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

[Kees Cook]

On Fri, Jun 05, 2020 at 07:54:36AM +0000, Sargun Dhillon wrote:
> 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@xxxxxxxxx>
> > > > > Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > > > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > > > > Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
> > > > > Cc: Daniel Wagner <daniel.wagner@xxxxxxxxxxxx>
> > > > > Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> > > > > Cc: Jann Horn <jannh@xxxxxxxxxx>,
> > > > > Cc: John Fastabend <john.r.fastabend@xxxxxxxxx>
> > > > > Cc: Tejun Heo <tj@xxxxxxxxxx>
> > > > > Cc: Tycho Andersen <tycho@xxxxxxxx>
> > > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > > Cc: cgroups@xxxxxxxxxxxxxxx
> > > > > Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> > > > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > > > ---
> > > > >  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);

IIUC, this means we can get the fdt into a state of an open fd with a
NULL file... is that okay? That feels like something Al might rebel at.
:)

> 
> 	...
> }
> 
> /*
>  * 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;

This seems like we're duplicating some checks and missing others -- I
really think we need to do this using the existing primitives. But I'd
really like some review or commentary from Al. We can do this a bunch of
ways, and I'd like to know which way looks best to him. :(

> 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.

I still wonder if we should refactor SCM_RIGHTS to just delay put_user
failures, which would simplify a bunch. It's a behavior change, but it
seems from Al and Jann that it just shouldn't matter. (And if it does,
we'll hear about it.)

-- 
Kees Cook