On Wed, Mar 11, 2020 at 7:12 AM Bernd Edlinger bernd.edlinger@hotmail.de 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@google.com wrote:
On Tue, Mar 10, 2020 at 8:29 PM Eric W. Biederman ebiederm@xmission.com wrote:
Jann Horn jannh@google.com writes:
On Tue, Mar 10, 2020 at 7:54 PM Eric W. Biederman ebiederm@xmission.com 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.