[PATCH v7 2/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

On 5/29/17 6:26 PM, Alan Cox wrote:

On Mon, 29 May 2017 17:38:00 -0400
Matt Brown [off-list ref] wrote:

quoted

This introduces the tiocsti_restrict sysctl, whose default is controlled
via CONFIG_SECURITY_TIOCSTI_RESTRICT. When activated, this control
restricts all TIOCSTI ioctl calls from non CAP_SYS_ADMIN users.

Which is really quite pointless as I keep pointing out and you keep
reposting this nonsense.

quoted

This patch depends on patch 1/2

This patch was inspired from GRKERNSEC_HARDEN_TTY.

This patch would have prevented
https://bugzilla.redhat.com/show_bug.cgi?id=1411256 under the following
conditions:
* non-privileged container
* container run inside new user namespace

And assuming no other ioctl could be used in an attack. Only there are
rather a lot of ways an app with access to a tty can cause mischief if
it's the same controlling tty as the higher privileged context that
launched it.

Can you give me an example of another ioctl that you can abuse to practically
gain code execution in the privileged context? Saying that the child process
could "cause mischief" is a bit vague.

Properly written code allocates a new pty/tty pair for the lower
privileged session. If the code doesn't do that then your change merely
modifies the degree of mayhem it can cause. If it does it right then your
patch is pointless.

quoted

Possible effects on userland:

There could be a few user programs that would be effected by this
change.

In other words, it's yet another weird config option that breaks stuff.

It doesn't break anything because it is default n. People that enable this
option will understand they are disabling the tiocsti ioctl for non privileged
processes.

NAK v7.

Rather than a NAK, could you explain how you would solve this problem in a way
that we can protect userspace from shooting itself in the foot.

See CVE lookup: https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=tiocsti

Matt
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