Re: [kernel-hardening] Re: [PATCH 1/2] sysctl: expand use of proc_dointvec_minmax_sysadmin

From: Jann Horn
Date: Sat Jan 23 2016 - 17:25:37 EST

On Fri, Jan 22, 2016 at 09:10:07PM -0600, Eric W. Biederman wrote:
> Kees Cook <keescook@xxxxxxxxxxxx> writes:
> > Several sysctls expect a state where the highest value (in extra2) is
> > locked once set for that boot. Yama does this, and kptr_restrict should
> > be doing it. This extracts Yama's logic and adds it to the existing
> > proc_dointvec_minmax_sysadmin, taking care to avoid the simple boolean
> > states (which do not get locked). Since Yama wants to be checking a
> > different capability, we build wrappers for both cases (CAP_SYS_ADMIN
> > and CAP_SYS_PTRACE).
> Sigh this sysctl appears susceptible to known attacks.
> In my quick skim I believe this sysctl implementation that checks
> capabilities is susceptible to attacks where the already open file
> descriptor is set as stdout on a setuid root application.
> Can we come up with an interface that isn't exploitable by an
> application that will act as a setuid cat?

Adding the struct file * to the parameters of all proc_handler
functions would work, right? (Or just filp->f_cred? That would be
less generic.)

A quick grep says that's just about 160 functions that'll need to
be changed. :/

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