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.
