Re: Hard-coding PTY device node numbers in userspace

From: Florian Weimer
Date: Sun Feb 19 2017 - 10:35:41 EST

* Greg KH:

> On Fri, Feb 17, 2017 at 12:02:52PM +0100, Florian Weimer wrote:
>> We want to reject PTY devices from other namespaces as valid input to
>> the ttyname and ttyname_r functions, while still providing a hint to
>> callers that the device is, in fact, a PTY. Christian Brauner wrote a
>> glibc patch for this:
>> <>
>> It hard-codes the major PTY device number range. Is this feasible?
>> Is it part of the stable userspace ABI for the TTY subsystem?
> What major numbers are you using in the patch '2' and '3'?

I think there is just one patch, and the check looks like this:

static inline int
is_pty (struct stat64 *sb)
int m = major (sb->st_rdev);
return (136 <= m && m <= 143);

> And yes,
> major numbers are static and you should be fine to rely on them. But
> can't you test that the device is a pty to verify it?

It's not entirely clear what exactly a PTY descriptor should be for
ttyname. Going forward, we only want to treat descriptors for PTY
devices which can be accessed using /dev/pts paths in the current
namespace as PTYs. Christian's patch adds a separate error code for
the case where the descriptor is a PTY, but it comes from a different

I'm concerned that some software out there assumes that if standard
input is a PTY according to ttyname, it is safe to chown it. There
have been security issues related to that a long time ago on some UNIX
systems, and I want us to be conservative here.