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. Bernd.
