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.
