Re: [PATCH 5/7] overlay filesystem (negative dentries cause OOPS on NULL inode)

From: Erez Zadok
Date: Fri May 20 2011 - 01:59:08 EST


I continued to test your overlayfs.v9 git repo w/ racer, using two separate empty ext3 filesystems (one for lowerdir and another for upperdir). I got a couple of NULL derefs in different places. Both appear to be due to negative dentries being passed around, where the code using them assumes a positive dentry.

The first is in ovl_getattr(). The realpath.dentry->d_inode can apparently be NULL after calling ovl_path_real, and when that's passed to vfs_getattr, the NULL inode is deref'ed in security_inode_getattr (calling IS_PRIVATE on the inode). I verified it by sticking the BUG_ON(!inode), seeing it triggered, and then replaced it with "if (!inode) return -ENOENT". After that, racer didn't trigger this bug, but I don't know if returning ENOENT is correct there. There may be a more fundamental problem in that the lower dentry in ovl_getattr should never be negative.

The second instance of a similar problem is in ovl_follow_link. Again, here realinode can be NULL, causing the WARN_ON(!realinode->…) to oops. I verified this by placing a BUG_ON, triggering it, and then adding some code to return NULL if the realinode is NULL. As with the first instance, I don't know if this is correct either. But I include a simple patch below.

With these two fixes, the error-path bug in ovl_permission, and the fs/namei.c:1362 bug, racer ran for 60 minutes without triggering an OOPS.


diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 3c15d54..6c70f57 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -42,6 +42,8 @@ static int ovl_getattr(struct vfsmount *mnt, struct dentry *dentry,
struct path realpath;

ovl_path_real(dentry, &realpath);
+ if (!realpath.dentry->d_inode)
+ return -ENOENT;
return vfs_getattr(realpath.mnt, realpath.dentry, stat);

@@ -127,6 +130,9 @@ static void *ovl_follow_link(struct dentry *dentry, struct nameidata *nd)

realdentry = ovl_dentry_real(dentry);
realinode = realdentry->d_inode;
+ if (!realinode)
+ return NULL;

if (WARN_ON(!realinode->i_op->follow_link))
return ERR_PTR(-EPERM);

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