[PATCH] ARC: Incorrect mm reference used in vmalloc fault handler

From: Vineet Gupta
Date: Sat Nov 02 2013 - 08:18:54 EST

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@xxxxxxxxxxxx>
Cc: Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx>
Cc: Noam Camus <noamc@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx #3.10 and 3.11
Cc: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
arch/arc/mm/fault.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
index 81554ded1260..9c69552350c4 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(unsigned long address, struct pt_regs *regs)
* 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;

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