ovl: fix oopses in ovl_fill_super() failure paths

ovl_free_fs() dereferences ofs->workbasedir and ofs->upper_mnt in cases when
those might not have been initialized yet.

Fix the initialization order for these fields.

Reported-by: syzbot+c75f181dc8429d2eb887@syzkaller.appspotmail.com
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc:  <stable@vger.kernel.org> # v4.15
Fixes: 95e6d4177cb7 ("ovl: grab reference to workbasedir early")
Fixes: a9075cdb467d ("ovl: factor out ovl_free_fs() helper")

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 2e0fc93c2c06646ccf5c67c43836f9d4989da501..30adc9d408a0df84455b86811ee468faf439c0c1 100644 (file)
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -982,16 +982,6 @@ static int ovl_get_upper(struct ovl_fs *ofs, struct path *upperpath)
        if (err)
                goto out;
 
-       err = -EBUSY;
-       if (ovl_inuse_trylock(upperpath->dentry)) {
-               ofs->upperdir_locked = true;
-       } else if (ofs->config.index) {
-               pr_err("overlayfs: upperdir is in-use by another mount, mount with '-o index=off' to override exclusive upperdir protection.\n");
-               goto out;
-       } else {
-               pr_warn("overlayfs: upperdir is in-use by another mount, accessing files from both mounts will result in undefined behavior.\n");
-       }
-
        upper_mnt = clone_private_mount(upperpath);
        err = PTR_ERR(upper_mnt);
        if (IS_ERR(upper_mnt)) {
@@ -1002,6 +992,17 @@ static int ovl_get_upper(struct ovl_fs *ofs, struct path *upperpath)
        /* Don't inherit atime flags */
        upper_mnt->mnt_flags &= ~(MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME);
        ofs->upper_mnt = upper_mnt;
+
+       err = -EBUSY;
+       if (ovl_inuse_trylock(ofs->upper_mnt->mnt_root)) {
+               ofs->upperdir_locked = true;
+       } else if (ofs->config.index) {
+               pr_err("overlayfs: upperdir is in-use by another mount, mount with '-o index=off' to override exclusive upperdir protection.\n");
+               goto out;
+       } else {
+               pr_warn("overlayfs: upperdir is in-use by another mount, accessing files from both mounts will result in undefined behavior.\n");
+       }
+
        err = 0;
 out:
        return err;
@@ -1101,8 +1102,10 @@ static int ovl_get_workdir(struct ovl_fs *ofs, struct path *upperpath)
                goto out;
        }
 
+       ofs->workbasedir = dget(workpath.dentry);
+
        err = -EBUSY;
-       if (ovl_inuse_trylock(workpath.dentry)) {
+       if (ovl_inuse_trylock(ofs->workbasedir)) {
                ofs->workdir_locked = true;
        } else if (ofs->config.index) {
                pr_err("overlayfs: workdir is in-use by another mount, mount with '-o index=off' to override exclusive workdir protection.\n");
@@ -1111,7 +1114,6 @@ static int ovl_get_workdir(struct ovl_fs *ofs, struct path *upperpath)
                pr_warn("overlayfs: workdir is in-use by another mount, accessing files from both mounts will result in undefined behavior.\n");
        }
 
-       ofs->workbasedir = dget(workpath.dentry);
        err = ovl_make_workdir(ofs, &workpath);
        if (err)
                goto out;