[PATCH net] sctp: add wait_buf flag in asoc to avoid the peeloff and wait sndbuf race

[Xin Long]

Commit dfcb9f4f99f1 ("sctp: deny peeloff operation on asocs with threads
sleeping on it") fixed the race between peeloff and wait sndbuf by
checking waitqueue_active(&asoc->wait) in sctp_do_peeloff().

But it actually doesn't work as even if waitqueue_active returns false
the waiting sndbuf thread may still not yet hold sk lock.

This patch is to fix this by adding wait_buf flag in asoc, and setting it
before going the waiting loop, clearing it until the waiting loop breaks,
and checking it in sctp_do_peeloff instead.

Fixes: dfcb9f4f99f1 ("sctp: deny peeloff operation on asocs with threads 
sleeping on it")
Suggested-by: Marcelo Ricardo Leitner <marcelo.leit...@gmail.com>
Signed-off-by: Xin Long <lucien....@gmail.com>
---
 include/net/sctp/structs.h | 1 +
 net/sctp/socket.c          | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
index 0477945..446350e 100644
--- a/include/net/sctp/structs.h
+++ b/include/net/sctp/structs.h
@@ -1883,6 +1883,7 @@ struct sctp_association {
 
        __u8 need_ecne:1,       /* Need to send an ECNE Chunk? */
             temp:1,            /* Is it a temporary association? */
+            wait_buf:1,
             force_delay:1,
             prsctp_enable:1,
             reconf_enable:1;
diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index 6f45d17..1b2c78c 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -4946,7 +4946,7 @@ int sctp_do_peeloff(struct sock *sk, sctp_assoc_t id, 
struct socket **sockp)
        /* If there is a thread waiting on more sndbuf space for
         * sending on this asoc, it cannot be peeled.
         */
-       if (waitqueue_active(&asoc->wait))
+       if (asoc->wait_buf)
                return -EBUSY;
 
        /* An association cannot be branched off from an already peeled-off
@@ -7835,6 +7835,7 @@ static int sctp_wait_for_sndbuf(struct sctp_association 
*asoc, long *timeo_p,
        /* Increment the association's refcnt.  */
        sctp_association_hold(asoc);
 
+       asoc->wait_buf = 1;
        /* Wait on the association specific sndbuf space. */
        for (;;) {
                prepare_to_wait_exclusive(&asoc->wait, &wait,
@@ -7860,6 +7861,7 @@ static int sctp_wait_for_sndbuf(struct sctp_association 
*asoc, long *timeo_p,
        }
 
 out:
+       asoc->wait_buf = 0;
        finish_wait(&asoc->wait, &wait);
 
        /* Release the association's refcnt.  */
-- 
2.1.0