Al Viro <viro@xxxxxxxxxxxxxxxxxx> writes:
> On Sat, Sep 22, 2012 at 08:50:44PM -0700, Eric W. Biederman wrote:
>
>> +struct inode *devpts_redirect(struct file *filp)
>> +{
>> + struct inode *inode;
>> + struct file *filp2;
>> +
>> + /* Is the inode already a devpts inode? */
>> + inode = filp->f_dentry->d_inode;
>> + if (filp->f_dentry->d_sb->s_magic == DEVPTS_SUPER_MAGIC)
>> + goto out;
>> +
>> + /* Is f_dentry->d_parent usable? */
>> + inode = ERR_PTR(-ENODEV);
>> + if (filp->f_vfsmnt->mnt_root == filp->f_dentry)
>> + goto out;
>> +
>> + /* Is there a devpts inode we can use instead? */
>> +
>> + filp2 = file_open_root(filp->f_dentry->d_parent, filp->f_vfsmnt,
>> + "pts/ptmx", O_PATH);
>> + if (!IS_ERR(filp2)) {
>> + if (filp2->f_dentry->d_sb->s_magic == DEVPTS_SUPER_MAGIC) {
>> + struct path old;
>> + old = filp->f_path;
>> + filp->f_path = filp2->f_path;
>> + inode = filp->f_dentry->d_inode;
>> + path_get(&filp->f_path);
>> + path_put(&old);
>
> You are welcome to supply an analysis of the reasons why ->open() pulling
> such tricks will not break all kinds of code in VFS.
>> + }
>> + fput(filp2);
>
> ... starting with "what happens when some joker binds /dev/ptmx on
> /dev/pts/ptmx"
The test:
>> + if (filp->f_vfsmnt->mnt_root == filp->f_dentry)
kicks in and no redirection is performed.
> Or does mknod /tmp/ptmx c 5 2; mkdir /tmp/pts; ln /tmp/ptmx /tmp/pts/ptmx,
> for that matter...
The test:
>> + if (filp2->f_dentry->d_sb->s_magic == DEVPTS_SUPER_MAGIC) {n
fails and no redirection is performed.
> NAK. This violates asserts made by VFS (namely, that ->f_path is not
> changed since dentry_open() has set it and until __fput() rips the thing
> out) *and* by your own code (attack mentioned above, just from looking
> at it for a minute). Far too brittle...
This code seems much more robust than your quick analysis.
But if the constraint that the path in struct file must not be
changed between dentry_open and __fput that is doable to it just
needs a little more reorganizing of the data structures. It is
definitely not a fundamental limitation.
Eric
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