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    Subject[PATCH 3.11 49/94] ARC: Incorrect mm reference used in vmalloc fault handler
    Date
    3.11-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Vineet Gupta <Vineet.Gupta1@synopsys.com>

    commit 9c41f4eeb9d51f3ece20428d35a3ea32cf3b5622 upstream.

    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: Peter Hurley <peter@hurleysoftware.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/arc/mm/fault.c | 6 +++---
    1 file changed, 3 insertions(+), 3 deletions(-)

    --- 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 m
    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))
    @@ -73,7 +73,7 @@ void do_page_fault(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;
    else



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