From: Brian Ellis 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 --- 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