In commit 9b9c8195f3f0 ("tty: n_gsm: fix UAF in gsm_cleanup_mux"), the UAF
problem is not completely fixed. There is a race condition in
gsm_cleanup_mux(), which caused this UAF.
The UAF problem is triggered by the following race:
task[5046] task[5054]
----------------------- -----------------------
gsm_cleanup_mux();
dlci = gsm->dlci[0];
mutex_lock(&gsm->mutex);
gsm_cleanup_mux();
dlci = gsm->dlci[0]; //Didn't take the lock
gsm_dlci_release(gsm->dlci[i]);
gsm->dlci[i] = NULL;
mutex_unlock(&gsm->mutex);
mutex_lock(&gsm->mutex);
dlci->dead = true; //UAF
Fix it by assigning values after mutex_lock().
Link: https://syzkaller.appspot.com/text?tag=CrashReport&x=176188b5a80000
Fixes: 9b9c8195f3f0 ("tty: n_gsm: fix UAF in gsm_cleanup_mux")
Fixes: aa371e96f05d ("tty: n_gsm: fix restart handling via CLD command")
Signed-off-by: Yi Yang <yiyang13@xxxxxxxxxx>
Co-developed-by: Qiumiao Zhang <zhangqiumiao1@xxxxxxxxxx>
Signed-off-by: Qiumiao Zhang <zhangqiumiao1@xxxxxxxxxx>
---
drivers/tty/n_gsm.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index c7a787f10a9c..b32d91ccddda 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -3042,12 +3042,13 @@ static void gsm_error(struct gsm_mux *gsm)
static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
{
int i;
- struct gsm_dlci *dlci = gsm->dlci[0];
+ struct gsm_dlci *dlci;
struct gsm_msg *txq, *ntxq;
gsm->dead = true;
mutex_lock(&gsm->mutex);
+ dlci = gsm->dlci[0];
if (dlci) {
if (disc && dlci->state != DLCI_CLOSED) {
gsm_dlci_begin_close(dlci);
--
2.17.1
