On Wed, Mar 06, 2019 at 01:14:13PM -0700, Tycho Andersen wrote:
As the comment notes, the return codes for TSYNC and NEW_LISTENER conflict, because they both return positive values, one in the case of success and one in the case of error. So, let's disallow both of these flags together.
While this is technically a userspace break, all the users I know of are still waiting on me to land this feature in libseccomp, so I think it'll be safe. Also, at present my use case doesn't require TSYNC at all, so this isn't a big deal to disallow. If someone wanted to support this, a path forward would be to add a new flag like TSYNC_AND_LISTENER_YES_I_UNDERSTAND_THAT_TSYNC_WILL_JUST_RETURN_EAGAIN, but the use cases are so different I don't see it really happening.
Finally, it's worth noting that this does actually fix a UAF issue: at the end of seccomp_set_mode_filter(), we have:
if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) { if (ret < 0) { listener_f->private_data = NULL; fput(listener_f); put_unused_fd(listener); } else { fd_install(listener, listener_f); ret = listener; } }out_free: seccomp_filter_free(prepared);
But if ret > 0 because TSYNC raced, we'll install the listener fd and then free the filter out from underneath it, causing a UAF when the task closes it or dies. This patch also switches the condition to be simply if (ret), so that if someone does add the flag mentioned above, they won't have to remember to fix this too.
Signed-off-by: Tycho Andersen tycho@tycho.ws Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace") CC: stable@vger.kernel.org # v5.0+
kernel/seccomp.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-)
diff --git a/kernel/seccomp.c b/kernel/seccomp.c index d0d355ded2f4..79bada51091b 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -500,7 +500,10 @@ seccomp_prepare_user_filter(const char __user *user_filter)
- Caller must be holding current->sighand->siglock lock.
- Returns 0 on success, -ve on error.
- Returns 0 on success, -ve on error, or
- in TSYNC mode: the pid of a thread which was either not in the correct
seccomp mode or did not have an ancestral seccomp filter*/
- in NEW_LISTENER mode: the fd of the new listener
static long seccomp_attach_filter(unsigned int flags, struct seccomp_filter *filter) @@ -1256,6 +1259,16 @@ static long seccomp_set_mode_filter(unsigned int flags, if (flags & ~SECCOMP_FILTER_FLAG_MASK) return -EINVAL;
- /*
* In the successful case, NEW_LISTENER returns the new listener fd.* But in the failure case, TSYNC returns the thread that died. If you* combine these two flags, there's no way to tell whether something* succeded or failed. So, let's disallow this combination.*/- if ((flags & SECCOMP_FILTER_FLAG_TSYNC) &&
(flags && SECCOMP_FILTER_FLAG_NEW_LISTENER))return -EINVAL;
May license a manpage entry that this makes it potentially unsafe to use with multiple threads. But I don't see a use-case for this right now so it looks sane to me. :)
(Though one simple question below.)
Acked-by: Christian Brauner christian.brauner@ubuntu.com
- /* Prepare the new filter before holding any locks. */ prepared = seccomp_prepare_user_filter(filter); if (IS_ERR(prepared))
@@ -1302,7 +1315,7 @@ static long seccomp_set_mode_filter(unsigned int flags, mutex_unlock(¤t->signal->cred_guard_mutex); out_put_fd: if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
if (ret < 0) {
if (ret) {
Why that change but keep checking if (ret < 0) further up?
listener_f->private_data = NULL; fput(listener_f); put_unused_fd(listener);-- 2.19.1