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Tue, 04 Feb 2020 10:05:54 -0800 (PST) Date: Tue, 04 Feb 2020 10:05:53 -0800 From: Christoph Paasch To: Florian Westphal Cc: netdev@vger.kernel.org, Paolo Abeni Subject: Re: [PATCH net] mptcp: fix use-after-free on tcp fallback Message-id: <20200204180553.GG33105@MacBook-Pro-64.local> References: <20200204171230.618-1-fw@strlen.de> MIME-version: 1.0 Content-type: text/plain; charset=us-ascii Content-disposition: inline In-reply-to: <20200204171230.618-1-fw@strlen.de> User-Agent: Mutt/1.12.2 (2019-09-21) X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2020-02-04_06:,, signatures=0 Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 04/02/20 - 18:12:30, Florian Westphal wrote: > When an mptcp socket connects to a tcp peer or when a middlebox interferes > with tcp options, mptcp needs to fall back to plain tcp. > Problem is that mptcp is trying to be too clever in this case: > > It attempts to close the mptcp meta sk and transparently replace it with > the (only) subflow tcp sk. > > Unfortunately, this is racy -- the socket is already exposed to userspace. > Any parallel calls to send/recv/setsockopt etc. can cause use-after-free: > > BUG: KASAN: use-after-free in atomic_try_cmpxchg include/asm-generic/atomic-instrumented.h:693 [inline] > CPU: 1 PID: 2083 Comm: syz-executor.1 Not tainted 5.5.0 #2 > atomic_try_cmpxchg include/asm-generic/atomic-instrumented.h:693 [inline] > queued_spin_lock include/asm-generic/qspinlock.h:78 [inline] > do_raw_spin_lock include/linux/spinlock.h:181 [inline] > __raw_spin_lock_bh include/linux/spinlock_api_smp.h:136 [inline] > _raw_spin_lock_bh+0x71/0xd0 kernel/locking/spinlock.c:175 > spin_lock_bh include/linux/spinlock.h:343 [inline] > __lock_sock+0x105/0x190 net/core/sock.c:2414 > lock_sock_nested+0x10f/0x140 net/core/sock.c:2938 > lock_sock include/net/sock.h:1516 [inline] > mptcp_setsockopt+0x2f/0x1f0 net/mptcp/protocol.c:800 > __sys_setsockopt+0x152/0x240 net/socket.c:2130 > __do_sys_setsockopt net/socket.c:2146 [inline] > __se_sys_setsockopt net/socket.c:2143 [inline] > __x64_sys_setsockopt+0xba/0x150 net/socket.c:2143 > do_syscall_64+0xb7/0x3d0 arch/x86/entry/common.c:294 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > While the use-after-free can be resolved, there is another problem: > sock->ops and sock->sk assignments are not atomic, i.e. we may get calls > into mptcp functions with sock->sk already pointing at the subflow socket, > or calls into tcp functions with a mptcp meta sk. > > Remove the fallback code and call the relevant functions for the (only) > subflow in case the mptcp socket is connected to tcp peer. > > Reported-by: Christoph Paasch > Diagnosed-by: Paolo Abeni > Signed-off-by: Florian Westphal > --- > net/mptcp/protocol.c | 76 ++++---------------------------------------- > 1 file changed, 6 insertions(+), 70 deletions(-) Tested-by: Christoph Paasch