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SubjectRe: Q: check_unsafe_exec() races (Was: [PATCH 2/4] fix setuid sometimes doesn't)
On 03/29, Al Viro wrote:
>
> > In principle, we can mark these threads as "-EAGAIN on such clone()" and
> > clean that on exec failure.

We can't. We can miss the new subthread if we race with clone(CLONE_THREAD).
Unless we add the additional locking, of course.

We can set current->signal->flags |= SIGNAL_DO_NOT_CLONE_FS. But this is
really nasty. For examlpe, what if this flag is already set when
check_unsafe_exec() takes ->siglock ? We should return -ESOMETHING, not
good. Or schedule_timeout_uninterruptible(1) until it is cleared?
This also means copy_process()->copy_fs() path should take ->siglock too,
otherwise we we don't have a barrier.

> ... or just do that to fs_struct. After finding that there's no outside
> users. Commenst?

This is even worse. Not only we race with our sub-threads, we race
with CLONE_FS processes.

We can't mark fs_struct after finding that there's no outside users
lockless. Because we can't know whether this is "after" or not, we
can't trust "atomic_read(fs->count) <= n_fs".

Unless we re-use fs_struct->lock. In this case copy_fs() should take
it too. But again, ->fs can be already marked when we enter
check_unsafe_exec().


And btw check_unsafe_exec() seem to have another hole. Another thread
(which shares ->fs with us) can do exit_fs() right before we read
fs->count. Since this thread was already accounted in n_fs, we can
miss the fact we share ->fs with another process.


Perhaps I missed something...


Not that I like this idea (actually I hate), but perhaps we can change
the meaning of LSM_UNSAFE_SHARE,

selinux_bprm_set_creds:

if (new_tsec->sid != old_tsec->sid) {
...
if (avc_has_perm(...))
bprm->unsafe |= LSM_UNSAFE_SHARE;
}

Then we modify de_thread(). It sends SIGKILL to all subthreads, this
means that another thread can't clone() after we drop ->siglock. So we
can add this code to the ->siglock protected section

if (unlikely(bprm->unsafe & LSM_UNSAFE_SHARE)) {
if (fs_struct_is_shared())
return -EPERM;
}
...
zap_other_threads();

Oh, ugly.

I'd better hope I missed something ;)

Oleg.



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