Re: [PATCH] pidfd: Stop taking cred_guard_mutex

[Jann Horn]

On Wed, Mar 11, 2020 at 7:12 AM Bernd Edlinger
<bernd.edlinger@xxxxxxxxxx> wrote:
> On 3/10/20 9:22 PM, Bernd Edlinger wrote:
> > On 3/10/20 9:10 PM, Jann Horn wrote:
> >> On Tue, Mar 10, 2020 at 9:00 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> >>> On Tue, Mar 10, 2020 at 8:29 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> >>>> Jann Horn <jannh@xxxxxxxxxx> writes:
> >>>>> On Tue, Mar 10, 2020 at 7:54 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> >>>>>> During exec some file descriptors are closed and the files struct is
> >>>>>> unshared.  But all of that can happen at other times and it has the
> >>>>>> same protections during exec as at ordinary times.  So stop taking the
> >>>>>> cred_guard_mutex as it is useless.
> >>>>>>
> >>>>>> Furthermore he cred_guard_mutex is a bad idea because it is deadlock
> >>>>>> prone, as it is held in serveral while waiting possibly indefinitely
> >>>>>> for userspace to do something.
> >> [...]
> >>>>> If you make this change, then if this races with execution of a setuid
> >>>>> program that afterwards e.g. opens a unix domain socket, an attacker
> >>>>> will be able to steal that socket and inject messages into
> >>>>> communication with things like DBus. procfs currently has the same
> >>>>> race, and that still needs to be fixed, but at least procfs doesn't
> >>>>> let you open things like sockets because they don't have a working
> >>>>> ->open handler, and it enforces the normal permission check for
> >>>>> opening files.
> >>>>
> >>>> It isn't only exec that can change credentials.  Do we need a lock for
> >>>> changing credentials?
> >> [...]
> >>>> If we need a lock around credential change let's design and build that.
> >>>> Having a mismatch between what a lock is designed to do, and what
> >>>> people use it for can only result in other bugs as people get confused.
> >>>
> >>> Hmm... what benefits do we get from making it a separate lock? I guess
> >>> it would allow us to make it a per-task lock instead of a
> >>> signal_struct-wide one? That might be helpful...
> >>
> >> But actually, isn't the core purpose of the cred_guard_mutex to guard
> >> against concurrent credential changes anyway? That's what almost
> >> everyone uses it for, and it's in the name...
> >>
> >
> > The main reason d'etre of exec_update_mutex is to get a consitent
> > view of task->mm and task credentials.
> > > The reason why you want the cred_guard_mutex, is that some action
> > is changing the resulting credentials that the execve is about
> > to install, and that is the data flow in the opposite direction.
> >
>
> So in other words, you need the exec_update_mutex when you
> access another thread's credentials and possibly the mmap at the
> same time.

Or the file descriptor table, or register state, ...

> You need no mutex at all when you are just accessing or
> even changing the credentials of the current thread.  (If another
> thread is doing execve, your task will be killed, and wether
> or not the credentials were changed does not matter any more)

Only if the only access checks you care about are those related to mm access.