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tty: n_gsm: fix the UAF caused by race condition in gsm_cleanup_mux
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commit 3c4f833 upstream.

In commit 9b9c819 ("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
Cc: stable <stable@kernel.org>
Fixes: 9b9c819 ("tty: n_gsm: fix UAF in gsm_cleanup_mux")
Fixes: aa371e9 ("tty: n_gsm: fix restart handling via CLD command")
Signed-off-by: Yi Yang <yiyang13@huawei.com>
Co-developed-by: Qiumiao Zhang <zhangqiumiao1@huawei.com>
Signed-off-by: Qiumiao Zhang <zhangqiumiao1@huawei.com>
Link: https://lore.kernel.org/r/20230811031121.153237-1-yiyang13@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Yi Yang authored and gregkh committed Aug 23, 2023
1 parent d6aa03b commit 31311a9
Showing 1 changed file with 2 additions and 1 deletion.
3 changes: 2 additions & 1 deletion drivers/tty/n_gsm.c
Expand Up @@ -2481,12 +2481,13 @@ 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);
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