On 06/25, Kees Cook wrote: > > On Wed, Jun 25, 2014 at 7:21 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > > > > But. Doesn't this change add a new security hole? > > > > Obviously, we should not allow to install a filter and then (say) exec > > a suid binary, that is why we have no_new_privs/LSM_UNSAFE_NO_NEW_PRIVS. > > > > But what if "thread->seccomp.filter = caller->seccomp.filter" races with > > any user of task_no_new_privs() ? Say, suppose this thread has already > > passed check_unsafe_exec/etc and it is going to exec the suid binary? > > Oh, ew. Yeah. It looks like there's a cred lock to be held to combat this? Yes, cred_guard_mutex looks like an obvious choice... Hmm, but somehow initially I thought that the fix won't be simple. Not sure why. Yes, at least this should close the race with suid-exec. And there are no other users. Except apparmor, and I hope you will check it because I simply do not know what it does ;) > I wonder if changes to nnp need to "flushed" during syscall entry > instead of getting updated externally/asynchronously? That way it > won't be out of sync with the seccomp mode/filters. > > Perhaps secure computing needs to check some (maybe seccomp-only) > atomic flags and flip on the "real" nnp if found? Not sure I understand you, could you clarify? But I was also worried that task_no_new_privs(current) is no longer stable inside the syscall paths, perhaps this is what you meant? However I do not see something bad here... And this has nothing to do with the race above. Also. Even ignoring no_new_privs, SECCOMP_FILTER_FLAG_TSYNC is not atomic and we can do nothing with this fact (unless it try to freeze the thread group somehow), perhaps it makes sense to document this somehow. I mean, suppose you want to ensure write-to-file is not possible, so you do seccomp(SECCOMP_FILTER_FLAG_TSYNC, nack_write_to_file_filter). You can't assume that this has effect right after seccomp() returns, this can obviously race with a sub-thread which has already entered sys_write(). Once again, I am not arguing, just I think it makes sense to at least mention the limitations during the discussion. Oleg.
