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tun(4): Don't allow open of open or dying devices

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Previously, a pid check was used to prevent open of the tun(4); this works,
but may not make the most sense as we don't prevent the owner process from
opening the tun device multiple times.

The potential race described near tun_pid should not be an issue: if a
tun(4) is to be handed off, its fd has to have been sent via control message
or some other mechanism that duplicates the fd to the receiving process so
that it may set the pid. Otherwise, the pid gets cleared when the original
process closes it and you have no effective handoff mechanism.

Close up another potential issue with handing a tun(4) off by not clobbering
state if the closer isn't the controller anymore. If we want some state to
be cleared, we should do that a little more surgically.

Additionally, nothing prevents a dying tun(4) from being "reopened" in the
middle of tun_destroy as soon as the mutex is unlocked, quickly leading to a
bad time. Return EBUSY if we're marked for destruction, as well, and the
consumer will need to deal with it. The associated character device will be
destroyed in short order.

MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D20033
This commit is contained in:
kevans 2019-04-25 13:46:12 +00:00
parent d61e108233
commit 43aa0eb3b2
1 changed files with 14 additions and 15 deletions
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29
sys/net/if_tun.c
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@ -81,16 +81,10 @@ struct tun_softc {
#define TUN_RWAIT 0x0040
#define TUN_ASYNC 0x0080
#define TUN_IFHEAD 0x0100
#define TUN_DYING 0x0200
#define TUN_READY (TUN_OPEN | TUN_INITED)
/*
* XXXRW: tun_pid is used to exclusively lock /dev/tun. Is this
* actually needed? Can we just return EBUSY if already open?
* Problem is that this involved inherent races when a tun device
* is handed off from one process to another, as opposed to just
* being slightly stale informationally.
*/
pid_t tun_pid; /* owning pid */
struct ifnet *tun_ifp; /* the interface */
struct sigio *tun_sigio; /* information for async I/O */
@ -277,6 +271,7 @@ tun_destroy(struct tun_softc *tp)
struct cdev *dev;
mtx_lock(&tp->tun_mtx);
tp->tun_flags |= TUN_DYING;
if ((tp->tun_flags & TUN_OPEN) != 0)
cv_wait_unlock(&tp->tun_cv, &tp->tun_mtx);
else
@ -473,19 +468,13 @@ tunopen(struct cdev *dev, int flag, int mode, struct thread *td)
tp = dev->si_drv1;
}
/*
* XXXRW: This use of tun_pid is subject to error due to the
* fact that a reference to the tunnel can live beyond the
* death of the process that created it. Can we replace this
* with a simple busy flag?
*/
mtx_lock(&tp->tun_mtx);
if (tp->tun_pid != 0 && tp->tun_pid != td->td_proc->p_pid) {
if ((tp->tun_flags & (TUN_OPEN | TUN_DYING)) != 0) {
mtx_unlock(&tp->tun_mtx);
return (EBUSY);
}
tp->tun_pid = td->td_proc->p_pid;
tp->tun_pid = td->td_proc->p_pid;
tp->tun_flags |= TUN_OPEN;
ifp = TUN2IFP(tp);
if_link_state_change(ifp, LINK_STATE_UP);
@ -509,6 +498,16 @@ tunclose(struct cdev *dev, int foo, int bar, struct thread *td)
ifp = TUN2IFP(tp);
mtx_lock(&tp->tun_mtx);
/*
* Simply close the device if this isn't the controlling process. This
* may happen if, for instance, the tunnel has been handed off to
* another process. The original controller should be able to close it
* without putting us into an inconsistent state.
*/
if (td->td_proc->p_pid != tp->tun_pid) {
mtx_unlock(&tp->tun_mtx);
return (0);
}
/*
* junk all pending output