linux-kernel - Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

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Date:	Fri, 11 Dec 2015 12:50:23 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Greg KH <greg@...ah.com>, Jiri Slaby <jslaby@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Aurelien Jarno <aurelien@...el32.net>,
	Andy Lutomirski <luto@...capital.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security@...nel.org" <security@...nel.org>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	security@...ian.org, Willy Tarreau <w@....eu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

On Fri, Dec 11, 2015 at 11:40 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
>
> +struct inode *devpts_ptmx(struct inode *inode, struct file *filp)
> +{
> +#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
> +       struct path path, old;
> +       struct super_block *sb;
> +       struct dentry *root;
> +
> +       if (inode->i_sb->s_magic == DEVPTS_SUPER_MAGIC)
> +               return inode;
> +
> +       old = filp->f_path;
> +       path = old;
> +       path_get(&path);
> +       if (kern_path_pts(&path)) {
> +               path_put(&path);
> +               return ERR_PTR(-EINVAL);
> +       }

So this is definitely crap.

You can't return an error. You should just return the old inode. If
somebody doesn't have /dev/pts/ mounted there, the legacy /dev/ptmx
should still work, not return ENOENT or whatever.

> +       sb = path.mnt->mnt_sb;
> +       if (sb->s_magic != DEVPTS_SUPER_MAGIC) {
> +               path_put(&path);
> +               return ERR_PTR(-EINVAL);
> +       }

Same deal. Returning an error is wrong.

Of, alternatively, make the caller not consider an error an error, but
fall back to the old behavior in the caller.

> +       /*
> +        * Update filp with the new path so that userspace can use
> +        * fstat to know which instance of devpts is open, and so
> +        * userspace can use readlink /proc/self/fd/NNN to find the
> +        * path to the devpts filesystem for reporting slave inodes.
> +        */

Hmm. I'm not 100% convinced about this. Normally we do *not* allow
f_path and f_inode to change. I guess this file descriptor hasn't been
exposed yet, so it might be ok, but it makes me a bit nervous that
this code violates the basic filp rules..

                 Linus
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