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SubjectRe: [PATCH] ioctl_tty.2: add TIOCGPTPEER documentation
> A couple of things to note on the bigger picture.
> The glibc library on all distributions has been changed to not have a
> setuid binary pt_chown, that uses ptsname. This was the primary fix
> for the security issue.
> The behavior of opening /dev/ptmx has been changed to perform a path
> lookup relative to the location of /dev/ptmx of ./pts/ptmx and open
> it it is a devpts filesystem and to fail otherwise. This further
> makes it hard to confuse userspace this way as /dev/ptmx always
> corresponds to /dev/pts/ptmx. Even in chroots and in other mount
> namespaces.

I have a feeling that there might be a way to trick glibc if you use
FUSE, but I haven't actually tried to create a PoC for it. Fair point

> That makes TIOCGPTPEER a very nice addition, but not something people
> have to scramble to use to ensure their system is secure. As a hostile
> environment now has to work very hard to confuse the existing mechanisms.

There are usecases where you simply need TIOCGPTPEER, and no other
userspace alternative will do, but maybe if we modified the paragraph to
read (as suggested):

Security-conscious programs interacting with namespaces may
wish to use this operation rather than open(2) with the
pathname returned by ptsname(3).

This would clarify that there are usecases where you need this
particular feature, without saying causing people to panic over
inaccurate claims of glibc being broken. Does that sound better?

Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH

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