On Mon, Nov 13, 2017 at 10:46 PM, Neil Horman <nhorman@xxxxxxxxxxxxx> 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@xxxxxxxxxx>
>> Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
>> ---
>> 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;
>
> Neil
>
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