Re: [PATCH v4 02/11] fs: Move __scm_install_fd() to __fd_install_received()

Kees Cook
2020-06-18T20:05:22Z

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@chromium.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