[PATCH] sctp: fix use-after-free in pr_debug statement

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

Couldn't get syzkaller working over here, so I still need your help on
testing this. I expect this will be the last cycle, though.

If it does generate another trace, I'll need the reproducer too because
I can't find anything else just with code review.

Thanks

--8<--

Dmitry Vyukov reported a use-after-free in the code expanded by the
macro debug_post_sfx, which is caused by the use of the asoc pointer
after it was freed within sctp_side_effect() scope.

This patch fixes it by allowing sctp_side_effect to clear that asoc
pointer when the TCB is freed.

As Vlad explained, we also have to cover the SCTP_DISPOSITION_ABORT case
because it will trigger DELETE_TCB too on that same loop.

Also, there were places issuing SCTP_CMD_INIT_FAILED and ASSOC_FAILED
but returning SCTP_DISPOSITION_CONSUME, which would fool the scheme
above. Fix it by returning SCTP_DISPOSITION_ABORT instead.

The macro is already prepared to handle such NULL pointer.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
---
 net/sctp/sm_sideeffect.c | 11 ++++++-----
 net/sctp/sm_statefuns.c  | 17 ++++-------------
 2 files changed, 10 insertions(+), 18 deletions(-)

diff --git a/net/sctp/sm_sideeffect.c b/net/sctp/sm_sideeffect.c
index 4f170ad38ff4f7d345d8e3a3fee7d691df64d9cb..2e21384697c2a6a5fd045142bcd9c39992d3867f 100644
--- a/net/sctp/sm_sideeffect.c
+++ b/net/sctp/sm_sideeffect.c
@@ -63,7 +63,7 @@ static int sctp_cmd_interpreter(sctp_event_t event_type,
 static int sctp_side_effects(sctp_event_t event_type, sctp_subtype_t subtype,
 			     sctp_state_t state,
 			     struct sctp_endpoint *ep,
-			     struct sctp_association *asoc,
+			     struct sctp_association **asoc,
 			     void *event_arg,
 			     sctp_disposition_t status,
 			     sctp_cmd_seq_t *commands,
@@ -1125,7 +1125,7 @@ int sctp_do_sm(struct net *net, sctp_event_t event_type, sctp_subtype_t subtype,
 	debug_post_sfn();
 
 	error = sctp_side_effects(event_type, subtype, state,
-				  ep, asoc, event_arg, status,
+				  ep, &asoc, event_arg, status,
 				  &commands, gfp);
 	debug_post_sfx();
 
@@ -1138,7 +1138,7 @@ int sctp_do_sm(struct net *net, sctp_event_t event_type, sctp_subtype_t subtype,
 static int sctp_side_effects(sctp_event_t event_type, sctp_subtype_t subtype,
 			     sctp_state_t state,
 			     struct sctp_endpoint *ep,
-			     struct sctp_association *asoc,
+			     struct sctp_association **asoc,
 			     void *event_arg,
 			     sctp_disposition_t status,
 			     sctp_cmd_seq_t *commands,
@@ -1153,7 +1153,7 @@ static int sctp_side_effects(sctp_event_t event_type, sctp_subtype_t subtype,
 	 * disposition SCTP_DISPOSITION_CONSUME.
 	 */
 	if (0 != (error = sctp_cmd_interpreter(event_type, subtype, state,
-					       ep, asoc,
+					       ep, *asoc,
 					       event_arg, status,
 					       commands, gfp)))
 		goto bail;
@@ -1176,11 +1176,12 @@ static int sctp_side_effects(sctp_event_t event_type, sctp_subtype_t subtype,
 		break;
 
 	case SCTP_DISPOSITION_DELETE_TCB:
+	case SCTP_DISPOSITION_ABORT:
 		/* This should now be a command. */
+		*asoc = NULL;
 		break;
 
 	case SCTP_DISPOSITION_CONSUME:
-	case SCTP_DISPOSITION_ABORT:
 		/*
 		 * We should no longer have much work to do here as the
 		 * real work has been done as explicit commands above.
diff --git a/net/sctp/sm_statefuns.c b/net/sctp/sm_statefuns.c
index 22c2bf367d7e8c7025065f33eabfd7e93a7f4021..f1f08c8f277bd8719299d1ed21eb23e36d55f7e2 100644
--- a/net/sctp/sm_statefuns.c
+++ b/net/sctp/sm_statefuns.c
@@ -2976,7 +2976,7 @@ sctp_disposition_t sctp_sf_eat_data_6_2(struct net *net,
 		SCTP_INC_STATS(net, SCTP_MIB_IN_DATA_CHUNK_DISCARDS);
 		goto discard_force;
 	case SCTP_IERROR_NO_DATA:
-		goto consume;
+		return SCTP_DISPOSITION_ABORT;
 	case SCTP_IERROR_PROTO_VIOLATION:
 		return sctp_sf_abort_violation(net, ep, asoc, chunk, commands,
 			(u8 *)chunk->subh.data_hdr, sizeof(sctp_datahdr_t));
@@ -3043,9 +3043,6 @@ discard_noforce:
 		sctp_add_cmd_sf(commands, SCTP_CMD_GEN_SACK, force);
 
 	return SCTP_DISPOSITION_DISCARD;
-consume:
-	return SCTP_DISPOSITION_CONSUME;
-
 }
 
 /*
@@ -3093,7 +3090,7 @@ sctp_disposition_t sctp_sf_eat_data_fast_4_4(struct net *net,
 	case SCTP_IERROR_BAD_STREAM:
 		break;
 	case SCTP_IERROR_NO_DATA:
-		goto consume;
+		return SCTP_DISPOSITION_ABORT;
 	case SCTP_IERROR_PROTO_VIOLATION:
 		return sctp_sf_abort_violation(net, ep, asoc, chunk, commands,
 			(u8 *)chunk->subh.data_hdr, sizeof(sctp_datahdr_t));
@@ -3119,7 +3116,6 @@ sctp_disposition_t sctp_sf_eat_data_fast_4_4(struct net *net,
 				SCTP_TO(SCTP_EVENT_TIMEOUT_T2_SHUTDOWN));
 	}
 
-consume:
 	return SCTP_DISPOSITION_CONSUME;
 }
 
@@ -4825,9 +4821,6 @@ sctp_disposition_t sctp_sf_do_9_1_prm_abort(
 	 * if necessary to fill gaps.
 	 */
 	struct sctp_chunk *abort = arg;
-	sctp_disposition_t retval;
-
-	retval = SCTP_DISPOSITION_CONSUME;
 
 	if (abort)
 		sctp_add_cmd_sf(commands, SCTP_CMD_REPLY, SCTP_CHUNK(abort));
@@ -4845,7 +4838,7 @@ sctp_disposition_t sctp_sf_do_9_1_prm_abort(
 	SCTP_INC_STATS(net, SCTP_MIB_ABORTEDS);
 	SCTP_DEC_STATS(net, SCTP_MIB_CURRESTAB);
 
-	return retval;
+	return SCTP_DISPOSITION_ABORT;
 }
 
 /* We tried an illegal operation on an association which is closed.  */
@@ -4960,12 +4953,10 @@ sctp_disposition_t sctp_sf_cookie_wait_prm_abort(
 	sctp_cmd_seq_t *commands)
 {
 	struct sctp_chunk *abort = arg;
-	sctp_disposition_t retval;
 
 	/* Stop T1-init timer */
 	sctp_add_cmd_sf(commands, SCTP_CMD_TIMER_STOP,
 			SCTP_TO(SCTP_EVENT_TIMEOUT_T1_INIT));
-	retval = SCTP_DISPOSITION_CONSUME;
 
 	if (abort)
 		sctp_add_cmd_sf(commands, SCTP_CMD_REPLY, SCTP_CHUNK(abort));
@@ -4985,7 +4976,7 @@ sctp_disposition_t sctp_sf_cookie_wait_prm_abort(
 	sctp_add_cmd_sf(commands, SCTP_CMD_INIT_FAILED,
 			SCTP_PERR(SCTP_ERROR_USER_ABORT));
 
-	return retval;
+	return SCTP_DISPOSITION_ABORT;
 }
 
 /*
-- 
2.5.0