[PATCH v4 2/2] tty: add TIOCGPTPEER ioctl

[Aleksa Sarai]

>> When opening the slave end of a PTY, it is not possible for userspace to
>> safely ensure that /dev/pts/$num is actually a slave (in cases where the
>> mount namespace in which devpts was mounted is controlled by an
>> untrusted process). In addition, there are several unresolvable
>> race conditions if userspace were to attempt to detect attacks through
>> stat(2) and other similar methods [in addition it is not clear how
>> userspace could detect attacks involving FUSE].
>>
>> Resolve this by providing an interface for userpace to safely open the
>> "peer" end of a PTY file descriptor by using the dentry cached by
>> devpts. Since it is not possible to have an open master PTY without
>> having its slave exposed in /dev/pts this interface is safe. This
>> interface currently does not provide a way to get the master pty (since
>> it is not clear whether such an interface is safe or even useful).
>>
>> Cc: Christian Brauner <christian.brauner at ubuntu.com>
>> Cc: Valentin Rothberg <vrothberg at suse.com>
>> Signed-off-by: Aleksa Sarai <asarai at suse.de>
> 
> Is this going to be documented anywhere?  Is there a man page update
> that also goes along with this?

I will add one, I didn't know where the man-pages project is hosted / 
where patches get pushed? What is the ML?

> What userspace program wants to use this?

LXC (Christian is on Cc) will use this, runC will most likely use it, 
pending on some design discussions (as well as some future container 
runtimes I'm planning on working on). Effectively any container runtime 
that wants to safely create terminals and spawn containers inside an 
existing container's namespaces will likely want to use this.

[ As an aside, I /would/ argue this is a security fix (it fixes an 
interface problem that made doing certain operations securely possible) 
but I didn't want to Cc stable@ because it's a feature and not a strict 
bugfix. ]

-- 
Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/