[PATCH 15/15] overlayfs: make use of ->layers safe in rcu pathwalk

[Al Viro <viro@zeniv.linux.org.uk>]

ovl_permission() accesses ->layers[...].mnt; we can't have ->layers
freed without an RCU delay on fs shutdown.  Fortunately, kern_unmount_array()
used to drop those mounts does include an RCU delay, so freeing is
delayed; unfortunately, the array passed to kern_unmount_array() is
formed by mangling ->layers contents and that happens without any
delays.

Use a separate array instead; local if we have a few layers,
kmalloc'ed if there's a lot of them.  If allocation fails,
fall back to kern_unmount() for individual mounts; it's
not a fast path by any stretch of imagination.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/overlayfs/ovl_entry.h |  1 -
 fs/overlayfs/params.c    | 26 ++++++++++++++++++++------
 2 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index e9539f98e86a..618b63bb7987 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -30,7 +30,6 @@ struct ovl_sb {
 };
 
 struct ovl_layer {
-	/* ovl_free_fs() relies on @mnt being the first member! */
 	struct vfsmount *mnt;
 	/* Trap in ovl inode cache */
 	struct inode *trap;
diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c
index b9355bb6d75a..ab594fd407b4 100644
--- a/fs/overlayfs/params.c
+++ b/fs/overlayfs/params.c
@@ -738,8 +738,15 @@ int ovl_init_fs_context(struct fs_context *fc)
 void ovl_free_fs(struct ovl_fs *ofs)
 {
 	struct vfsmount **mounts;
+	struct vfsmount *m[16];
+	unsigned n = ofs->numlayer;
 	unsigned i;
 
+	if (n > 16)
+		mounts = kmalloc_array(n, sizeof(struct mount *), GFP_KERNEL);
+	else
+		mounts = m;
+
 	iput(ofs->workbasedir_trap);
 	iput(ofs->indexdir_trap);
 	iput(ofs->workdir_trap);
@@ -752,14 +759,21 @@ void ovl_free_fs(struct ovl_fs *ofs)
 	if (ofs->upperdir_locked)
 		ovl_inuse_unlock(ovl_upper_mnt(ofs)->mnt_root);
 
-	/* Hack!  Reuse ofs->layers as a vfsmount array before freeing it */
-	mounts = (struct vfsmount **) ofs->layers;
-	for (i = 0; i < ofs->numlayer; i++) {
+	for (i = 0; i < n; i++) {
 		iput(ofs->layers[i].trap);
-		mounts[i] = ofs->layers[i].mnt;
-		kfree(ofs->layers[i].name);
+		if (unlikely(!mounts))
+			kern_unmount(ofs->layers[i].mnt);
+		else
+			mounts[i] = ofs->layers[i].mnt;
 	}
-	kern_unmount_array(mounts, ofs->numlayer);
+	if (mounts) {
+		kern_unmount_array(mounts, n);
+		if (mounts != m)
+			kfree(mounts);
+	}
+	// by this point we had an RCU delay from kern_unmount{_array,}()
+	for (i = 0; i < n; i++)
+		kfree(ofs->layers[i].name);
 	kfree(ofs->layers);
 	for (i = 0; i < ofs->numfs; i++)
 		free_anon_bdev(ofs->fs[i].pseudo_dev);
-- 
2.39.2