On Wed, Mar 12, 2025 at 02:21:41PM -0700, Ryan Lee wrote:
> Currently, opening O_PATH file descriptors completely bypasses the LSM
> infrastructure. Invoking the LSM file_open hook for O_PATH fds will
> be necessary for e.g. mediating the fsmount() syscall.
>
> Signed-off-by: Ryan Lee <ryan.lee@xxxxxxxxxxxxx>
> ---
> fs/open.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/open.c b/fs/open.c
> index 30bfcddd505d..0f8542bf6cd4 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -921,8 +921,13 @@ static int do_dentry_open(struct file *f,
> if (unlikely(f->f_flags & O_PATH)) {
> f->f_mode = FMODE_PATH | FMODE_OPENED;
> file_set_fsnotify_mode(f, FMODE_NONOTIFY);
> f->f_op = &empty_fops;
> - return 0;
> + /*
> + * do_o_path in fs/namei.c unconditionally invokes path_put
> + * after this function returns, so don't path_put the path
> + * upon LSM rejection of O_PATH opening
> + */
> + return security_file_open(f);
Unconditional path_put() in do_o_path() has nothing to do with that -
what gets dropped there is the reference acquired there; the reference
acquired (and not dropped) here is the one that went into ->f_path.
Since you are leaving FMODE_OPENED set, you will have __fput() drop
that reference.
Basically, you are simulating behaviour on the O_DIRECT open of
something that does not support O_DIRECT - return an error, with
->f_path and FMODE_OPENED left in place.
Said that, what I do not understand is the point of that exercise -
why does LSM need to veto anything for those and why is security_file_open()
the right place for such checks?
The second part is particularly interesting...
