Re: [PATCH] pidfd: Stop taking cred_guard_mutex

[Christian Brauner]

On Tue, Mar 10, 2020 at 03:57:35PM -0500, Eric W. Biederman wrote:
> Jann Horn <jannh@xxxxxxxxxx> writes:
> 
> > 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...
> 
> Having been through all of the users nope.
> 
> Maybe someone tried to repurpose for that.  I haven't traced through
> when it went the it was renamed from cred_exec_mutex to
> cred_guard_mutex.
> 
> The original purpose was to make make exec and ptrace deadlock.  But it
> was seen as being there to allow safely calculating the new credentials
> before the point of now return.  Because if a process is ptraced or not
> affects the new credential calculations.  Unfortunately offering that
> guarantee fundamentally leads to deadlock.
> 
> So ptrace_attach and seccomp use the cred_guard_mutex to guarantee
> a deadlock.
> 
> The common use is to take cred_guard_mutex to guard the window when
> credentials and process details are out of sync in exec.  But there
> is at least do_io_accounting that seems to have the same justification
> for holding __pidfd_fget.
> 
> With effort I suspect we can replace exec_change_mutex with task_lock.
> When we are guaranteed to be single threaded placing exec_change_mutex
> in signal_struct doesn't really help us (except maybe in some races?).
> 
> The deep problem is no one really understands cred_guard_mutex so it is
> a mess.  Code with poorly defined semantics is always wrong somewhere

This is a good point. When discussing patches sensitive to credential
changes cred_guard_mutex was always introduced as having the purpose to
guard against concurrent credential changes. And I'm pretty sure that
that's how most people have been using it for quite a long time. I mean,
it's at least the case for seccomp and proc and probably quite a few
more. So the problem seems to me that it has clear _intended_ semantics
that runs into issues in all sorts of cases. So if cred_guard_mutex is
not that then we seem to need to provide something that serves it's
intended purpose.