Bad things can happen if several concurrent overlay mounts try to
use the same upperdir path and/or workdir path.
Try to get the 'inuse' advisory lock on upper and work dir.
Fail mount if another overlay mount instance or another user
holds the 'inuse' lock.
Note that this provides no protection for concurrent overlay
mount that use overlapping (i.e. descendant) upper dirs or
work dirs.
Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
---
fs/overlayfs/super.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 67 insertions(+), 3 deletions(-)
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 4882ffb..ac9212d 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -165,12 +165,42 @@ static const struct dentry_operations ovl_reval_dentry_operations = {
.d_weak_revalidate = ovl_dentry_weak_revalidate,
};
+/* Get exclusive ownership on upper/work dir among overlay mounts */
+static bool ovl_dir_lock(struct dentry *dentry)
+{
+ struct inode *inode;
+
+ if (!dentry)
+ return false;
+
+ inode = d_inode(dentry);
+ if (!inode || inode_inuse(inode))
+ return false;
+
+ return inode_inuse_trylock(inode);
+}
+
+static void ovl_dir_unlock(struct dentry *dentry)
+{
+ struct inode *inode;
+
+ if (!dentry)
+ return;
+
+ inode = d_inode(dentry);
+ if (inode && inode_inuse(inode))
+ inode_inuse_unlock(inode);
+}
+
static void ovl_put_super(struct super_block *sb)
{
struct ovl_fs *ufs = sb->s_fs_info;
unsigned i;
+ ovl_dir_unlock(ufs->workdir);
dput(ufs->workdir);
+ if (ufs->upper_mnt)
+ ovl_dir_unlock(ufs->upper_mnt->mnt_root);
mntput(ufs->upper_mnt);
for (i = 0; i < ufs->numlower; i++)
mntput(ufs->lower_mnt[i]);
@@ -407,6 +437,14 @@ static struct dentry *ovl_workdir_create(struct vfsmount *mnt,
if (retried)
goto out_dput;
+ /*
+ * We have parent i_mutex, so this test is race free
+ * w.r.t. ovl_dir_lock() below by another overlay mount.
+ */
+ err = -EBUSY;
+ if (inode_inuse(work->d_inode))
+ goto out_dput;
+
retried = true;
ovl_workdir_cleanup(dir, mnt, work, 0);
dput(work);
@@ -446,6 +484,14 @@ static struct dentry *ovl_workdir_create(struct vfsmount *mnt,
inode_unlock(work->d_inode);
if (err)
goto out_dput;
+
+ /*
+ * Protect our work dir from being deleted/renamed and from
+ * being reused by another overlay mount.
+ */
+ err = -EBUSY;
+ if (!ovl_dir_lock(work))
+ goto out_dput;
}
out_unlock:
inode_unlock(dir);
@@ -849,6 +895,16 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
pr_err("overlayfs: failed to clone upperpath\n");
goto out_put_lowerpath;
}
+ /*
+ * Protect our upper dir from being deleted/renamed and from
+ * being reused by another overlay mount.
+ */
+ err = -EBUSY;
+ if (!ovl_dir_lock(upperpath.dentry)) {
+ pr_err("overlayfs: upperdir in-use by another overlay mount?\n");
+ goto out_put_upper_mnt;
+ }
+
/* Don't inherit atime flags */
ufs->upper_mnt->mnt_flags &= ~(MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME);
@@ -857,6 +913,10 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
ufs->workdir = ovl_workdir_create(ufs->upper_mnt, workpath.dentry);
err = PTR_ERR(ufs->workdir);
if (IS_ERR(ufs->workdir)) {
+ if (err == -EBUSY) {
+ pr_err("overlayfs: workdir in-use by another overlay mount?\n");
+ goto out_unlock_upperdir;
+ }
pr_warn("overlayfs: failed to create directory %s/%s (errno: %i); mounting read-only\n",
ufs->config.workdir, OVL_WORKDIR_NAME, -err);
sb->s_flags |= MS_RDONLY;
@@ -874,7 +934,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
err = ovl_check_d_type_supported(&workpath);
if (err < 0)
- goto out_put_workdir;
+ goto out_unlock_workdir;
/*
* We allowed this configuration and don't want to
@@ -910,7 +970,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
err = -ENOMEM;
ufs->lower_mnt = kcalloc(numlower, sizeof(struct vfsmount *), GFP_KERNEL);
if (ufs->lower_mnt == NULL)
- goto out_put_workdir;
+ goto out_unlock_workdir;
for (i = 0; i < numlower; i++) {
struct vfsmount *mnt = clone_private_mount(&stack[i]);
@@ -1002,8 +1062,12 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
for (i = 0; i < ufs->numlower; i++)
mntput(ufs->lower_mnt[i]);
kfree(ufs->lower_mnt);
-out_put_workdir:
+out_unlock_workdir:
+ ovl_dir_unlock(ufs->workdir);
dput(ufs->workdir);
+out_unlock_upperdir:
+ ovl_dir_unlock(upperpath.dentry);
+out_put_upper_mnt:
mntput(ufs->upper_mnt);
out_put_lowerpath:
for (i = 0; i < numlower; i++)
--
2.7.4
--
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