[PATCH v3] usb: gadget: u_serial: fix NULL deref in gs_close() after failed open

[Brian Ellis via B4 Relay <devnull+brianellis.gotenna.com@kernel.org>]

From: Brian Ellis <brianellis@gotenna.com>

gs_close() opens with

	struct gs_port *port = tty->driver_data;
	...
	spin_lock_irq(&port->port_lock);

and never checks port.  gs_open() assigns tty->driver_data only on the
port.count 0 -> 1 edge, so any failure earlier in gs_open() leaves it NULL,
and tty_open() calls tty_release(), and hence ->close(), when ->open()
fails.  Acquiring the spinlock is a write, so this faults outright rather
than reading garbage.

syzbot reproduces it on arm64 as a KASAN null-ptr-deref write at offset
0x74, which is port_lock within struct gs_port:

  BUG: unable to handle kernel paging request in gs_close
  KASAN: null-ptr-deref Write in gs_close
  Call trace:
   gs_close+0xac/0x65c      drivers/usb/gadget/function/u_serial.c:698
   tty_release+0x2c4/0xffc  drivers/tty/tty_io.c:1745
   tty_open+0x46c/0x89c     drivers/tty/tty_io.c:2146

Return early instead.  There is nothing to release when the open never got
far enough to claim the port.

Unlike the check reverted by commit f6c7bc4a6823 ("Revert "usb: gadget:
u_serial: Add null pointer check in gs_start_io""), this is not hiding a
race: the tty core calls ->close() for an ->open() that failed by design,
so driver_data is legitimately unset on that path.

Fixes: c1dca562be8a ("usb gadget: split out serial core")
Reported-by: syzbot+97aa71f55869d71bc94a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=97aa71f55869d71bc94a
Cc: stable@vger.kernel.org
Signed-off-by: Brian Ellis <brianellis@gotenna.com>
---
Reported by syzbot (still open, with C and syz reproducers) and hit
independently on a downstream 6.6-adi BSP kernel, where gserial_free_line()
clearing ports[n].port makes the same path reachable.

Compile-tested against usb-linus with allmodconfig (CONFIG_USB_U_SERIAL=m).
Not boot-tested on mainline and the syzbot reproducer has not been run here:
the board this was hit on needs a vendor BSP device tree to boot.
---
Changes in v3:
- Restore the Reported-by:/Closes: syzbot tags and the KASAN trace. They were
  dropped from v1 and v2 in error when the commit message was rewritten. The
  report is still open, and without Closes: syzbot cannot associate this fix
  with it.
- Say why this differs from the check reverted in f6c7bc4a6823, since that
  revert's reasoning is the obvious objection here.
- Link to v2: https://lore.kernel.org/r/20260927-u_serial-gs-close-null-v2-1-bdedf57fe96f@gotenna.com

Changes in v2:
- Remove the comment above the check, rather than reword it as Prashanth
  suggested. The commit message already explains the path that leaves
  tty->driver_data NULL, so the comment only restated it. Happy to add a
  reworded one instead if that is preferred.
- Link to v1: https://lore.kernel.org/r/20260910-u_serial-gs-close-null-v1-1-9a5e848b1dfb@gotenna.com
---
 drivers/usb/gadget/function/u_serial.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
index cdd1dfc66..892d2673e 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -695,6 +695,9 @@ static void gs_close(struct tty_struct *tty, struct file *file)
 	struct gs_port *port = tty->driver_data;
 	struct gserial	*gser;
 
+	if (!port)
+		return;
+
 	spin_lock_irq(&port->port_lock);
 
 	if (port->port.count != 1) {

---
base-commit: be4219dd98608736e13e0b790ef742b76a13254d
change-id: 20260910-u_serial-gs-close-null-a80a436666e8

Best regards,
-- 
Brian Ellis <brianellis@gotenna.com>