From: "Steven Rostedt (Google)" rostedt@goodmis.org

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.l...

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: 7e8358edf503e ("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 | 4 ++++ 1 file changed, 4 insertions(+)

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c index 62524b20964e..bc86ffdb103b 100644 --- a/fs/tracefs/inode.c +++ b/fs/tracefs/inode.c @@ -215,6 +215,10 @@ static void set_gid(struct dentry *parent, kgid_t gid) 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);