with commit 20bafb3d23d108bc
   "n_tty: Move buffers into n_tty_data"

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: Noam Camus <noamc@ezchip.com>
Cc: stable@vger.kernel.org    #3.10 and 3.11
Cc: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
index d63f3de0cd5b..0c14d8a52683 100644
--- a/arch/arc/mm/fault.c
+++ b/arch/arc/mm/fault.c
@@ -17,7 +17,7 @@
 #include <asm/pgalloc.h>
 #include <asm/mmu.h>
 
-static int handle_vmalloc_fault(struct mm_struct *mm, unsigned long address)
+static int handle_vmalloc_fault(unsigned long address)
 {
         /*
          * Synchronize this task's top level page-table
@@ -27,7 +27,7 @@ static int handle_vmalloc_fault(struct mm_struct *mm, unsigned long address)
         pud_t *pud, *pud_k;
         pmd_t *pmd, *pmd_k;
 
-        pgd = pgd_offset_fast(mm, address);
+        pgd = pgd_offset_fast(current->active_mm, address);
         pgd_k = pgd_offset_k(address);
 
         if (!pgd_present(*pgd_k))
@@ -72,7 +72,7 @@ void do_page_fault(struct pt_regs *regs, unsigned long address)
          * nothing more.
          */
         if (address >= VMALLOC_START && address <= VMALLOC_END) {
-                ret = handle_vmalloc_fault(mm, address);
+                ret = handle_vmalloc_fault(address);
                 if (unlikely(ret))
                         goto bad_area_nosemaphore;
                 else