tracefs: Check for dentry->d_inode exists in set_gid()

If a getdents() is called on the tracefs directory but does not get all
the files, it can leave a "cursor" dentry in the d_subdirs list of tracefs
dentry. This cursor dentry does not have a d_inode for it. Before
referencing tracefs_inode from the dentry, the d_inode must first be
checked if it has content. If not, then it's not a tracefs_inode and can
be ignored.

The following caused a crash:

 #define getdents64(fd, dirp, count) syscall(SYS_getdents64, fd, dirp, count)
 #define BUF_SIZE 256
 #define TDIR "/tmp/file0"

 int main(void)
 {
	char buf[BUF_SIZE];
	int fd;
       	int n;

       	mkdir(TDIR, 0777);
	mount(NULL, TDIR, "tracefs", 0, NULL);
       	fd = openat(AT_FDCWD, TDIR, O_RDONLY);
       	n = getdents64(fd, buf, BUF_SIZE);
       	ret = mount(NULL, TDIR, NULL, MS_NOSUID|MS_REMOUNT|MS_RELATIME|MS_LAZYTIME,
		    "gid=1000");
	return 0;
 }

That's because the 256 BUF_SIZE was not big enough to read all the
dentries of the tracefs file system and it left a "cursor" dentry in the
subdirs of the tracefs root inode. Then on remounting with "gid=1000",
it would cause an iteration of all dentries which hit:

	ti = get_tracefs(dentry->d_inode);
	if (ti && (ti->flags & TRACEFS_EVENT_INODE))
		eventfs_update_gid(dentry, gid);

Which crashed because of the dereference of the cursor dentry which had a NULL
d_inode.

In the subdir loop of the dentry lookup of set_gid(), if a child has a
NULL d_inode, simply skip it.

Link: https://lore.kernel.org/all/20240102135637.3a21fb10@gandalf.local.home/
Link: https://lore.kernel.org/linux-trace-kernel/20240102151249.05da244d@gandalf.local.home

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Fixes: 7e8358edf5 ("eventfs: Fix file and directory uid and gid ownership")
Reported-by: "Ubisectech Sirius" <bugreport@ubisectech.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

fs/tracefs/inode.c

@@ -215,6 +215,10 @@ resume:
 		struct dentry *dentry = list_entry(tmp, struct dentry, d_child);
 		next = tmp->next;
 
+		/* Note, getdents() can add a cursor dentry with no inode */
+		if (!dentry->d_inode)
+			continue;
+
 		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
 
 		change_gid(dentry, gid);