sctp: deny peeloff operation on asocs with threads sleeping on it (CVE-2017-6353)

debian/changelog

@@ -94,6 +94,8 @@ linux (4.9.13-1) UNRELEASED; urgency=medium
   * [x86] kvm: fix page struct leak in handle_vmon (CVE-2017-2596)
   * ipc/shm: Fix shmat mmap nil-page protection (CVE-2017-5669)
   * time: Disable TIMER_STATS (CVE-2017-5967)
+  * sctp: deny peeloff operation on asocs with threads sleeping on it
+    (CVE-2017-6353)
 
  -- Ben Hutchings <ben@decadent.org.uk>  Sat, 18 Feb 2017 00:38:10 +0000
 

debian/patches/bugfix/all/sctp-deny-peeloff-operation-on-asocs-with-threads-sl.patch

@@ -0,0 +1,61 @@
+From: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
+Date: Thu, 23 Feb 2017 09:31:18 -0300
+Subject: sctp: deny peeloff operation on asocs with threads sleeping on it
+Origin: https://git.kernel.org/cgit/linux/kernel/git/davem/net.git/commit?id=dfcb9f4f99f1e9a49e43398a7bfbf56927544af1
+Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2017-6353
+
+commit 2dcab5984841 ("sctp: avoid BUG_ON on sctp_wait_for_sndbuf")
+attempted to avoid a BUG_ON call when the association being used for a
+sendmsg() is blocked waiting for more sndbuf and another thread did a
+peeloff operation on such asoc, moving it to another socket.
+
+As Ben Hutchings noticed, then in such case it would return without
+locking back the socket and would cause two unlocks in a row.
+
+Further analysis also revealed that it could allow a double free if the
+application managed to peeloff the asoc that is created during the
+sendmsg call, because then sctp_sendmsg() would try to free the asoc
+that was created only for that call.
+
+This patch takes another approach. It will deny the peeloff operation
+if there is a thread sleeping on the asoc, so this situation doesn't
+exist anymore. This avoids the issues described above and also honors
+the syscalls that are already being handled (it can be multiple sendmsg
+calls).
+
+Joint work with Xin Long.
+
+Fixes: 2dcab5984841 ("sctp: avoid BUG_ON on sctp_wait_for_sndbuf")
+Cc: Alexander Popov <alex.popov@linux.com>
+Cc: Ben Hutchings <ben@decadent.org.uk>
+Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
+Signed-off-by: Xin Long <lucien.xin@gmail.com>
+Signed-off-by: David S. Miller <davem@davemloft.net>
+---
+ net/sctp/socket.c | 8 ++++++--
+ 1 file changed, 6 insertions(+), 2 deletions(-)
+
+--- a/net/sctp/socket.c
++++ b/net/sctp/socket.c
+@@ -4735,6 +4735,12 @@ int sctp_do_peeloff(struct sock *sk, sct
+ 	if (!asoc)
+ 		return -EINVAL;
+ 
++	/* If there is a thread waiting on more sndbuf space for
++	 * sending on this asoc, it cannot be peeled.
++	 */
++	if (waitqueue_active(&asoc->wait))
++		return -EBUSY;
++
+ 	/* An association cannot be branched off from an already peeled-off
+ 	 * socket, nor is this supported for tcp style sockets.
+ 	 */
+@@ -7427,8 +7433,6 @@ static int sctp_wait_for_sndbuf(struct s
+ 		 */
+ 		release_sock(sk);
+ 		current_timeo = schedule_timeout(current_timeo);
+-		if (sk != asoc->base.sk)
+-			goto do_error;
+ 		lock_sock(sk);
+ 
+ 		*timeo_p = current_timeo;

debian/patches/series

@@ -111,6 +111,7 @@ debian/i386-686-pae-pci-set-pci-nobios-by-default.patch
 bugfix/x86/kvm-fix-page-struct-leak-in-handle_vmon.patch
 bugfix/all/ipc-shm-fix-shmat-mmap-nil-page-protection.patch
 debian/time-mark-timer_stats-as-broken.patch
+bugfix/all/sctp-deny-peeloff-operation-on-asocs-with-threads-sl.patch
 
 # Fix exported symbol versions
 bugfix/ia64/revert-ia64-move-exports-to-definitions.patch