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

[Kees Cook]

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(-)
> > 
> > diff --git a/fs/file.c b/fs/file.c
> > index abb8b7081d7a..5afd76fca8c2 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -18,6 +18,9 @@
> >  #include <linux/bitops.h>
> >  #include <linux/spinlock.h>
> >  #include <linux/rcupdate.h>
> > +#include <net/sock.h>
> > +#include <net/netprio_cgroup.h>
> > +#include <net/cls_cgroup.h>
> >  
> >  unsigned int sysctl_nr_open __read_mostly = 1024*1024;
> >  unsigned int sysctl_nr_open_min = BITS_PER_LONG;
> > @@ -931,6 +934,38 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
> >  	return err;
> >  }
> >  
> > +/*
> > + * File Receive - Receive a file from another process
> > + *
> > + * This function is designed to receive files from other tasks. It encapsulates
> > + * logic around security and cgroups. The file descriptor provided must be a
> > + * freshly allocated (unused) file descriptor.
> > + *
> > + * This helper does not consume a reference to the file, so the caller must put
> > + * their reference.
> > + *
> > + * Returns 0 upon success.
> > + */
> > +int file_receive(int fd, struct file *file)
> 
> 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)
{
	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