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Date: Tue, 14 Nov 2017 13:19:20 -0500
From: Neil Horman <nhorman@...driver.com>
To: Xin Long <lucien.xin@...il.com>
Cc: network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
davem <davem@...emloft.net>,
Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] sctp: do not free asoc when it is already dead in
sctp_sendmsg
On Tue, Nov 14, 2017 at 08:52:56PM +0800, Xin Long wrote:
> On Tue, Nov 14, 2017 at 8:43 PM, Neil Horman <nhorman@...driver.com> wrote:
> > On Mon, Nov 13, 2017 at 11:29:49PM +0800, Xin Long wrote:
> >> On Mon, Nov 13, 2017 at 10:46 PM, Neil Horman <nhorman@...driver.com> wrote:
> >> > On Mon, Nov 13, 2017 at 12:43:50PM +0800, Xin Long wrote:
> >> >> Now in sctp_sendmsg sctp_wait_for_sndbuf could schedule out without
> >> >> holding sock sk. It means the current asoc can be freed elsewhere,
> >> >> like when receiving an abort packet.
> >> >>
> >> >> If the asoc is just created in sctp_sendmsg and sctp_wait_for_sndbuf
> >> >> returns err, the asoc will be freed again due to new_asoc is not nil.
> >> >> An use-after-free issue would be triggered by this.
> >> >>
> >> >> This patch is to fix it by setting new_asoc with nil if the asoc is
> >> >> already dead when cpu schedules back, so that it will not be freed
> >> >> again in sctp_sendmsg.
> >> >>
> >> >> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
> >> >> Signed-off-by: Xin Long <lucien.xin@...il.com>
> >> >> ---
> >> >> net/sctp/socket.c | 15 +++++++++------
> >> >> 1 file changed, 9 insertions(+), 6 deletions(-)
> >> >>
> >> >> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> >> >> index 6f45d17..f575976 100644
> >> >> --- a/net/sctp/socket.c
> >> >> +++ b/net/sctp/socket.c
> >> >> @@ -83,8 +83,8 @@
> >> >> /* Forward declarations for internal helper functions. */
> >> >> static int sctp_writeable(struct sock *sk);
> >> >> static void sctp_wfree(struct sk_buff *skb);
> >> >> -static int sctp_wait_for_sndbuf(struct sctp_association *, long *timeo_p,
> >> >> - size_t msg_len);
> >> >> +static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
> >> >> + size_t msg_len, struct sctp_association **new);
> >> >> static int sctp_wait_for_packet(struct sock *sk, int *err, long *timeo_p);
> >> >> static int sctp_wait_for_connect(struct sctp_association *, long *timeo_p);
> >> >> static int sctp_wait_for_accept(struct sock *sk, long timeo);
> >> >> @@ -1962,7 +1962,7 @@ static int sctp_sendmsg(struct sock *sk, struct msghdr *msg, size_t msg_len)
> >> >>
> >> >> timeo = sock_sndtimeo(sk, msg->msg_flags & MSG_DONTWAIT);
> >> >> if (!sctp_wspace(asoc)) {
> >> >> - err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len);
> >> >> + err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len, &new_asoc);
> >> >> if (err)
> >> >> goto out_free;
> >> >> }
> >> >> @@ -7822,7 +7822,7 @@ void sctp_sock_rfree(struct sk_buff *skb)
> >> >>
> >> >> /* Helper function to wait for space in the sndbuf. */
> >> >> static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
> >> >> - size_t msg_len)
> >> >> + size_t msg_len, struct sctp_association **new)
> >> >> {
> >> >> struct sock *sk = asoc->base.sk;
> >> >> int err = 0;
> >> >> @@ -7839,10 +7839,13 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
> >> >> for (;;) {
> >> >> prepare_to_wait_exclusive(&asoc->wait, &wait,
> >> >> TASK_INTERRUPTIBLE);
> >> >> + if (asoc->base.dead) {
> >> >> + *new = NULL;
> >> >> + goto do_error;
> >> >> + }
> >> >> if (!*timeo_p)
> >> >> goto do_nonblock;
> >> >> - if (sk->sk_err || asoc->state >= SCTP_STATE_SHUTDOWN_PENDING ||
> >> >> - asoc->base.dead)
> >> >> + if (sk->sk_err || asoc->state >= SCTP_STATE_SHUTDOWN_PENDING)
> >> >> goto do_error;
> >> >> if (signal_pending(current))
> >> >> goto do_interrupted;
> >> >> --
> >> >> 2.1.0
> >> >>
> >> >>
> >> > Why pass a pointer to a pointer into the wait function? It seems like you could
> >> > just check the return code for err == -EPIPE and set the association to null in
> >> > sctp_sendmsg. That would avoid passing another parameter, and cut down on some
> >> > complexity here.
> >> "if (sk->sk_err || asoc->state >= SCTP_STATE_SHUTDOWN_PENDING)"
> >> also goes to err == -EPIPE path, with it, the new_asoc are supposed to
> >> be freed in old codes.
> >>
> >> do you think it's good to not free it now when sk->sk_err or SHUTDOWN_PENDING ?
> >> or I can add another err path like:
> >> +do_dead:
> >> + err = -ESRCH;
> >> + goto out;
> >>
> > That would work, but it also seems just as easy to check in sctp_sendmsg. that
> > is to say, if sctp_wait_for_sndbuf return -EPIPE sctp_sndmsg jumps to out_free,
> > where we can check to see if (new_assoc && new_assoc->base.dead) as a gating
> > factor on free.
> we can't do that, as nowhere holds new_assoc, it may be already destroyed.
> to dereference it (new_assoc->base.dead) will cause use-after-free panic.
>
> I may do like this:
> err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len);
> - if (err)
> + if (err) {
> + if (err == -ESRCH) {
> + /* asoc is already dead; */
> + new_asoc = NULL;
> + err = -EPIPE;
> + }
> goto out_free;
> + }
>
> agree ?
Yes, I concur. Thanks!
Neil
>
> >
> > Neil
> >> >
> >> > Neil
> >> >
> >>
>
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