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

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Date:	Fri, 20 May 2011 01:58:57 -0400
From:	Erez Zadok <ezk@....cs.sunysb.edu>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	"viro@...IV.linux.org.uk" <viro@...IV.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"apw@...onical.com" <apw@...onical.com>,
	"nbd@...nwrt.org" <nbd@...nwrt.org>,
	"neilb@...e.de" <neilb@...e.de>,
	"hramrach@...trum.cz" <hramrach@...trum.cz>,
	"jordipujolp@...il.com" <jordipujolp@...il.com>,
	"mszeredi@...e.cz" <mszeredi@...e.cz>
Subject: Re: [PATCH 5/7] overlay filesystem (negative dentries cause OOPS on NULL inode)

Miklos,

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.

Cheers,
Erez.

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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