Any time after inode allocation, destroy_inode can be called.  The
hugetlbfs inode contains a shared_policy structure, and
mpol_free_shared_policy is unconditionally called as part of
hugetlbfs_destroy_inode.  Initialize the policy as part of inode
allocation so that any quick (error path) calls to destroy_inode will be
handed an initialized policy.

syzkaller fuzzer found this bug, that resulted in the following:

    BUG: KASAN: user-memory-access in atomic_inc
    include/asm-generic/atomic-instrumented.h:87 [inline] at addr
    000000131730bd7a
    BUG: KASAN: user-memory-access in __lock_acquire+0x21a/0x3a80
    kernel/locking/lockdep.c:3239 at addr 000000131730bd7a
    Write of size 4 by task syz-executor6/14086
    CPU: 3 PID: 14086 Comm: syz-executor6 Not tainted 4.11.0-rc3+ #364
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
    Call Trace:
     atomic_inc include/asm-generic/atomic-instrumented.h:87 [inline]
     __lock_acquire+0x21a/0x3a80 kernel/locking/lockdep.c:3239
     lock_acquire+0x1ee/0x590 kernel/locking/lockdep.c:3762
     __raw_write_lock include/linux/rwlock_api_smp.h:210 [inline]
     _raw_write_lock+0x33/0x50 kernel/locking/spinlock.c:295
     mpol_free_shared_policy+0x43/0xb0 mm/mempolicy.c:2536
     hugetlbfs_destroy_inode+0xca/0x120 fs/hugetlbfs/inode.c:952
     alloc_inode+0x10d/0x180 fs/inode.c:216
     new_inode_pseudo+0x69/0x190 fs/inode.c:889
     new_inode+0x1c/0x40 fs/inode.c:918
     hugetlbfs_get_inode+0x40/0x420 fs/hugetlbfs/inode.c:734
     hugetlb_file_setup+0x329/0x9f0 fs/hugetlbfs/inode.c:1282
     newseg+0x422/0xd30 ipc/shm.c:575
     ipcget_new ipc/util.c:285 [inline]
     ipcget+0x21e/0x580 ipc/util.c:639
     SYSC_shmget ipc/shm.c:673 [inline]
     SyS_shmget+0x158/0x230 ipc/shm.c:657
     entry_SYSCALL_64_fastpath+0x1f/0xc2

Analysis provided by Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

Link: http://lkml.kernel.org/r/1490477850-7944-1-git-send-email-mike.kravetz@oracle.comSigned-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>