Re: [PATCH net-next v2 2/3] net: core: add getsockopt SO_PEERPIDFD

[Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>]

On Wed, Mar 22, 2023 at 4:35 PM Christian Brauner <brauner@kernel.org> wrote:
>
> On Tue, Mar 21, 2023 at 07:33:41PM +0100, Alexander Mikhalitsyn wrote:
> > Add SO_PEERPIDFD which allows to get pidfd of peer socket holder pidfd.
> > This thing is direct analog of SO_PEERCRED which allows to get plain PID.
> >
> > Cc: "David S. Miller" <davem@davemloft.net>
> > Cc: Eric Dumazet <edumazet@google.com>
> > Cc: Jakub Kicinski <kuba@kernel.org>
> > Cc: Paolo Abeni <pabeni@redhat.com>
> > Cc: Leon Romanovsky <leon@kernel.org>
> > Cc: David Ahern <dsahern@kernel.org>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Christian Brauner <brauner@kernel.org>
> > Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
> > Cc: Lennart Poettering <mzxreary@0pointer.de>
> > Cc: linux-kernel@vger.kernel.org
> > Cc: netdev@vger.kernel.org
> > Cc: linux-arch@vger.kernel.org
> > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
> > ---
> > v2:
> >       According to review comments from Kuniyuki Iwashima and Christian Brauner:
> >       - use pidfd_create(..) retval as a result
> >       - whitespace change
> > ---
> >  arch/alpha/include/uapi/asm/socket.h    |  1 +
> >  arch/mips/include/uapi/asm/socket.h     |  1 +
> >  arch/parisc/include/uapi/asm/socket.h   |  1 +
> >  arch/sparc/include/uapi/asm/socket.h    |  1 +
> >  include/uapi/asm-generic/socket.h       |  1 +
> >  net/core/sock.c                         | 21 +++++++++++++++++++++
> >  tools/include/uapi/asm-generic/socket.h |  1 +
> >  7 files changed, 27 insertions(+)
> >
> > diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> > index ff310613ae64..e94f621903fe 100644
> > --- a/arch/alpha/include/uapi/asm/socket.h
> > +++ b/arch/alpha/include/uapi/asm/socket.h
> > @@ -138,6 +138,7 @@
> >  #define SO_RCVMARK           75
> >
> >  #define SO_PASSPIDFD         76
> > +#define SO_PEERPIDFD         77
> >
> >  #if !defined(__KERNEL__)
> >
> > diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
> > index 762dcb80e4ec..60ebaed28a4c 100644
> > --- a/arch/mips/include/uapi/asm/socket.h
> > +++ b/arch/mips/include/uapi/asm/socket.h
> > @@ -149,6 +149,7 @@
> >  #define SO_RCVMARK           75
> >
> >  #define SO_PASSPIDFD         76
> > +#define SO_PEERPIDFD         77
> >
> >  #if !defined(__KERNEL__)
> >
> > diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
> > index df16a3e16d64..be264c2b1a11 100644
> > --- a/arch/parisc/include/uapi/asm/socket.h
> > +++ b/arch/parisc/include/uapi/asm/socket.h
> > @@ -130,6 +130,7 @@
> >  #define SO_RCVMARK           0x4049
> >
> >  #define SO_PASSPIDFD         0x404A
> > +#define SO_PEERPIDFD         0x404B
> >
> >  #if !defined(__KERNEL__)
> >
> > diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
> > index 6e2847804fea..682da3714686 100644
> > --- a/arch/sparc/include/uapi/asm/socket.h
> > +++ b/arch/sparc/include/uapi/asm/socket.h
> > @@ -131,6 +131,7 @@
> >  #define SO_RCVMARK               0x0054
> >
> >  #define SO_PASSPIDFD             0x0055
> > +#define SO_PEERPIDFD             0x0056
> >
> >  #if !defined(__KERNEL__)
> >
> > diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
> > index b76169fdb80b..8ce8a39a1e5f 100644
> > --- a/include/uapi/asm-generic/socket.h
> > +++ b/include/uapi/asm-generic/socket.h
> > @@ -133,6 +133,7 @@
> >  #define SO_RCVMARK           75
> >
> >  #define SO_PASSPIDFD         76
> > +#define SO_PEERPIDFD         77
> >
> >  #if !defined(__KERNEL__)
> >
> > diff --git a/net/core/sock.c b/net/core/sock.c
> > index 3f974246ba3e..85c269ca9d8a 100644
> > --- a/net/core/sock.c
> > +++ b/net/core/sock.c
> > @@ -1763,6 +1763,27 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
> >               goto lenout;
> >       }
> >
> > +     case SO_PEERPIDFD:
> > +     {
> > +             struct pid *peer_pid;
> > +             int pidfd;
> > +
> > +             if (len > sizeof(pidfd))
> > +                     len = sizeof(pidfd);
> > +
> > +             spin_lock(&sk->sk_peer_lock);
> > +             peer_pid = get_pid(sk->sk_peer_pid);
> > +             spin_unlock(&sk->sk_peer_lock);
> > +
> > +             pidfd = pidfd_create(peer_pid, 0);
> > +
> > +             put_pid(peer_pid);
> > +
> > +             if (copy_to_sockptr(optval, &pidfd, len))
> > +                     return -EFAULT;
>
> This leaks the pidfd. We could do:
>
>         if (copy_to_sockptr(optval, &pidfd, len)) {
>                 close_fd(pidfd);
>                 return -EFAULT;
>         }

Ah, my bad. Thanks for pointing this out!

>
> but it's a nasty anti-pattern to install the fd in the caller's fdtable
> and then close it again. So let's avoid it if we can. Since you can only
> set one socket option per setsockopt() sycall we should be able to
> reserve an fd and pidfd_file, do the stuff that might fail, and then
> call fd_install. So that would roughly be:
>
>         peer_pid = get_pid(sk->sk_peer_pid);
>         pidfd_file = pidfd_file_create(peer_pid, 0, &pidfd);
>         f (copy_to_sockptr(optval, &pidfd, len))
>                return -EFAULT;
>         goto lenout:
>
>         .
>         .
>         .
>
> lenout:
>         if (copy_to_sockptr(optlen, &len, sizeof(int)))
>                 return -EFAULT;
>
>         // Made it safely, install pidfd now.
>         fd_install(pidfd, pidfd_file)
>
> (See below for the associated api I'm going to publish independent of
> this as kernel/fork.c and fanotify both could use it.)
>
> But now, let's look at net/socket.c there's another wrinkle. So let's say you
> have successfully installed the pidfd then it seems you can still fail later:
>
>         if (level == SOL_SOCKET)
>                 err = sock_getsockopt(sock, level, optname, optval, optlen);
>         else if (unlikely(!sock->ops->getsockopt))
>                 err = -EOPNOTSUPP;
>         else
>                 err = sock->ops->getsockopt(sock, level, optname, optval,
>                                             optlen);
>
>         if (!in_compat_syscall())
>                 err = BPF_CGROUP_RUN_PROG_GETSOCKOPT(sock->sk, level, optname,
>                                                      optval, optlen, max_optlen,
>                                                      err);
>
> out_put:
>         fput_light(sock->file, fput_needed);
>         return err;
>
> If the bpf hook returns an error we've placed an fd into the caller's sockopt
> buffer without their knowledge.

yes, so we need to postpone fd_install to the end of __sys_getsockopt.
I'll think about that.

>
> From 4fee16f0920308bee2531fd3b08484f607eb5830 Mon Sep 17 00:00:00 2001
> From: Christian Brauner <brauner@kernel.org>
> Date: Wed, 22 Mar 2023 15:59:02 +0100
> Subject: [PATCH 1/3] [HERE BE DRAGONS - DRAFT - __UNTESTED__] pid: add
>  pidfd_file_create()
>
> Reserve and fd and pidfile, do stuff that might fail, install fd when
> point of no return.
>
> [HERE BE DRAGONS - DRAFT - __UNTESTED__] pid: add pidfd_file_create()
>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  include/linux/pid.h |  1 +
>  kernel/pid.c        | 45 +++++++++++++++++++++++++++++++++------------
>  2 files changed, 34 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 343abf22092e..c486dbc4d7b6 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -80,6 +80,7 @@ extern struct pid *pidfd_pid(const struct file *file);
>  struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
>  struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags);
>  int pidfd_create(struct pid *pid, unsigned int flags);
> +struct file *pidfd_file_create(struct pid *pid, unsigned int flags, int *pidfd);
>
>  static inline struct pid *get_pid(struct pid *pid)
>  {
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 3fbc5e46b721..8d0924f1dbf6 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -576,6 +576,32 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
>         return task;
>  }
>
> +struct file *pidfd_file_create(struct pid *pid, unsigned int flags, int *pidfd)
> +{
> +       int fd;
> +       struct file *pidfile;
> +
> +       if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
> +               return ERR_PTR(-EINVAL);
> +
> +       if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
> +               return ERR_PTR(-EINVAL);
> +
> +       fd = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
> +       if (fd < 0)
> +               return ERR_PTR(fd);
> +
> +       pidfile = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
> +                                    flags | O_RDWR | O_CLOEXEC);
> +       if (IS_ERR(pidfile)) {
> +               put_unused_fd(fd);
> +               return pidfile;
> +       }
> +       get_pid(pid); /* held by pidfile now */
> +       *pidfd = fd;
> +       return pidfile;
> +}
> +
>  /**
>   * pidfd_create() - Create a new pid file descriptor.
>   *
> @@ -594,20 +620,15 @@ struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
>   */
>  int pidfd_create(struct pid *pid, unsigned int flags)
>  {
> -       int fd;
> +       int pidfd;
> +       struct file *pidfile;
>
> -       if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
> -               return -EINVAL;
> +       pidfile = pidfd_file_create(pid, flags, &pidfd);
> +       if (IS_ERR(pidfile))
> +               return PTR_ERR(pidfile);
>
> -       if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
> -               return -EINVAL;
> -
> -       fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
> -                             flags | O_RDWR | O_CLOEXEC);
> -       if (fd < 0)
> -               put_pid(pid);
> -
> -       return fd;
> +       fd_install(pidfd, pidfile);
> +       return pidfd;
>  }
>
>  /**
> --
> 2.34.1
>
> From c336f1c6cc39faa5aef4fbedd3c4f8eca51d8436 Mon Sep 17 00:00:00 2001
> From: Christian Brauner <brauner@kernel.org>
> Date: Wed, 22 Mar 2023 15:59:54 +0100
> Subject: [PATCH 2/3] [HERE BE DRAGONS - DRAFT - __UNTESTED__] fork: use
>  pidfd_file_create()
>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  kernel/fork.c | 11 +----------
>  1 file changed, 1 insertion(+), 10 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index f68954d05e89..c8dc78ee0a74 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2296,20 +2296,11 @@ static __latent_entropy struct task_struct *copy_process(
>          * if the fd table isn't shared).
>          */
>         if (clone_flags & CLONE_PIDFD) {
> -               retval = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
> -               if (retval < 0)
> -                       goto bad_fork_free_pid;
> -
> -               pidfd = retval;
> -
> -               pidfile = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
> -                                             O_RDWR | O_CLOEXEC);
> +               pidfile = pidfd_file_create(pid, O_RDWR | O_CLOEXEC, &pidfd);
>                 if (IS_ERR(pidfile)) {
> -                       put_unused_fd(pidfd);
>                         retval = PTR_ERR(pidfile);
>                         goto bad_fork_free_pid;
>                 }
> -               get_pid(pid);   /* held by pidfile now */
>
>                 retval = put_user(pidfd, args->pidfd);
>                 if (retval)
> --
> 2.34.1
>
> From 0897f68fe06a8777d8ec600fdc719143f76095b1 Mon Sep 17 00:00:00 2001
> From: Christian Brauner <brauner@kernel.org>
> Date: Wed, 22 Mar 2023 16:02:50 +0100
> Subject: [PATCH 3/3] [HERE BE DRAGONS - DRAFT - __UNTESTED__] fanotify: use
>  pidfd_file_create()
>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  fs/notify/fanotify/fanotify_user.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index 8f430bfad487..4a8db6b5f690 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -665,6 +665,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>         unsigned int pidfd_mode = info_mode & FAN_REPORT_PIDFD;
>         struct file *f = NULL;
>         int ret, pidfd = FAN_NOPIDFD, fd = FAN_NOFD;
> +       struct file *pidfd_file = NULL;
>
>         pr_debug("%s: group=%p event=%p\n", __func__, group, event);
>
> @@ -718,9 +719,11 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>                     !pid_has_task(event->pid, PIDTYPE_TGID)) {
>                         pidfd = FAN_NOPIDFD;
>                 } else {
> -                       pidfd = pidfd_create(event->pid, 0);
> -                       if (pidfd < 0)
> +                       pidfd_file = pidfd_file_create(event->pid, 0, &pidfd);
> +                       if (IS_ERR(pidfd_file)) {
>                                 pidfd = FAN_EPIDFD;
> +                               pidfd_file = NULL;
> +                       }
>                 }
>         }
>
> @@ -750,6 +753,8 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>
>         if (f)
>                 fd_install(fd, f);
> +       if (pidfd_file)
> +               fd_install(pidfd, pidfd_file);
>
>         return metadata.event_len;
>
> @@ -759,8 +764,10 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
>                 fput(f);
>         }
>
> -       if (pidfd >= 0)
> -               close_fd(pidfd);
> +       if (pidfd >= 0) {
> +               put_unused_fd(pidfd);
> +               fput(pidfd_file);
> +       }
>
>         return ret;
>  }
> --
> 2.34.1
>