eCryptfs: check readlink result was not an error before using it

The result from readlink is being used to index into the link name
buffer without checking whether it is a valid length. If readlink
returns an error this will fault or cause memory corruption.

Cc: Tyler Hicks <tyhicks@linux.vnet.ibm.com>
Cc: Dustin Kirkland <kirkland@canonical.com>
Cc: ecryptfs-devel@lists.launchpad.net
Signed-off-by: Duane Griffin <duaneg@dghda.com>
Acked-by: Michael Halcrow <mhalcrow@us.ibm.com>
Acked-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/ecryptfs/inode.c b/fs/ecryptfs/inode.c
index 89209f00f9c..5e78fc17988 100644
--- a/fs/ecryptfs/inode.c
+++ b/fs/ecryptfs/inode.c
@@ -673,10 +673,11 @@ static void *ecryptfs_follow_link(struct dentry *dentry, struct nameidata *nd)
         ecryptfs_printk(KERN_DEBUG, "Calling readlink w/ "
                         "dentry->d_name.name = [%s]\n", dentry->d_name.name);
         rc = dentry->d_inode->i_op->readlink(dentry, (char __user *)buf, len);
-        buf[rc] = '\0';
         set_fs(old_fs);
         if (rc < 0)
                 goto out_free;
+        else
+                buf[rc] = '\0';
         rc = 0;
         nd_set_link(nd, buf);
         goto out;