• Dave Chinner's avatar
    fs: don't scan the inode cache before SB_BORN is set · b9659ff3
    Dave Chinner authored
    commit 79f546a6 upstream.
    
    We recently had an oops reported on a 4.14 kernel in
    xfs_reclaim_inodes_count() where sb->s_fs_info pointed to garbage
    and so the m_perag_tree lookup walked into lala land.  It produces
    an oops down this path during the failed mount:
    
      radix_tree_gang_lookup_tag+0xc4/0x130
      xfs_perag_get_tag+0x37/0xf0
      xfs_reclaim_inodes_count+0x32/0x40
      xfs_fs_nr_cached_objects+0x11/0x20
      super_cache_count+0x35/0xc0
      shrink_slab.part.66+0xb1/0x370
      shrink_node+0x7e/0x1a0
      try_to_free_pages+0x199/0x470
      __alloc_pages_slowpath+0x3a1/0xd20
      __alloc_pages_nodemask+0x1c3/0x200
      cache_grow_begin+0x20b/0x2e0
      fallback_alloc+0x160/0x200
      kmem_cache_alloc+0x111/0x4e0
    
    The problem is that the superblock shrinker is running before the
    filesystem structures it depends on have been fully set up. i.e.
    the shrinker is registered in sget(), before ->fill_super() has been
    called, and the shrinker can call into the filesystem before
    fill_super() does it's setup work. Essentially we are exposed to
    both use-after-free and use-before-initialisation bugs here.
    
    To fix this, add a check for the SB_BORN flag in super_cache_count.
    In general, this flag is not set until ->fs_mount() completes
    successfully, so we know that it is set after the filesystem
    setup has completed. This matches the trylock_super() behaviour
    which will not let super_cache_scan() run if SB_BORN is not set, and
    hence will not allow the superblock shrinker from entering the
    filesystem while it is being set up or after it has failed setup
    and is being torn down.
    
    Cc: stable@kernel.org
    Signed-Off-By: 's avatarDave Chinner <dchinner@redhat.com>
    Signed-off-by: 's avatarAl Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    b9659ff3