llc: fix sk_buff leak in llc_conn_service()

syzbot reported:

    BUG: memory leak
    unreferenced object 0xffff88811eb3de00 (size 224):
       comm "syz-executor559", pid 7315, jiffies 4294943019 (age 10.300s)
       hex dump (first 32 bytes):
         00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
         00 a0 38 24 81 88 ff ff 00 c0 f2 15 81 88 ff ff  ..8$............
       backtrace:
         [<000000008d1c66a1>] kmemleak_alloc_recursive  include/linux/kmemleak.h:55 [inline]
         [<000000008d1c66a1>] slab_post_alloc_hook mm/slab.h:439 [inline]
         [<000000008d1c66a1>] slab_alloc_node mm/slab.c:3269 [inline]
         [<000000008d1c66a1>] kmem_cache_alloc_node+0x153/0x2a0 mm/slab.c:3579
         [<00000000447d9496>] __alloc_skb+0x6e/0x210 net/core/skbuff.c:198
         [<000000000cdbf82f>] alloc_skb include/linux/skbuff.h:1058 [inline]
         [<000000000cdbf82f>] llc_alloc_frame+0x66/0x110 net/llc/llc_sap.c:54
         [<000000002418b52e>] llc_conn_ac_send_sabme_cmd_p_set_x+0x2f/0x140  net/llc/llc_c_ac.c:777
         [<000000001372ae17>] llc_exec_conn_trans_actions net/llc/llc_conn.c:475  [inline]
         [<000000001372ae17>] llc_conn_service net/llc/llc_conn.c:400 [inline]
         [<000000001372ae17>] llc_conn_state_process+0x1ac/0x640  net/llc/llc_conn.c:75
         [<00000000f27e53c1>] llc_establish_connection+0x110/0x170  net/llc/llc_if.c:109
         [<00000000291b2ca0>] llc_ui_connect+0x10e/0x370 net/llc/af_llc.c:477
         [<000000000f9c740b>] __sys_connect+0x11d/0x170 net/socket.c:1840
         [...]

The bug is that most callers of llc_conn_send_pdu() assume it consumes a
reference to the skb, when actually due to commit b85ab56c3f81 ("llc:
properly handle dev_queue_xmit() return value") it doesn't.

Revert most of that commit, and instead make the few places that need
llc_conn_send_pdu() to *not* consume a reference call skb_get() before.

Fixes: b85ab56c3f81 ("llc: properly handle dev_queue_xmit() return value")
Reported-by: syzbot+6b825a6494a04cc0e3f7@syzkaller.appspotmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

3 files changed, 16 insertions, 26 deletions

diff --git a/include/net/llc_conn.h b/include/net/llc_conn.h
index df528a623548..ea985aa7a6c5 100644
--- a/include/net/llc_conn.h
+++ b/include/net/llc_conn.h
@@ -104,7 +104,7 @@ void llc_sk_reset(struct sock *sk);
 
 /* Access to a connection */
 int llc_conn_state_process(struct sock *sk, struct sk_buff *skb);
-int llc_conn_send_pdu(struct sock *sk, struct sk_buff *skb);
+void llc_conn_send_pdu(struct sock *sk, struct sk_buff *skb);
 void llc_conn_rtn_pdu(struct sock *sk, struct sk_buff *skb);
 void llc_conn_resend_i_pdu_as_cmd(struct sock *sk, u8 nr, u8 first_p_bit);
 void llc_conn_resend_i_pdu_as_rsp(struct sock *sk, u8 nr, u8 first_f_bit);
diff --git a/net/llc/llc_c_ac.c b/net/llc/llc_c_ac.c
index 4d78375f9872..647c0554d04c 100644
--- a/net/llc/llc_c_ac.c
+++ b/net/llc/llc_c_ac.c
@@ -372,6 +372,7 @@ int llc_conn_ac_send_i_cmd_p_set_1(struct sock *sk, struct sk_buff *skb)
         llc_pdu_init_as_i_cmd(skb, 1, llc->vS, llc->vR);
         rc = llc_mac_hdr_init(skb, llc->dev->dev_addr, llc->daddr.mac);
         if (likely(!rc)) {
+                skb_get(skb);
                 llc_conn_send_pdu(sk, skb);
                 llc_conn_ac_inc_vs_by_1(sk, skb);
         }
@@ -389,7 +390,8 @@ static int llc_conn_ac_send_i_cmd_p_set_0(struct sock *sk, struct sk_buff *skb)
         llc_pdu_init_as_i_cmd(skb, 0, llc->vS, llc->vR);
         rc = llc_mac_hdr_init(skb, llc->dev->dev_addr, llc->daddr.mac);
         if (likely(!rc)) {
-                rc = llc_conn_send_pdu(sk, skb);
+                skb_get(skb);
+                llc_conn_send_pdu(sk, skb);
                 llc_conn_ac_inc_vs_by_1(sk, skb);
         }
         return rc;
@@ -406,6 +408,7 @@ int llc_conn_ac_send_i_xxx_x_set_0(struct sock *sk, struct sk_buff *skb)
         llc_pdu_init_as_i_cmd(skb, 0, llc->vS, llc->vR);
         rc = llc_mac_hdr_init(skb, llc->dev->dev_addr, llc->daddr.mac);
         if (likely(!rc)) {
+                skb_get(skb);
                 llc_conn_send_pdu(sk, skb);
                 llc_conn_ac_inc_vs_by_1(sk, skb);
         }
@@ -916,7 +919,8 @@ static int llc_conn_ac_send_i_rsp_f_set_ackpf(struct sock *sk,
         llc_pdu_init_as_i_cmd(skb, llc->ack_pf, llc->vS, llc->vR);
         rc = llc_mac_hdr_init(skb, llc->dev->dev_addr, llc->daddr.mac);
         if (likely(!rc)) {
-                rc = llc_conn_send_pdu(sk, skb);
+                skb_get(skb);
+                llc_conn_send_pdu(sk, skb);
                 llc_conn_ac_inc_vs_by_1(sk, skb);
         }
         return rc;
diff --git a/net/llc/llc_conn.c b/net/llc/llc_conn.c
index 4ff89cb7c86f..ed2aca12460c 100644
--- a/net/llc/llc_conn.c
+++ b/net/llc/llc_conn.c
@@ -30,7 +30,7 @@
 #endif
 
 static int llc_find_offset(int state, int ev_type);
-static int llc_conn_send_pdus(struct sock *sk, struct sk_buff *skb);
+static void llc_conn_send_pdus(struct sock *sk);
 static int llc_conn_service(struct sock *sk, struct sk_buff *skb);
 static int llc_exec_conn_trans_actions(struct sock *sk,
                                        struct llc_conn_state_trans *trans,
@@ -193,11 +193,11 @@ out_skb_put:
         return rc;
 }
 
-int llc_conn_send_pdu(struct sock *sk, struct sk_buff *skb)
+void llc_conn_send_pdu(struct sock *sk, struct sk_buff *skb)
 {
         /* queue PDU to send to MAC layer */
         skb_queue_tail(&sk->sk_write_queue, skb);
-        return llc_conn_send_pdus(sk, skb);
+        llc_conn_send_pdus(sk);
 }
 
 /**
@@ -255,7 +255,7 @@ void llc_conn_resend_i_pdu_as_cmd(struct sock *sk, u8 nr, u8 first_p_bit)
         if (howmany_resend > 0)
                 llc->vS = (llc->vS + 1) % LLC_2_SEQ_NBR_MODULO;
         /* any PDUs to re-send are queued up; start sending to MAC */
-        llc_conn_send_pdus(sk, NULL);
+        llc_conn_send_pdus(sk);
 out:;
 }
 
@@ -296,7 +296,7 @@ void llc_conn_resend_i_pdu_as_rsp(struct sock *sk, u8 nr, u8 first_f_bit)
         if (howmany_resend > 0)
                 llc->vS = (llc->vS + 1) % LLC_2_SEQ_NBR_MODULO;
         /* any PDUs to re-send are queued up; start sending to MAC */
-        llc_conn_send_pdus(sk, NULL);
+        llc_conn_send_pdus(sk);
 out:;
 }
 
@@ -340,16 +340,12 @@ out:
 /**
  *      llc_conn_send_pdus - Sends queued PDUs
  *      @sk: active connection
- *      @hold_skb: the skb held by caller, or NULL if does not care
  *
- *      Sends queued pdus to MAC layer for transmission. When @hold_skb is
- *      NULL, always return 0. Otherwise, return 0 if @hold_skb is sent
- *      successfully, or 1 for failure.
+ *      Sends queued pdus to MAC layer for transmission.
  */
-static int llc_conn_send_pdus(struct sock *sk, struct sk_buff *hold_skb)
+static void llc_conn_send_pdus(struct sock *sk)
 {
         struct sk_buff *skb;
-        int ret = 0;
 
         while ((skb = skb_dequeue(&sk->sk_write_queue)) != NULL) {
                 struct llc_pdu_sn *pdu = llc_pdu_sn_hdr(skb);
@@ -361,20 +357,10 @@ static int llc_conn_send_pdus(struct sock *sk, struct sk_buff *hold_skb)
                         skb_queue_tail(&llc_sk(sk)->pdu_unack_q, skb);
                         if (!skb2)
                                 break;
-                        dev_queue_xmit(skb2);
-                } else {
-                        bool is_target = skb == hold_skb;
-                        int rc;
-
-                        if (is_target)
-                                skb_get(skb);
-                        rc = dev_queue_xmit(skb);
-                        if (is_target)
-                                ret = rc;
+                        skb = skb2;
                 }
+                dev_queue_xmit(skb);
         }
-
-        return ret;
 }
 
 /**