Re: Hard-coding PTY device node numbers in userspace
From: Greg KH
Date: Tue Feb 21 2017 - 10:04:47 EST
On Sun, Feb 19, 2017 at 04:35:12PM +0100, Florian Weimer wrote:
> * 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:
> >> <https://sourceware.org/ml/libc-alpha/2017-01/msg00531.html>
> >> 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);
Ah, yes, 136-143 are the right ones, sorry, I was looking at the
"legacy" ones in devices.txt.
> > 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.
Yeah, it's tricky. And putting namespaces in the mix makes it messier.