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Date:   Mon, 17 Apr 2023 19:16:10 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Cc:     davem@davemloft.net, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, daniel@iogearbox.net,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Leon Romanovsky <leon@kernel.org>,
        David Ahern <dsahern@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Kees Cook <keescook@chromium.org>,
        Kuniyuki Iwashima <kuniyu@amazon.com>,
        Lennart Poettering <mzxreary@0pointer.de>,
        Luca Boccassi <bluca@debian.org>, linux-arch@vger.kernel.org
Subject: Re: [PATCH net-next v4 1/4] scm: add SO_PASSPIDFD and SCM_PIDFD
Message-ID: <20230417-irren-performanz-3c18de02e6e2@brauner>
References: <20230413133355.350571-1-aleksandr.mikhalitsyn@canonical.com>
 <20230413133355.350571-2-aleksandr.mikhalitsyn@canonical.com>
 <20230417-bahnanlagen-fixstern-bccf5afe6fa0@brauner>
 <CAEivzxcAeBPJYTVTN7cRik7AXo3y-Ox1yffPG2bvwxXsH2WWDg@mail.gmail.com>
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On Mon, Apr 17, 2023 at 06:01:16PM +0200, Aleksandr Mikhalitsyn wrote:
> 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.

So we discussed this a bit offline and I think there's still an issue.
If put_cmsg() fails

          if (msg->msg_control_is_user) {
                  struct cmsghdr __user *cm = msg->msg_control_user;

                  check_object_size(data, cmlen - sizeof(*cm), true);

                  if (!user_write_access_begin(cm, cmlen))
                          goto efault;

		  // This succeeds so cm->cmsg_len == sizeof(int)
                  unsafe_put_user(cmlen, &cm->cmsg_len, efault_end);

		  // This succeeds so cm->cmsg_level == SOL_SOCKET
                  unsafe_put_user(level, &cm->cmsg_level, efault_end);

		  // This succeeds so cm->cmsg_type == SCM_PIDFD
                  unsafe_put_user(type, &cm->cmsg_type, efault_end);

		  // This fails and leaves all bits set to 0
                  unsafe_copy_to_user(CMSG_USER_DATA(cm), data,
                                      cmlen - sizeof(*cm), efault_end);
                  user_write_access_end();

so now we hit

          if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) {
                  if (pidfd_file) {
                          put_unused_fd(pidfd);
                          fput(pidfd_file);
                  }

                  return;
          }

and return early. Afaict, userspace would now receive:

	if (cmsg && cmsg->cmsg_len == CMSG_LEN(sizeof(int)) &&
	    cmsg->cmsg_level == SOL_SOCKET &&
	    cmsg->cmsg_type == SCM_PIDFD) {
		memcpy(&pidfd, CMSG_DATA(cmsg), sizeof(int));

		// pidfd is now 0 which is a valid fd number
		// it'll likely refer to /dev/stdin or whatever and so
		// will fail or, worst case, 0 refers to another pidfd :)
		pidfd_send_signal(pidfd, SIGKILL);

so we need to address this. So one way I think that would solve this is:

diff --git a/net/core/scm.c b/net/core/scm.c
index 3cd7dd377e53..d1f4cd135c5a 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -236,9 +236,9 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)

                unsafe_put_user(cmlen, &cm->cmsg_len, efault_end);
                unsafe_put_user(level, &cm->cmsg_level, efault_end);
-               unsafe_put_user(type, &cm->cmsg_type, efault_end);
                unsafe_copy_to_user(CMSG_USER_DATA(cm), data,
                                    cmlen - sizeof(*cm), efault_end);
+               unsafe_put_user(type, &cm->cmsg_type, efault_end);
                user_write_access_end();
        } else {
                struct cmsghdr *cm = msg->msg_control;

such that we only copy cm->cmsg_type after we transfered the data.