xfs: fix NULL pointer dereference in xfs_getbmap()

Reproducer:
 1. fallocate -l 100M image
 2. mkfs.xfs -f image
 3. mount image /mnt
 4. setxattr("/mnt", "trusted.overlay.upper", NULL, 0, XATTR_CREATE)
 5. char arg[32] = "\x01\xff\x00\x00\x00\x00\x03\x00\x00\x00\x00\x00\x00"
                   "\x00\x00\x00\x00\x00\x08\x00\x00\x00\xc6\x2a\xf7";
    fd = open("/mnt", O_RDONLY|O_DIRECTORY);
    ioctl(fd, _IOC(_IOC_READ|_IOC_WRITE, 0x58, 0x2c, 0x20), arg);

NULL pointer dereference will occur when race happens between xfs_getbmap()
and xfs_bmap_set_attrforkoff():

         ioctl               |       setxattr
 ----------------------------|---------------------------
 xfs_getbmap                 |
   xfs_ifork_ptr             |
     xfs_inode_has_attr_fork |
       ip->i_forkoff == 0    |
     return NULL             |
   ifp == NULL               |
                             | xfs_bmap_set_attrforkoff
                             |   ip->i_forkoff > 0
   xfs_inode_has_attr_fork   |
     ip->i_forkoff > 0       |
   ifp == NULL               |
   ifp->if_format            |

Fix this by locking i_lock before xfs_ifork_ptr().

Fixes: abbf9e8a45 ("xfs: rewrite getbmap using the xfs_iext_* helpers")
Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
Signed-off-by: Guo Xuenan <guoxuenan@huawei.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: added fixes tag]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

fs/xfs/xfs_bmap_util.c

@@ -439,29 +439,28 @@ xfs_getbmap(
 		whichfork = XFS_COW_FORK;
 	else
 		whichfork = XFS_DATA_FORK;
-	ifp = xfs_ifork_ptr(ip, whichfork);
 
 	xfs_ilock(ip, XFS_IOLOCK_SHARED);
 	switch (whichfork) {
 	case XFS_ATTR_FORK:
+		lock = xfs_ilock_attr_map_shared(ip);
 		if (!xfs_inode_has_attr_fork(ip))
-			goto out_unlock_iolock;
+			goto out_unlock_ilock;
 
 		max_len = 1LL << 32;
-		lock = xfs_ilock_attr_map_shared(ip);
 		break;
 	case XFS_COW_FORK:
+		lock = XFS_ILOCK_SHARED;
+		xfs_ilock(ip, lock);
+
 		/* No CoW fork? Just return */
-		if (!ifp)
-			goto out_unlock_iolock;
+		if (!xfs_ifork_ptr(ip, whichfork))
+			goto out_unlock_ilock;
 
 		if (xfs_get_cowextsz_hint(ip))
 			max_len = mp->m_super->s_maxbytes;
 		else
 			max_len = XFS_ISIZE(ip);
-
-		lock = XFS_ILOCK_SHARED;
-		xfs_ilock(ip, lock);
 		break;
 	case XFS_DATA_FORK:
 		if (!(iflags & BMV_IF_DELALLOC) &&
@@ -491,6 +490,8 @@ xfs_getbmap(
 		break;
 	}
 
+	ifp = xfs_ifork_ptr(ip, whichfork);
+
 	switch (ifp->if_format) {
 	case XFS_DINODE_FMT_EXTENTS:
 	case XFS_DINODE_FMT_BTREE: