fs: limit filesystem stacking depth

Add a simple read-only counter to super_block that indicates how deep this
is in the stack of filesystems.  Previously ecryptfs was the only stackable
filesystem and it explicitly disallowed multiple layers of itself.

Overlayfs, however, can be stacked recursively and also may be stacked
on top of ecryptfs or vice versa.

To limit the kernel stack usage we must limit the depth of the
filesystem stack.  Initially the limit is set to 2.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

1 files changed, 9 insertions, 0 deletions

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 7dcc24e84417..08b704cebfc4 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -677,6 +677,15 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
         }
         ufs->lower_namelen = statfs.f_namelen;
 
+        sb->s_stack_depth = max(upperpath.mnt->mnt_sb->s_stack_depth,
+                                lowerpath.mnt->mnt_sb->s_stack_depth) + 1;
+
+        err = -EINVAL;
+        if (sb->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
+                pr_err("overlayfs: maximum fs stacking depth exceeded\n");
+                goto out_put_workpath;
+        }
+
         ufs->upper_mnt = clone_private_mount(&upperpath);
         err = PTR_ERR(ufs->upper_mnt);
         if (IS_ERR(ufs->upper_mnt)) {