On Sun, Mar 01, 2020 at 09:00:22PM +0100, Jann Horn wrote: > On Sun, Mar 1, 2020 at 7:52 PM Christian Brauner > <christian.brauner@xxxxxxxxxx> wrote: > > On Sun, Mar 01, 2020 at 07:21:03PM +0100, Jann Horn wrote: > > > On Sun, Mar 1, 2020 at 12:27 PM Bernd Edlinger > > > <bernd.edlinger@xxxxxxxxxx> wrote: > > > > The proposed solution is to have a second mutex that is > > > > used in mm_access, so it is allowed to continue while the > > > > dying threads are not yet terminated. > > > > > > Just for context: When I proposed something similar back in 2016, > > > https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@xxxxxxxxxx/ > > > was the resulting discussion thread. At least back then, I looked > > > through the various existing users of cred_guard_mutex, and the only > > > places that couldn't be converted to the new second mutex were > > > PTRACE_ATTACH and SECCOMP_FILTER_FLAG_TSYNC. > > > > > > > > > The ideal solution would IMO be something like this: Decide what the > > > new task's credentials should be *before* reaching de_thread(), > > > install them into a second cred* on the task (together with the new > > > dumpability), drop the cred_guard_mutex, and let ptrace_may_access() > > > check against both. After that, some further restructuring might even > > > > Hm, so essentially a private ptrace_access_cred member in task_struct? > > And a second dumpability field, because that changes together with the > creds during execve. (Btw, currently the dumpability is in the > mm_struct, but that's kinda wrong. The mm_struct is removed from a > task on exit while access checks can still be performed against it, and > currently ptrace_may_access() just lets the access go through in that > case, which weakens the protection offered by PR_SET_DUMPABLE when > used for security purposes. I think it ought to be moved over into the > task_struct.) > > > That would presumably also involve altering various LSM hooks to look at > > ptrace_access_cred. > > When I tried to implement this in the past, I changed the LSM hook to > take the target task's cred* as an argument, and then called the LSM > hook twice from ptrace_may_access(). IIRC having the target task's > creds as an argument works for almost all the LSMs, with the exception > of Yama, which doesn't really care about the target task's creds, so > you have to pass in both the task_struct* and the cred*. It seems we should try PoCing this. Christian
