[PATCH 6.1.y] usb: gadget: f_tcm: Fix NULL pointer dereferences in nexus handling

[Sasha Levin <sashal@kernel.org>]

From: Jiasheng Jiang <jiashengjiangcool@gmail.com>

[ Upstream commit b9fde507355342a2d64225d582dc8b98ff5ecb19 ]

The `tpg->tpg_nexus` pointer in the USB Target driver is dynamically
managed and tied to userspace configuration via ConfigFS. It can be
NULL if the USB host sends requests before the nexus is fully
established or immediately after it is dropped.

Currently, functions like `bot_submit_command()` and the data
transfer paths retrieve `tv_nexus = tpg->tpg_nexus` and immediately
dereference `tv_nexus->tvn_se_sess` without any validation. If a
malicious or misconfigured USB host sends a BOT (Bulk-Only Transport)
command during this race window, it triggers a NULL pointer
dereference, leading to a kernel panic (local DoS).

This exposes an inconsistent API usage within the module, as peer
functions like `usbg_submit_command()` and `bot_send_bad_response()`
correctly implement a NULL check for `tv_nexus` before proceeding.

Fix this by bringing consistency to the nexus handling. Add the
missing `if (!tv_nexus)` checks to the vulnerable BOT command and
request processing paths, aborting the command gracefully with an
error instead of crashing the system.

Fixes: c52661d60f63 ("usb-gadget: Initial merge of target module for UASP + BOT")
Cc: stable <stable@kernel.org>
Signed-off-by: Jiasheng Jiang <jiashengjiangcool@gmail.com>
Reviewed-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://patch.msgid.link/20260219023834.17976-1-jiashengjiangcool@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/usb/gadget/function/f_tcm.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/usb/gadget/function/f_tcm.c b/drivers/usb/gadget/function/f_tcm.c
index 55a81ad6837b6..2325d04db263d 100644
--- a/drivers/usb/gadget/function/f_tcm.c
+++ b/drivers/usb/gadget/function/f_tcm.c
@@ -1032,6 +1032,13 @@ static void usbg_cmd_work(struct work_struct *work)
 	se_cmd = &cmd->se_cmd;
 	tpg = cmd->fu->tpg;
 	tv_nexus = tpg->tpg_nexus;
+	if (!tv_nexus) {
+		struct usb_gadget *gadget = fuas_to_gadget(cmd->fu);
+
+		dev_err(&gadget->dev, "Missing nexus, ignoring command\n");
+		return;
+	}
+
 	dir = get_cmd_dir(cmd->cmd_buf);
 	if (dir < 0) {
 		__target_init_cmd(se_cmd,
@@ -1160,6 +1167,13 @@ static void bot_cmd_work(struct work_struct *work)
 	se_cmd = &cmd->se_cmd;
 	tpg = cmd->fu->tpg;
 	tv_nexus = tpg->tpg_nexus;
+	if (!tv_nexus) {
+		struct usb_gadget *gadget = fuas_to_gadget(cmd->fu);
+
+		dev_err(&gadget->dev, "Missing nexus, ignoring command\n");
+		return;
+	}
+
 	dir = get_cmd_dir(cmd->cmd_buf);
 	if (dir < 0) {
 		__target_init_cmd(se_cmd,
-- 
2.51.0