af_unix: Fix uninit-value in __unix_walk_scc()

KMSAN reported uninit-value access in __unix_walk_scc() [1].

In the list_for_each_entry_reverse() loop, when the vertex's index
equals it's scc_index, the loop uses the variable vertex as a
temporary variable that points to a vertex in scc. And when the loop
is finished, the variable vertex points to the list head, in this case
scc, which is a local variable on the stack (more precisely, it's not
even scc and might underflow the call stack of __unix_walk_scc():
container_of(&scc, struct unix_vertex, scc_entry)).

However, the variable vertex is used under the label prev_vertex. So
if the edge_stack is not empty and the function jumps to the
prev_vertex label, the function will access invalid data on the
stack. This causes the uninit-value access issue.

Fix this by introducing a new temporary variable for the loop.

[1]
BUG: KMSAN: uninit-value in __unix_walk_scc net/unix/garbage.c:478 [inline]
BUG: KMSAN: uninit-value in unix_walk_scc net/unix/garbage.c:526 [inline]
BUG: KMSAN: uninit-value in __unix_gc+0x2589/0x3c20 net/unix/garbage.c:584
 __unix_walk_scc net/unix/garbage.c:478 [inline]
 unix_walk_scc net/unix/garbage.c:526 [inline]
 __unix_gc+0x2589/0x3c20 net/unix/garbage.c:584
 process_one_work kernel/workqueue.c:3231 [inline]
 process_scheduled_works+0xade/0x1bf0 kernel/workqueue.c:3312
 worker_thread+0xeb6/0x15b0 kernel/workqueue.c:3393
 kthread+0x3c4/0x530 kernel/kthread.c:389
 ret_from_fork+0x6e/0x90 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

Uninit was stored to memory at:
 unix_walk_scc net/unix/garbage.c:526 [inline]
 __unix_gc+0x2adf/0x3c20 net/unix/garbage.c:584
 process_one_work kernel/workqueue.c:3231 [inline]
 process_scheduled_works+0xade/0x1bf0 kernel/workqueue.c:3312
 worker_thread+0xeb6/0x15b0 kernel/workqueue.c:3393
 kthread+0x3c4/0x530 kernel/kthread.c:389
 ret_from_fork+0x6e/0x90 arch/x86/kernel/process.c:147
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

Local variable entries created at:
 ref_tracker_free+0x48/0xf30 lib/ref_tracker.c:222
 netdev_tracker_free include/linux/netdevice.h:4058 [inline]
 netdev_put include/linux/netdevice.h:4075 [inline]
 dev_put include/linux/netdevice.h:4101 [inline]
 update_gid_event_work_handler+0xaa/0x1b0 drivers/infiniband/core/roce_gid_mgmt.c:813

CPU: 1 PID: 12763 Comm: kworker/u8:31 Not tainted 6.10.0-rc4-00217-g35bb670d65fc torvalds#32
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-2.fc40 04/01/2014
Workqueue: events_unbound __unix_gc

Fixes: 3484f06 ("af_unix: Detect Strongly Connected Components.")
Reported-by: syzkaller <syzkaller@googlegroups.com>
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20240702160428.10153-1-syoshida@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Shigeru Yoshida

net/unix/garbage.c

@@ -476,6 +476,7 @@ static void __unix_walk_scc(struct unix_vertex *vertex, unsigned long *last_inde
 	}
 
 	if (vertex->index == vertex->scc_index) {
+		struct unix_vertex *v;
 		struct list_head scc;
 		bool scc_dead = true;
 
@@ -486,15 +487,15 @@ static void __unix_walk_scc(struct unix_vertex *vertex, unsigned long *last_inde
 		 */
 		__list_cut_position(&scc, &vertex_stack, &vertex->scc_entry);
 
-		list_for_each_entry_reverse(vertex, &scc, scc_entry) {
+		list_for_each_entry_reverse(v, &scc, scc_entry) {
 			/* Don't restart DFS from this vertex in unix_walk_scc(). */
-			list_move_tail(&vertex->entry, &unix_visited_vertices);
+			list_move_tail(&v->entry, &unix_visited_vertices);
 
 			/* Mark vertex as off-stack. */
-			vertex->index = unix_vertex_grouped_index;
+			v->index = unix_vertex_grouped_index;
 
 			if (scc_dead)
-				scc_dead = unix_vertex_dead(vertex);
+				scc_dead = unix_vertex_dead(v);
 		}
 
 		if (scc_dead)