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...
