Re: [PATCH net-next v4 1/4] scm: add SO_PASSPIDFD and SCM_PIDFD

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

On Mon, Apr 17, 2023 at 5:18 PM Christian Brauner <brauner@kernel.org> wrote:
>
> On Thu, Apr 13, 2023 at 03:33:52PM +0200, Alexander Mikhalitsyn wrote:
> > Implement SCM_PIDFD, a new type of CMSG type analogical to SCM_CREDENTIALS,
> > but it contains pidfd instead of plain pid, which allows programmers not
> > to care about PID reuse problem.
> >
> > Idea comes from UAPI kernel group:
> > https://uapi-group.org/kernel-features/
> >
> > Big thanks to Christian Brauner and Lennart Poettering for productive
> > discussions about this.
> >
> > 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: Luca Boccassi <bluca@debian.org>
> > Cc: linux-kernel@vger.kernel.org
> > Cc: netdev@vger.kernel.org
> > Cc: linux-arch@vger.kernel.org
> > Tested-by: Luca Boccassi <bluca@debian.org>
> > Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
> > ---
> > v4:
> >       - fixed silent fd_install if writting of CMSG to the userspace fails (pointed by Christian)
> > 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    |  2 ++
> >  arch/mips/include/uapi/asm/socket.h     |  2 ++
> >  arch/parisc/include/uapi/asm/socket.h   |  2 ++
> >  arch/sparc/include/uapi/asm/socket.h    |  2 ++
> >  include/linux/net.h                     |  1 +
> >  include/linux/socket.h                  |  1 +
> >  include/net/scm.h                       | 39 +++++++++++++++++++++++--
> >  include/uapi/asm-generic/socket.h       |  2 ++
> >  net/core/sock.c                         | 11 +++++++
> >  net/mptcp/sockopt.c                     |  1 +
> >  net/unix/af_unix.c                      | 18 ++++++++----
> >  tools/include/uapi/asm-generic/socket.h |  2 ++
> >  12 files changed, 76 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> > index 739891b94136..ff310613ae64 100644
> > --- a/arch/alpha/include/uapi/asm/socket.h
> > +++ b/arch/alpha/include/uapi/asm/socket.h
> > @@ -137,6 +137,8 @@
> >
> >  #define SO_RCVMARK           75
> >
> > +#define SO_PASSPIDFD         76
> > +
> >  #if !defined(__KERNEL__)
> >
> >  #if __BITS_PER_LONG == 64
> > diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
> > index 18f3d95ecfec..762dcb80e4ec 100644
> > --- a/arch/mips/include/uapi/asm/socket.h
> > +++ b/arch/mips/include/uapi/asm/socket.h
> > @@ -148,6 +148,8 @@
> >
> >  #define SO_RCVMARK           75
> >
> > +#define SO_PASSPIDFD         76
> > +
> >  #if !defined(__KERNEL__)
> >
> >  #if __BITS_PER_LONG == 64
> > diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
> > index f486d3dfb6bb..df16a3e16d64 100644
> > --- a/arch/parisc/include/uapi/asm/socket.h
> > +++ b/arch/parisc/include/uapi/asm/socket.h
> > @@ -129,6 +129,8 @@
> >
> >  #define SO_RCVMARK           0x4049
> >
> > +#define SO_PASSPIDFD         0x404A
> > +
> >  #if !defined(__KERNEL__)
> >
> >  #if __BITS_PER_LONG == 64
> > diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
> > index 2fda57a3ea86..6e2847804fea 100644
> > --- a/arch/sparc/include/uapi/asm/socket.h
> > +++ b/arch/sparc/include/uapi/asm/socket.h
> > @@ -130,6 +130,8 @@
> >
> >  #define SO_RCVMARK               0x0054
> >
> > +#define SO_PASSPIDFD             0x0055
> > +
> >  #if !defined(__KERNEL__)
> >
> >
> > diff --git a/include/linux/net.h b/include/linux/net.h
> > index b73ad8e3c212..c234dfbe7a30 100644
> > --- a/include/linux/net.h
> > +++ b/include/linux/net.h
> > @@ -43,6 +43,7 @@ struct net;
> >  #define SOCK_PASSSEC         4
> >  #define SOCK_SUPPORT_ZC              5
> >  #define SOCK_CUSTOM_SOCKOPT  6
> > +#define SOCK_PASSPIDFD               7
> >
> >  #ifndef ARCH_HAS_SOCKET_TYPES
> >  /**
> > diff --git a/include/linux/socket.h b/include/linux/socket.h
> > index 13c3a237b9c9..6bf90f251910 100644
> > --- a/include/linux/socket.h
> > +++ b/include/linux/socket.h
> > @@ -177,6 +177,7 @@ static inline size_t msg_data_left(struct msghdr *msg)
> >  #define      SCM_RIGHTS      0x01            /* rw: access rights (array of int) */
> >  #define SCM_CREDENTIALS 0x02         /* rw: struct ucred             */
> >  #define SCM_SECURITY 0x03            /* rw: security label           */
> > +#define SCM_PIDFD    0x04            /* ro: pidfd (int)              */
> >
> >  struct ucred {
> >       __u32   pid;
> > diff --git a/include/net/scm.h b/include/net/scm.h
> > index 585adc1346bd..c67f765a165b 100644
> > --- a/include/net/scm.h
> > +++ b/include/net/scm.h
> > @@ -120,12 +120,44 @@ static inline bool scm_has_secdata(struct socket *sock)
> >  }
> >  #endif /* CONFIG_SECURITY_NETWORK */
> >
> > +static __inline__ void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm)
> > +{
> > +     struct file *pidfd_file = NULL;
> > +     int pidfd;
> > +
> > +     /*
> > +      * put_cmsg() doesn't return an error if CMSG is truncated,
> > +      * that's why we need to opencode these checks here.
> > +      */
> > +     if ((msg->msg_controllen <= sizeof(struct cmsghdr)) ||
> > +         (msg->msg_controllen - sizeof(struct cmsghdr)) < sizeof(int)) {
> > +             msg->msg_flags |= MSG_CTRUNC;
> > +             return;
>
> Hm, curious about this: We mark the message as truncated for SCM_PIDFD
> but if the same conditions were to apply for SCM_PASSCRED we don't mark
> the message as truncated. Am I reading this correct? And is so, you
> please briefly explain this difference?

Hi, Christian!

For SCM_CREDENTIALS we mark it too. Inside the put_cmsg function:
https://github.com/torvalds/linux/blob/6a8f57ae2eb07ab39a6f0ccad60c760743051026/net/core/scm.c#L225

The reason why I'm open-coding these checks is that I want to know
that the message
doesn't fit into the userspace buffer before doing pidfd_prepare and
other stuff and because
put_cmsg is not returning an error when message doesn't fit in the
userspace buffer and
we won't be able to properly do pidfd cleanup (put struct pid and fd index).

>
> > +     }
> > +
> > +     WARN_ON_ONCE(!scm->pid);
> > +     pidfd = pidfd_prepare(scm->pid, 0, &pidfd_file);
> > +
> > +     if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) {
>
> If the put_cmsg() of the pidfd fails userspace needs to be able to
> detect this. Otherwise they can't distinguish between the SCM_PIDFD
> value being zero because the put_cmsg() failed or put_cmsg() succeeded
> and the allocated fd nr was 0.

If pidfd_prepare fails then userspace will receive SCM_PIDFD message
with negative pidfd value.

>
> Looking at put_cmsg() it looks to me that userspace will receive a
> SCM_PIDFD message only if the put_cmsg() is completely successful. IIUC,
> then this change is fine.

Kind regards,
Alex