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Date:   Thu, 18 Jun 2020 13:05:14 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-kernel@...r.kernel.org, Sargun Dhillon <sargun@...gun.me>,
        Christian Brauner <christian@...uner.io>,
        "David S. Miller" <davem@...emloft.net>,
        Christoph Hellwig <hch@....de>,
        Tycho Andersen <tycho@...ho.ws>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Aleksa Sarai <cyphar@...har.com>,
        Matt Denton <mpdenton@...gle.com>,
        Jann Horn <jannh@...gle.com>, Chris Palmer <palmer@...gle.com>,
        Robert Sesek <rsesek@...gle.com>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, Shuah Khan <shuah@...nel.org>,
        netdev@...r.kernel.org, containers@...ts.linux-foundation.org,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 02/11] fs: Move __scm_install_fd() to
 __fd_install_received()

On Thu, Jun 18, 2020 at 10:56:14AM +0200, Christian Brauner wrote:
> On Mon, Jun 15, 2020 at 08:25:15PM -0700, Kees Cook wrote:
> > In preparation for users of the "install a received file" logic outside
> > of net/ (pidfd and seccomp), relocate and rename __scm_install_fd() from
> > net/core/scm.c to __fd_install_received() in fs/file.c, and provide a
> > wrapper named fd_install_received_user(), as future patches will change
> > the interface to __fd_install_received().
> > 
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  fs/file.c            | 47 ++++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/file.h |  8 ++++++++
> >  include/net/scm.h    |  1 -
> >  net/compat.c         |  2 +-
> >  net/core/scm.c       | 32 +-----------------------------
> >  5 files changed, 57 insertions(+), 33 deletions(-)
> > 
> > diff --git a/fs/file.c b/fs/file.c
> > index abb8b7081d7a..fcfddae0d252 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -11,6 +11,7 @@
> >  #include <linux/export.h>
> >  #include <linux/fs.h>
> >  #include <linux/mm.h>
> > +#include <linux/net.h>
> >  #include <linux/sched/signal.h>
> >  #include <linux/slab.h>
> >  #include <linux/file.h>
> > @@ -18,6 +19,8 @@
> >  #include <linux/bitops.h>
> >  #include <linux/spinlock.h>
> >  #include <linux/rcupdate.h>
> > +#include <net/cls_cgroup.h>
> > +#include <net/netprio_cgroup.h>
> >  
> >  unsigned int sysctl_nr_open __read_mostly = 1024*1024;
> >  unsigned int sysctl_nr_open_min = BITS_PER_LONG;
> > @@ -931,6 +934,50 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
> >  	return err;
> >  }
> >  
> > +/**
> > + * __fd_install_received() - Install received file into file descriptor table
> > + *
> > + * @fd: fd to install into (if negative, a new fd will be allocated)
> > + * @file: struct file that was received from another process
> > + * @ufd_required: true to use @ufd for writing fd number to userspace
> > + * @ufd: __user pointer to write new fd number to
> > + * @o_flags: the O_* flags to apply to the new fd entry
> > + *
> > + * Installs a received file into the file descriptor table, with appropriate
> > + * checks and count updates. Optionally writes the fd number to userspace.
> > + *
> > + * Returns -ve on error.
> > + */
> > +int __fd_install_received(struct file *file, int __user *ufd, unsigned int o_flags)
> > +{
> > +	struct socket *sock;
> > +	int new_fd;
> > +	int error;
> > +
> > +	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;
> > +	}
> > +
> > +	/* Bump the usage count and install the file. */
> > +	sock = sock_from_file(file, &error);
> > +	if (sock) {
> > +		sock_update_netprioidx(&sock->sk->sk_cgrp_data);
> > +		sock_update_classid(&sock->sk->sk_cgrp_data);
> > +	}
> > +	fd_install(new_fd, get_file(file));
> > +	return 0;
> > +}
> > +
> >  static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
> >  {
> >  	int err = -EBADF;
> > diff --git a/include/linux/file.h b/include/linux/file.h
> > index 122f80084a3e..fe18a1a0d555 100644
> > --- a/include/linux/file.h
> > +++ b/include/linux/file.h
> > @@ -91,6 +91,14 @@ extern void put_unused_fd(unsigned int fd);
> >  
> >  extern void fd_install(unsigned int fd, struct file *file);
> >  
> > +extern int __fd_install_received(struct file *file, int __user *ufd,
> > +				 unsigned int o_flags);
> > +static inline int fd_install_received_user(struct file *file, int __user *ufd,
> > +					   unsigned int o_flags)
> > +{
> > +	return __fd_install_received(file, ufd, o_flags);
> > +}
> 
> Shouldn't this be the other way around such that
> fd_install_received_user() is the workhorse that has a "ufd" argument
> and fd_install_received() is the static inline function that doesn't?
> 
> extern int fd_install_received_user(struct file *file, int __user *ufd, unsigned int o_flags)
> static inline int fd_install_received(struct file *file, unsigned int o_flags)
> {
> 	return fd_install_received_user(file, NULL, o_flags);
> }

So, I think it's all worked out in v5[1], so the helper argument handling
is better for the ufd case, as David pointed out earlier. (As in,
I think you're reacting to the same general problem here.)

> (So I'm on vacation this week some my reviews are selective and spotty
> but I promise to be back next week. :))

No worries!

-Kees

[1] https://lore.kernel.org/lkml/20200617220327.3731559-1-keescook@chromium.org/

-- 
Kees Cook

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