linux-kernel - [PATCH v2] usb: gadget: u_serial: Fix the issue that gs_start

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Message-ID:
 <TYUPR06MB62178D00DC96CC2702114CF5D2222@TYUPR06MB6217.apcprd06.prod.outlook.com>
Date: Thu, 21 Nov 2024 12:41:00 +0000
From: 胡连勤 <hulianqin@...o.com>
To: "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"quic_jjohnson@...cinc.com" <quic_jjohnson@...cinc.com>, "mwalle@...nel.org"
	<mwalle@...nel.org>, Prashanth K <quic_prashk@...cinc.com>
CC: "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	opensource.kernel <opensource.kernel@...o.com>, 胡连勤
	<hulianqin@...o.com>
Subject: [PATCH v2] usb: gadget: u_serial: Fix the issue that gs_start_io
 crashed due to accessing null pointer

From: Lianqin Hu <hulianqin@...o.com>

Considering that in some extreme cases,
when u_serial driver is accessed by multiple threads,
Thread A is executing the open operation and calling the gs_open,
Thread B is executing the disconnect operation and calling the
gserial_disconnect function,The port->port_usb pointer will be set to NULL.

E.g.
    Thread A                                 Thread B

    gs_open()                                gadget_unbind_driver()

    gs_start_io()                            composite_disconnect()

    gs_start_rx()                            gserial_disconnect()
    ...                                      ...
    spin_unlock(&port->port_lock)
    status = usb_ep_queue()                  spin_lock(&port->port_lock)
    spin_lock(&port->port_lock)              port->port_usb = NULL
    gs_free_requests(port->port_usb->in)     spin_unlock(&port->port_lock)
    Crash

This causes thread A to access a null pointer (port->port_usb is null)
when calling the gs_free_requests function, causing a crash.

To avoid this, add a null pointer check to gs_start_io before attempting
to access the value of the pointer port->port_usb.

Unable to handle kernel NULL pointer dereference at
virtual address 00000000000000e8
pc : gs_start_io+0x164/0x25c
lr : gs_start_io+0x238/0x25c
sp : ffffffc08b75ba00
x29: ffffffc08b75ba00 x28: ffffffed8ba01000 x27: 0000000000020902
x26: dead000000000100 x25: ffffff899f43a400 x24: ffffff8862325400
x23: ffffff88623256a4 x22: ffffff8862325690 x21: ffffff88623255ec
x20: ffffff88623255d8 x19: ffffff885e19d700 x18: ffffffed8c45ae40
x17: 00000000d48d30ad x16: 00000000d48d30ad x15: 0010000000000001
x14: ffffffed8c50fcc0 x13: 0000000040000000 x12: 0000000000000001
x11: 0000000080200012 x10: 0000000080200012 x9 : ffffff88623255d8
x8 : 0000000000000000 x7 : 0000000000000000 x6 : 000000000000003f
x5 : ffffffed8ae0b9a4 x4 : fffffffe267d0ea0 x3 : 0000000080200012
x2 : ffffff899f43a400 x1 : 0000000080200013 x0 : ffffff899f43b100
Call trace:
 gs_start_io+0x164/0x25c
 gs_open+0x108/0x13c
 tty_open+0x314/0x638
 chrdev_open+0x1b8/0x258
 do_dentry_open+0x2c4/0x700
 vfs_open+0x2c/0x3c
 path_openat+0xa64/0xc60
 do_filp_open+0xb8/0x164
 do_sys_openat2+0x84/0xf0
 __arm64_sys_openat+0x70/0x9c
 invoke_syscall+0x58/0x114
 el0_svc_common+0x80/0xe0
 do_el0_svc+0x1c/0x28
 el0_svc+0x38/0x68
 el0t_64_sync_handler+0x68/0xbc
 el0t_64_sync+0x1a8/0x1ac
Code: f2fbd5ba eb14013f 540004a1 f940e708 (f9407513)
---[ end trace 0000000000000000 ]---

Suggested-by: Prashanth K <quic_prashk@...cinc.com>
Signed-off-by: Lianqin Hu <hulianqin@...o.com>

v2:
 - Modify patch content and description according to "v1 suggestion"
 - Link to v1: https://lore.kernel.org/all/TYUPR06MB621737D16F68B5ABD6CF5772D2272@TYUPR06MB6217.apcprd06.prod.outlook.com/

 drivers/usb/gadget/function/u_serial.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
index 0a8c05b2746b..53d9fc41acc5 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -579,9 +579,12 @@ static int gs_start_io(struct gs_port *port)
 		 * we didn't in gs_start_tx() */
 		tty_wakeup(port->port.tty);
 	} else {
-		gs_free_requests(ep, head, &port->read_allocated);
-		gs_free_requests(port->port_usb->in, &port->write_pool,
-			&port->write_allocated);
+		/* Free reqs only if we are still connected */
+		if (port->port_usb) {
+			gs_free_requests(ep, head, &port->read_allocated);
+			gs_free_requests(port->port_usb->in, &port->write_pool,
+				&port->write_allocated);
+		}
 		status = -EIO;
 	}
 
-- 
2.39.0