Date: Thu, 18 Dec 2025 03:52:50 +0000 From: Kyle Evans <kevans@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-branches@FreeBSD.org Subject: git: 33a3ccfc1618 - stable/15 - if_tuntap: defer transient destroy_dev() to a taskqueue Message-ID: <69437a92.9dbe.202d4231@gitrepo.freebsd.org>
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The branch stable/15 has been updated by kevans: URL: https://cgit.FreeBSD.org/src/commit/?id=33a3ccfc1618cd7eabd1b38109b81b613b60c19f commit 33a3ccfc1618cd7eabd1b38109b81b613b60c19f Author: Kyle Evans <kevans@FreeBSD.org> AuthorDate: 2025-11-05 00:28:02 +0000 Commit: Kyle Evans <kevans@FreeBSD.org> CommitDate: 2025-12-18 03:51:56 +0000 if_tuntap: defer transient destroy_dev() to a taskqueue We're in the dtor, so we can't destroy it now without deadlocking after recent changes to make destroy_dev() provide a barrier. However, we know there isn't any other dtor to run, so we can go ahead and clean up our state and just prevent a use-after-free if someone races to open the device while we're trying to destroy it. tunopen() now uses the net epoch to protect against softc release by a concurrent tun_destroy(). While we're here, allow a destroy operation to proceed if we caught a signal in cv_wait_sig() but tun_busy dropped to 0 while we were waiting to acquire the lock. This was more of an inherent design flaw, rather than a bug in the below-refed commit. Reviewed by: kib, markj (cherry picked from commit 96c1d8db39dfeea78ea3f27d67649252a39bbf2e) --- sys/net/if_tuntap.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 57 insertions(+), 6 deletions(-) diff --git a/sys/net/if_tuntap.c b/sys/net/if_tuntap.c index 56bb90cce9bc..0dc3a58f6ae6 100644 --- a/sys/net/if_tuntap.c +++ b/sys/net/if_tuntap.c @@ -138,6 +138,7 @@ struct tuntap_softc { #define TUN_READY (TUN_OPEN | TUN_INITED) pid_t tun_pid; /* owning pid */ + struct epoch_context tun_epoch_ctx; struct ifnet *tun_ifp; /* the interface */ struct sigio *tun_sigio; /* async I/O info */ struct tuntap_driver *tun_drv; /* appropriate driver */ @@ -630,6 +631,18 @@ out: CURVNET_RESTORE(); } +static void +tunfree(struct epoch_context *ctx) +{ + struct tuntap_softc *tp; + + tp = __containerof(ctx, struct tuntap_softc, tun_epoch_ctx); + + /* Any remaining resources that would be needed by a concurrent open. */ + mtx_destroy(&tp->tun_mtx); + free(tp, M_TUN); +} + static int tun_destroy(struct tuntap_softc *tp, bool may_intr) { @@ -649,7 +662,7 @@ tun_destroy(struct tuntap_softc *tp, bool may_intr) error = cv_wait_sig(&tp->tun_cv, &tp->tun_mtx); else cv_wait(&tp->tun_cv, &tp->tun_mtx); - if (error != 0) { + if (error != 0 && tp->tun_busy != 0) { tp->tun_flags &= ~TUN_DYING; TUN_UNLOCK(tp); return (error); @@ -663,8 +676,18 @@ tun_destroy(struct tuntap_softc *tp, bool may_intr) TAILQ_REMOVE(&tunhead, tp, tun_list); mtx_unlock(&tunmtx); - /* destroy_dev will take care of any alias. */ - destroy_dev(tp->tun_dev); + /* + * destroy_dev will take care of any alias. For transient tunnels, + * we're being called from close(2) so we can't destroy it ourselves + * without deadlocking, but we already know that we can cleanup + * everything else and just continue to prevent it from being reopened. + */ + if ((tp->tun_flags & TUN_TRANSIENT) != 0) { + atomic_store_ptr(&tp->tun_dev->si_drv1, tp->tun_dev); + destroy_dev_sched(tp->tun_dev); + } else { + destroy_dev(tp->tun_dev); + } seldrain(&tp->tun_rsel); knlist_clear(&tp->tun_rsel.si_note, 0); knlist_destroy(&tp->tun_rsel.si_note); @@ -679,9 +702,8 @@ tun_destroy(struct tuntap_softc *tp, bool may_intr) sx_xunlock(&tun_ioctl_sx); free_unr(tp->tun_drv->unrhdr, TUN2IFP(tp)->if_dunit); if_free(TUN2IFP(tp)); - mtx_destroy(&tp->tun_mtx); cv_destroy(&tp->tun_cv); - free(tp, M_TUN); + NET_EPOCH_CALL(tunfree, &tp->tun_epoch_ctx); CURVNET_RESTORE(); return (0); @@ -742,9 +764,11 @@ tun_uninit(const void *unused __unused) mtx_unlock(&tunmtx); for (i = 0; i < nitems(tuntap_drivers); ++i) { drv = &tuntap_drivers[i]; + destroy_dev_drain(&drv->cdevsw); delete_unrhdr(drv->unrhdr); clone_cleanup(&drv->clones); } + NET_EPOCH_DRAIN_CALLBACKS(); mtx_destroy(&tunmtx); } SYSUNINIT(tun_uninit, SI_SUB_PROTO_IF, SI_ORDER_ANY, tun_uninit, NULL); @@ -1104,19 +1128,43 @@ out: static int tunopen(struct cdev *dev, int flag, int mode, struct thread *td) { + struct epoch_tracker et; struct ifnet *ifp; struct tuntap_softc *tp; + void *p; int error __diagused, tunflags; + /* + * Transient tunnels do deferred destroy of the tun device but want + * to immediately cleanup state, so they clobber si_drv1 to avoid a + * use-after-free in case someone does happen to open it in the interim. + * We avoid using NULL to be able to distinguish from an uninitialized + * cdev. + * + * We use the net epoch here to let a concurrent tun_destroy() schedule + * freeing our tuntap_softc, in case we entered here and loaded si_drv1 + * before it was swapped out. If we managed to load this while it was + * still a softc, then the concurrent tun_destroy() hasn't yet scheduled + * it to be free- that will take place sometime after the epoch we just + * entered, so we can safely use it. + */ + NET_EPOCH_ENTER(et); + p = atomic_load_ptr(&dev->si_drv1); + if (p == dev) { + NET_EPOCH_EXIT(et); + return (ENXIO); + } + tunflags = 0; CURVNET_SET(TD_TO_VNET(td)); error = tuntap_name2info(dev->si_name, NULL, &tunflags); if (error != 0) { CURVNET_RESTORE(); + NET_EPOCH_EXIT(et); return (error); /* Shouldn't happen */ } - tp = dev->si_drv1; + tp = p; KASSERT(tp != NULL, ("si_drv1 should have been initialized at creation")); @@ -1124,14 +1172,17 @@ tunopen(struct cdev *dev, int flag, int mode, struct thread *td) if ((tp->tun_flags & TUN_INITED) == 0) { TUN_UNLOCK(tp); CURVNET_RESTORE(); + NET_EPOCH_EXIT(et); return (ENXIO); } if ((tp->tun_flags & (TUN_OPEN | TUN_DYING)) != 0) { TUN_UNLOCK(tp); CURVNET_RESTORE(); + NET_EPOCH_EXIT(et); return (EBUSY); } + NET_EPOCH_EXIT(et); error = tun_busy_locked(tp); KASSERT(error == 0, ("Must be able to busy an unopen tunnel")); ifp = TUN2IFP(tp);help
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