Re: [PATCH v5 3/7] fs: Add fd_install_received() wrapper for __fd_install_received()

Kees Cook
2020-06-18T20:13:15Z

On Thu, Jun 18, 2020 at 05:49:19AM +0000, Sargun Dhillon wrote:
> On Wed, Jun 17, 2020 at 03:03:23PM -0700, Kees Cook wrote:
> > [...]
> > static inline int fd_install_received_user(struct file *file, int __user *ufd,
> > unsigned int o_flags)
> > {
> > + if (ufd == NULL)
> > + return -EFAULT;
> Isn't this *technically* a behvaiour change? Nonetheless, I think this is a much better
> approach than forcing everyone to do null checking, and avoids at least one error case
> where the kernel installs FDs for SCM_RIGHTS, and they're not actualy usable.

So, the only behavior change I see is that the order of sanity checks is
changed.

The loop in scm_detach_fds() is:

for (i = 0; i < fdmax; i++) {
err = __scm_install_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
if (err < 0)
break;
}

Before, __scm_install_fd() does:

error = security_file_receive(file);
if (error)
return error;

new_fd = get_unused_fd_flags(o_flags);
if (new_fd < 0)
return new_fd;

error = put_user(new_fd, ufd);
if (error) {
put_unused_fd(new_fd);
return error;
}
...

After, fd_install_received_user() and __fd_install_received() does:

if (ufd == NULL)
return -EFAULT;
...
error = security_file_receive(file);
if (error)
return error;
...
new_fd = get_unused_fd_flags(o_flags);
if (new_fd < 0)
return new_fd;
...
error = put_user(new_fd, ufd);
if (error) {
put_unused_fd(new_fd);
return error;
}

i.e. if a caller attempts a receive that is rejected by LSM *and*
includes a NULL userpointer destination, they will get an EFAULT now
instead of an EPERM.

I struggle to imagine a situation where this could possible matter
(both fail, neither installs files). It is only the error code that
is different. I am comfortable making this change and seeing if anyone
screams. If they do, I can restore the v4 "ufd_required" way of doing it.

> Reviewed-by: Sargun Dhillon <sargun@sargun.me>

Thanks!

--
Kees Cook