[PATCH] tty: n_gsm: keep DLCI 0 object across mux restarts

[henrymei <ljp1205831794@gmail.com>]

From: Aohan Mei <henrymei@tencent.com>

gsmtty_install() takes a reference on the object occupying the
gsm->dlci[0] slot, and gsmtty_cleanup() drops a reference on whatever
object occupies the slot at cleanup time. The two are expected to
balance on the same object, but gsm_activate_mux() breaks that: on a
mux restart (GSMIOC_SETCONF with a need_restart change) it
unconditionally allocates a fresh DLCI 0 into the slot, orphaning the
old object which still has install-time references outstanding.

When the last of those old gsmttys is closed, gsmtty_cleanup() then
drops its slot reference on the *new* DLCI 0, driving its base kref
from 1 to 0: gsm_dlci_free() frees it and NULLs the slot while the
mux stays alive (dead == false), and the old object is leaked. The
next gsmtty open passes the dead check and dereferences the NULL slot
in gsmtty_install() (dlci_get(gsm->dlci[0])), crashing the kernel.

Keep the DLCI 0 object in the slot on reactivation when one is still
present: it can only still be there because open gsmttys hold
references on it, and gsm_dlci_free() is the only code that clears the
slot and only runs at kref 0, so the slot identity can no longer
change while install-time references are outstanding. Re-take the base
reference that gsm_cleanup_mux() dropped via gsm_dlci_release(), clear
the dead flag it set, and refresh the per-object config fields exactly
like a fresh gsm_dlci_alloc() would, since the restart may carry a
config change.

This also restores the invariant that dead == false implies
gsm->dlci[0] != NULL, making the NULL dereference structurally
impossible.

Fixes: 6ab8fba7fcb0 ("tty: n_gsm: Added refcount usage to gsm_mux and gsm_dlci structs")
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Cc: stable@vger.kernel.org
Assisted-by: CodeBuddy:Kimi-K3
Signed-off-by: Aohan Mei <henrymei@tencent.com>
---
 drivers/tty/n_gsm.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index c13e050de83b..2ca7e18addc5 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -3186,7 +3186,29 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
 	struct gsm_dlci *dlci;
 	int ret;
 
-	dlci = gsm_dlci_alloc(gsm, 0);
+	dlci = gsm->dlci[0];
+	if (dlci) {
+		/*
+		 * gsmtty_install() takes a reference on the object that
+		 * occupies the dlci[0] slot and gsmtty_cleanup() drops a
+		 * reference on the object occupying the slot at cleanup
+		 * time. Replacing the object here while such references
+		 * are still outstanding makes the cleanup reference drop
+		 * on the wrong (new) object, freeing it prematurely, and
+		 * leaks the old one. Keep the still-referenced object in
+		 * the slot and re-take the base reference that
+		 * gsm_cleanup_mux() dropped so the install and cleanup
+		 * references always balance on the same object.
+		 */
+		dlci_get(dlci);
+		dlci->dead = false;
+		dlci->adaption = gsm->adaption;
+		dlci->mtu = gsm->mtu;
+		dlci->ftype = gsm->ftype;
+		dlci->k = gsm->k;
+	} else {
+		dlci = gsm_dlci_alloc(gsm, 0);
+	}
 	if (dlci == NULL)
 		return -ENOMEM;
 
-- 
2.43.7