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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Roopa Prabhu , Nikolay Aleksandrov , Willem de Bruijn , Simon Horman , Kuniyuki Iwashima , netdev@vger.kernel.org, bridge@lists.linux.dev, syzkaller , yan kang , yue sun Subject: Re: [PATCH v1 net] net: Remove RTNL dance for SIOCBRADDIF and SIOCBRDELIF. Message-ID: References: <20250314010501.75798-1-kuniyu@amazon.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250314010501.75798-1-kuniyu@amazon.com> On Thu, Mar 13, 2025 at 05:59:55PM -0700, Kuniyuki Iwashima wrote: > SIOCBRDELIF is passed to dev_ioctl() first and later forwarded to > br_ioctl_call(), which causes unnecessary RTNL dance and the splat > below [0] under RTNL pressure. > > Let's say Thread A is trying to detach a device from a bridge and > Thread B is trying to remove the bridge. > > In dev_ioctl(), Thread A bumps the bridge device's refcnt by > netdev_hold() and releases RTNL because the following br_ioctl_call() > also re-acquires RTNL. > > In the race window, Thread B could acquire RTNL and try to remove > the bridge device. Then, rtnl_unlock() by Thread B will release RTNL > and wait for netdev_put() by Thread A. > > Thread A, however, must hold RTNL twice after the unlock in dev_ifsioc(), > which may take long under RTNL pressure, resulting in the splat by > Thread B. > > Thread A (SIOCBRDELIF) Thread B (SIOCBRDELBR) > ---------------------- ---------------------- > sock_ioctl sock_ioctl > `- sock_do_ioctl `- br_ioctl_call > `- dev_ioctl `- br_ioctl_stub > |- rtnl_lock | > |- dev_ifsioc ' > ' |- dev = __dev_get_by_name(...) > |- netdev_hold(dev, ...) . > / |- rtnl_unlock ------. | > | |- br_ioctl_call `---> |- rtnl_lock > Race | | `- br_ioctl_stub |- br_del_bridge > Window | | | |- dev = __dev_get_by_name(...) > | | | May take long | `- br_dev_delete(dev, ...) > | | | under RTNL pressure | `- unregister_netdevice_queue(dev, ...) > | | | | `- rtnl_unlock > | | |- rtnl_lock <--| `- netdev_run_todo > | | |- ... | `- netdev_run_todo > | | `- rtnl_unlock | |- __rtnl_unlock > | | | |- netdev_wait_allrefs_any > \ |- rtnl_lock <--------' | > |- netdev_put(dev, ...) <----------------' Wait refcnt decrement > and log splat below Isn't the race window a bit smaller? dev_ifsioc() does netdev_put() before rtnl_lock(). > > To avoid blocking SIOCBRDELBR unnecessarily, let's not call > dev_ioctl() for SIOCBRADDIF and SIOCBRDELIF. > > In the dev_ioctl() path, we do the following: > > 1. Copy struct ifreq by get_user_ifreq in sock_do_ioctl() > 2. Check CAP_NET_ADMIN in dev_ioctl() > 3. Call dev_load() in dev_ioctl() > 4. Fetch the master dev from ifr.ifr_name in dev_ifsioc() > > 3. can be done by request_module() in br_ioctl_call(), so we move > 1., 2., and 4. to br_ioctl_stub(). > > Note that 2. is also checked later in add_del_if(), but it's better > performed before RTNL. > > SIOCBRADDIF and SIOCBRDELIF have been processed in dev_ioctl() since > the pre-git era, and there seems to be no specific reason to process > them there. I couldn't find an explanation as well. Doesn't seem like we have any tests for the IOCTL path, but FWIW I verified that basic operations using brctl still work after this patch. > > [0]: > unregister_netdevice: waiting for wpan3 to become free. Usage count = 2 > ref_tracker: wpan3@ffff8880662d8608 has 1/1 users at > __netdev_tracker_alloc include/linux/netdevice.h:4282 [inline] > netdev_hold include/linux/netdevice.h:4311 [inline] > dev_ifsioc+0xc6a/0x1160 net/core/dev_ioctl.c:624 > dev_ioctl+0x255/0x10c0 net/core/dev_ioctl.c:826 > sock_do_ioctl+0x1ca/0x260 net/socket.c:1213 > sock_ioctl+0x23a/0x6c0 net/socket.c:1318 > vfs_ioctl fs/ioctl.c:51 [inline] > __do_sys_ioctl fs/ioctl.c:906 [inline] > __se_sys_ioctl fs/ioctl.c:892 [inline] > __x64_sys_ioctl+0x1a4/0x210 fs/ioctl.c:892 > do_syscall_x64 arch/x86/entry/common.c:52 [inline] > do_syscall_64+0xcb/0x250 arch/x86/entry/common.c:83 > entry_SYSCALL_64_after_hwframe+0x77/0x7f > > Fixes: 893b19587534 ("net: bridge: fix ioctl locking") > Reported-by: syzkaller > Reported-by: yan kang > Reported-by: yue sun > Closes: https://lore.kernel.org/netdev/SY8P300MB0421225D54EB92762AE8F0F2A1D32@SY8P300MB0421.AUSP300.PROD.OUTLOOK.COM/ > Signed-off-by: Kuniyuki Iwashima Thanks for the fix and the detailed commit message. One nit below. > --- > include/linux/if_bridge.h | 6 ++---- > net/bridge/br_ioctl.c | 39 ++++++++++++++++++++++++++++++++++++--- > net/bridge/br_private.h | 3 +-- > net/core/dev_ioctl.c | 19 ------------------- > net/socket.c | 19 +++++++++---------- > 5 files changed, 48 insertions(+), 38 deletions(-) > > diff --git a/include/linux/if_bridge.h b/include/linux/if_bridge.h > index 3ff96ae31bf6..c5fe3b2a53e8 100644 > --- a/include/linux/if_bridge.h > +++ b/include/linux/if_bridge.h > @@ -65,11 +65,9 @@ struct br_ip_list { > #define BR_DEFAULT_AGEING_TIME (300 * HZ) > > struct net_bridge; > -void brioctl_set(int (*hook)(struct net *net, struct net_bridge *br, > - unsigned int cmd, struct ifreq *ifr, > +void brioctl_set(int (*hook)(struct net *net, unsigned int cmd, > void __user *uarg)); > -int br_ioctl_call(struct net *net, struct net_bridge *br, unsigned int cmd, > - struct ifreq *ifr, void __user *uarg); > +int br_ioctl_call(struct net *net, unsigned int cmd, void __user *uarg); > > #if IS_ENABLED(CONFIG_BRIDGE) && IS_ENABLED(CONFIG_BRIDGE_IGMP_SNOOPING) > int br_multicast_list_adjacent(struct net_device *dev, > diff --git a/net/bridge/br_ioctl.c b/net/bridge/br_ioctl.c > index f213ed108361..b5a607f6da4e 100644 > --- a/net/bridge/br_ioctl.c > +++ b/net/bridge/br_ioctl.c > @@ -394,10 +394,29 @@ static int old_deviceless(struct net *net, void __user *data) > return -EOPNOTSUPP; > } > > -int br_ioctl_stub(struct net *net, struct net_bridge *br, unsigned int cmd, > - struct ifreq *ifr, void __user *uarg) > +int br_ioctl_stub(struct net *net, unsigned int cmd, void __user *uarg) > { > int ret = -EOPNOTSUPP; > + struct ifreq ifr; > + > + switch (cmd) { > + case SIOCBRADDIF: > + case SIOCBRDELIF: { Why not a simple if statement? Unlikely that we will add more commands to this switch statement. > + void __user *data; > + char *colon; > + > + if (!ns_capable(net->user_ns, CAP_NET_ADMIN)) > + return -EPERM; > + > + if (get_user_ifreq(&ifr, &data, uarg)) > + return -EFAULT; > + > + ifr.ifr_name[IFNAMSIZ - 1] = 0; > + colon = strchr(ifr.ifr_name, ':'); > + if (colon) > + *colon = 0; > + } > + } > > rtnl_lock();