[PATCH] tty: n_gsm: fix memory leak in gsm_activate_mux

[Minu Jin]

syzbot reported a memory leak in gsm_activate_mux().
The root cause is a missing cleanup path when gsm_register_devices()
fails. In this case, the previously allocated DLCI 0
and its associated kfifo remain allocated, leading to a memory leak.

And gsm_dlci_alloc() does not check for already allocated DLCIs.
Repeated calls to gsm_activate_mux() would overwrite the existing pointer
in gsm->dlci[addr], causing the original memory to be lost.

Fix this by:
1. Adding gsm_dlci_free() in the error path of gsm_activate_mux().
2. Adding a check in gsm_dlci_alloc() to return the existing DLCI
   if it is already allocated.

Reported-by: syzbot+b5d1f455d385b2c7da3c@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=b5d1f455d385b2c7da3c
Tested-by: syzbot+b5d1f455d385b2c7da3c@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 01aecd917114 ("tty: n_gsm: fix tty registration before control channel open")
Signed-off-by: Minu Jin <s9430939@xxxxxxxxx>
---
 drivers/tty/n_gsm.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index c13e050de83b..de3d30eac86e 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2645,7 +2645,12 @@ static int gsm_dlci_config(struct gsm_dlci *dlci, struct gsm_dlci_config *dc, in
 
 static struct gsm_dlci *gsm_dlci_alloc(struct gsm_mux *gsm, int addr)
 {
-	struct gsm_dlci *dlci = kzalloc_obj(struct gsm_dlci, GFP_ATOMIC);
+	struct gsm_dlci *dlci;
+
+	if (gsm->dlci[addr])
+		return gsm->dlci[addr];
+
+	dlci = kzalloc_obj(struct gsm_dlci, GFP_ATOMIC);
 	if (dlci == NULL)
 		return NULL;
 	spin_lock_init(&dlci->lock);
@@ -3196,8 +3201,10 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
 		gsm->receive = gsm1_receive;
 
 	ret = gsm_register_devices(gsm_tty_driver, gsm->num);
-	if (ret)
+	if (ret) {
+		gsm_dlci_free(&dlci->port);
 		return ret;
+	}
 
 	gsm->has_devices = true;
 	gsm->dead = false;		/* Tty opens are now permissible */
-- 
2.43.0