x86/amd_node: Fix PCI device reference counting in amd_smn_init()grafted

The local "root" pointer is a temporary variable used during the device
search. Therefore, refcount related to the search iterators should be cleaned
up after the search is complete.

Use the __free() cleanup macro to ensure the refcount is decremented when the
temporary pointer goes out of scope.

Additionally, increment the refcount when caching a root pointer. This ensures
the in-use refcount is separate from the temporary search refcounting.

Finally, drop the redundant "root = NULL" before the second search loop. The
pci_get_class() iterator always decrements the refcount of its "from"
argument, so the first loop can only fall through with "root" already NULL.

Fixes: 0a4b61d9c2e4 ("x86/amd_node: Fix AMD root device caching")
Closes: https://sashiko.dev/#/patchset/20260806160159.230453-1-jason.andryuk%40amd.com
Reported-by: Sashiko <sashiko-bot@kernel.org>
Assisted-by: LLM
Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Cc: <stable@kernel.org>
Link: https://patch.msgid.link/20260903154325.74343-1-yazen.ghannam@amd.com

1 files changed, 110 insertions, 0 deletions

diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
new file mode 100644
index 000000000..6b2f7208b
--- /dev/null
+++ b/drivers/tty/ttynull.c
@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2019 Axis Communications AB
+ *
+ * Based on ttyprintk.c:
+ *  Copyright (C) 2010 Samo Pogacnik
+ */
+
+#include <linux/console.h>
+#include <linux/module.h>
+#include <linux/tty.h>
+
+static const struct tty_port_operations ttynull_port_ops;
+static struct tty_driver *ttynull_driver;
+static struct tty_port ttynull_port;
+
+static int ttynull_open(struct tty_struct *tty, struct file *filp)
+{
+	return tty_port_open(&ttynull_port, tty, filp);
+}
+
+static void ttynull_close(struct tty_struct *tty, struct file *filp)
+{
+	tty_port_close(&ttynull_port, tty, filp);
+}
+
+static void ttynull_hangup(struct tty_struct *tty)
+{
+	tty_port_hangup(&ttynull_port);
+}
+
+static ssize_t ttynull_write(struct tty_struct *tty, const u8 *buf,
+			     size_t count)
+{
+	return count;
+}
+
+static unsigned int ttynull_write_room(struct tty_struct *tty)
+{
+	return 65536;
+}
+
+static const struct tty_operations ttynull_ops = {
+	.open = ttynull_open,
+	.close = ttynull_close,
+	.hangup = ttynull_hangup,
+	.write = ttynull_write,
+	.write_room = ttynull_write_room,
+};
+
+static struct tty_driver *ttynull_device(struct console *c, int *index)
+{
+	*index = 0;
+	return ttynull_driver;
+}
+
+static struct console ttynull_console = {
+	.name = "ttynull",
+	.device = ttynull_device,
+};
+
+static int __init ttynull_init(void)
+{
+	struct tty_driver *driver;
+	int ret;
+
+	driver = tty_alloc_driver(1,
+		TTY_DRIVER_RESET_TERMIOS |
+		TTY_DRIVER_REAL_RAW |
+		TTY_DRIVER_UNNUMBERED_NODE);
+	if (IS_ERR(driver))
+		return PTR_ERR(driver);
+
+	tty_port_init(&ttynull_port);
+	ttynull_port.ops = &ttynull_port_ops;
+
+	driver->driver_name = "ttynull";
+	driver->name = "ttynull";
+	driver->type = TTY_DRIVER_TYPE_CONSOLE;
+	driver->init_termios = tty_std_termios;
+	driver->init_termios.c_oflag = OPOST | OCRNL | ONOCR | ONLRET;
+	tty_set_operations(driver, &ttynull_ops);
+	tty_port_link_device(&ttynull_port, driver, 0);
+
+	ret = tty_register_driver(driver);
+	if (ret < 0) {
+		tty_driver_kref_put(driver);
+		tty_port_destroy(&ttynull_port);
+		return ret;
+	}
+
+	ttynull_driver = driver;
+	register_console(&ttynull_console);
+
+	return 0;
+}
+
+static void __exit ttynull_exit(void)
+{
+	unregister_console(&ttynull_console);
+	tty_unregister_driver(ttynull_driver);
+	tty_driver_kref_put(ttynull_driver);
+	tty_port_destroy(&ttynull_port);
+}
+
+module_init(ttynull_init);
+module_exit(ttynull_exit);
+
+MODULE_DESCRIPTION("NULL TTY driver");
+MODULE_LICENSE("GPL v2");