Re: [PATCH v3 1/4] fs, net: Standardize on file_receive helper to move fds across processes

Sargun Dhillon
2020-06-05T07:54:44Z

On Thu, Jun 04, 2020 at 02:52:26PM +0200, Christian Brauner wrote:
> On Wed, Jun 03, 2020 at 07:22:57PM -0700, Kees Cook wrote:
> > On Thu, Jun 04, 2020 at 03:24:52AM +0200, Christian Brauner wrote:
> > > On Tue, Jun 02, 2020 at 06:10:41PM -0700, Sargun Dhillon wrote:
> > > > Previously there were two chunks of code where the logic to receive file
> > > > descriptors was duplicated in net. The compat version of copying
> > > > file descriptors via SCM_RIGHTS did not have logic to update cgroups.
> > > > Logic to change the cgroup data was added in:
> > > > commit 48a87cc26c13 ("net: netprio: fd passed in SCM_RIGHTS datagram not set correctly")
> > > > commit d84295067fc7 ("net: net_cls: fd passed in SCM_RIGHTS datagram not set correctly")
> > > >
> > > > This was not copied to the compat path. This commit fixes that, and thus
> > > > should be cherry-picked into stable.
> > > >
> > > > This introduces a helper (file_receive) which encapsulates the logic for
> > > > handling calling security hooks as well as manipulating cgroup information.
> > > > This helper can then be used other places in the kernel where file
> > > > descriptors are copied between processes
> > > >
> > > > I tested cgroup classid setting on both the compat (x32) path, and the
> > > > native path to ensure that when moving the file descriptor the classid
> > > > is set.
> > > >
> > > > Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> > > > Suggested-by: Kees Cook <keescook@chromium.org>
> > > > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > > > Cc: Christian Brauner <christian.brauner@ubuntu.com>
> > > > Cc: Daniel Wagner <daniel.wagner@bmw-carit.de>
> > > > Cc: David S. Miller <davem@davemloft.net>
> > > > Cc: Jann Horn <jannh@google.com>,
> > > > Cc: John Fastabend <john.r.fastabend@intel.com>
> > > > Cc: Tejun Heo <tj@kernel.org>
> > > > Cc: Tycho Andersen <tycho@tycho.ws>
> > > > Cc: stable@vger.kernel.org
> > > > Cc: cgroups@vger.kernel.org
> > > > Cc: linux-fsdevel@vger.kernel.org
> > > > Cc: linux-kernel@vger.kernel.org
> > > > ---
> > > > fs/file.c | 35 +++++++++++++++++++++++++++++++++++
> > > > include/linux/file.h | 1 +
> > > > net/compat.c | 10 +++++-----
> > > > net/core/scm.c | 14 ++++----------
> > > > 4 files changed, 45 insertions(+), 15 deletions(-)
> > > >
> > >
> > > This is all just a remote version of fd_install(), yet it deviates from
> > > fd_install()'s semantics and naming. That's not great imho. What about
> > > naming this something like:
> > >
> > > fd_install_received()
> > >
> > > and move the get_file() out of there so it has the same semantics as
> > > fd_install(). It seems rather dangerous to have a function like
> > > fd_install() that consumes a reference once it returned and another
> > > version of this that is basically the same thing but doesn't consume a
> > > reference because it takes its own. Seems an invitation for confusion.
> > > Does that make sense?
> >
> > We have some competing opinions on this, I guess. What I really don't
> > like is the copy/pasting of the get_unused_fd_flags() and
> > put_unused_fd() needed by (nearly) all the callers. If it's a helper, it
> > should help. Specifically, I'd like to see this:
> >
> > int file_receive(int fd, unsigned long flags, struct file *file,
> > int __user *fdptr)
>
> I still fail to see what this whole put_user() handling buys us at all
> and why this function needs to be anymore complicated then simply:
>
> fd_install_received(int fd, struct file *file)
> {
> security_file_receive(file);
>
> sock = sock_from_file(fd, &err);
> if (sock) {
> sock_update_netprioidx(&sock->sk->sk_cgrp_data);
> sock_update_classid(&sock->sk->sk_cgrp_data);
> }
>
> fd_install();
> return;
> }
>
> exactly like fd_install() but for received files.
>
> For scm you can fail somewhere in the middle of putting any number of
> file descriptors so you're left in a state with only a subset of
> requested file descriptors installed so it's not really useful there.
> And if you manage to install an fd but then fail to put_user() it
> userspace can simply check it's fds via proc and has to anyway on any
> scm message error. If you fail an scm message userspace better check
> their fds.
> For seccomp maybe but even there I doubt it and I still maintain that
> userspace screwing this up is on them which is how we do this most of
> the time. And for pidfd_getfd() this whole put_user() thing doesn't
> matter at all.
>
> It's much easier and clearer if we simply have a fd_install() -
> fd_install_received() parallelism where we follow an established
> convention. _But_ if that blocks you from making this generic enough
> then at least the replace_fd() vs fd_install() logic seems it shouldn't
> be in there.
>
> And the function name really needs to drive home the point that it
> installs an fd into the tasks fdtable no matter what version you go
> with. file_receive() is really not accurate enough for this at all.
>
> > {
> > struct socket *sock;
> > int err;
> >
> > err = security_file_receive(file);
> > if (err)
> > return err;
> >
> > if (fd < 0) {
> > /* Install new fd. */
> > int new_fd;
> >
> > err = get_unused_fd_flags(flags);
> > if (err < 0)
> > return err;
> > new_fd = err;
> >
> > /* Copy fd to any waiting user memory. */
> > if (fdptr) {
> > err = put_user(new_fd, fdptr);
> > if (err < 0) {
> > put_unused_fd(new_fd);
> > return err;
> > }
> > }
> > fd_install(new_fd, get_file(file));
> > fd = new_fd;
> > } else {
> > /* Replace existing fd. */
> > err = replace_fd(fd, file, flags);
> > if (err)
> > return err;
> > }
> >
> > /* Bump the cgroup usage counts. */
> > sock = sock_from_file(fd, &err);
> > if (sock) {
> > sock_update_netprioidx(&sock->sk->sk_cgrp_data);
> > sock_update_classid(&sock->sk->sk_cgrp_data);
> > }
> >
> > return fd;
> > }
> >
> > If everyone else *really* prefers keeping the get_unused_fd_flags() /
> > put_unused_fd() stuff outside the helper, then I guess I'll give up,
> > but I think it is MUCH cleaner this way -- all 4 users trim down lots
> > of code duplication.
> >
> > --
> > Kees Cook
How about this:

static int do_dup2(struct files_struct *files,
struct file *file, unsigned fd, unsigned flags)
__releases(&files->file_lock)
{
struct file *tofree;
struct fdtable *fdt;

...

/*
* New bit, allowing the file to be null. Doesn't have the same
* "sanity check" bits from __alloc_fd
*/
if (likely(file))
get_file(file);
rcu_assign_pointer(fdt->fd[fd], file);

__set_open_fd(fd, fdt);

...
}

/*
* File Receive - Receive a file from another process
*
* Encapsulates the logic to handle receiving a file from another task. It
* does not install the file descriptor. That is delegated to the user. If
* an error occurs that results in the file descriptor not being installed,
* they must put_unused_fd.
*
* fd should be >= 0 if you intend on replacing a file descriptor, or
* alternatively -1 if you want file_receive to allocate an FD for you
*
* Returns the fd number on success.
* Returns negative error code on failure.
*
*/
int file_receive(int fd, unsigned int flags, struct file *file)
{
int err;
struct socket *sock;
struct files_struct *files = current->files;

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

if (fd >= 0) {
if (fd >= rlimit(RLIMIT_NOFILE))
return -EBADF;

spin_lock(&files->file_lock);
err = expand_files(files, fd);
if (err < 0) {
goto out_unlock;
}

err = do_dup2(files, NULL, fd, flags);
if (err)
return err;
} else {
fd = get_unused_fd_flags(flags);
if (fd < 0)
return fd;
}

sock = sock_from_file(file, &err);
if (sock) {
sock_update_netprioidx(&sock->sk->sk_cgrp_data);
sock_update_classid(&sock->sk->sk_cgrp_data);
}

return fd;

out_unlock:
spin_unlock(&files->file_lock);
return err;
}

---

then the code in scm.c:
err = file_receive(-1, flags, fp[i]);
if (err < 0)
break;

new_fd = err;
err = put_user(new_fd, cmfptr);
if (err) {
put_unused_fd(new_fd);
break;
}

/* Bump the usage count and install the file. */
fd_install(new_fd, get_file(fp[i]));

And addfd:
ret = file_receive(addfd->fd, addfd->flags, addfd->file);
if (ret >= 0)
fd_install(ret, get_file(addfd->file));
addfd->ret = ret;

----

This way there is:
1. No "put_user" logic in file_receive
2. Minimal (single) branching logic, unless there's something in between
the file_receive and installing the FD, such as put_user.
3. Doesn't implement fd_install, so there's no ambiguity about it being
file_install_received vs. just the receive logic.