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

From: Christian Brauner
Date: Tue Apr 18 2023 - 04:16:23 EST

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On Mon, Apr 17, 2023 at 07:43:19PM +0200, Eric Dumazet wrote:
> On Mon, Apr 17, 2023 at 7:16 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > > > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > > > > Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> > > > > Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> > > > > Cc: Leon Romanovsky <leon@xxxxxxxxxx>
> > > > > Cc: David Ahern <dsahern@xxxxxxxxxx>
> > > > > Cc: Arnd Bergmann <arnd@xxxxxxxx>
> > > > > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > > > > Cc: Christian Brauner <brauner@xxxxxxxxxx>
> > > > > Cc: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> > > > > Cc: Lennart Poettering <mzxreary@xxxxxxxxxxx>
> > > > > Cc: Luca Boccassi <bluca@xxxxxxxxxx>
> > > > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > > > Cc: netdev@xxxxxxxxxxxxxxx
> > > > > Cc: linux-arch@xxxxxxxxxxxxxxx
> > > > > Tested-by: Luca Boccassi <bluca@xxxxxxxxxx>
> > > > > Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> > > > > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@xxxxxxxxxxxxx>
> > > > > ---
> > > > > 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.
>
> This looks wrong to me.
>
> if put_cmsg() returns -EFAULT, then msg->msg_control and
> msg->msg_controllen were not changed.
>
> So the user application should not attempt to read this part of the
> control buffer, this could contain garbage.

Thanks for the review, Eric. That's reassuring.

I've done a bit of container related networking before but I'm fumbling
my way through the reviews here. So any additional reviews here would be
very helpful.

Next message: Laurent Pinchart: "Re: [PATCH] media: nxp: ignore unused suspend operations"
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In reply to: Eric Dumazet: "Re: [PATCH net-next v4 1/4] scm: add SO_PASSPIDFD and SCM_PIDFD"
Next in thread: Christian Brauner: "Re: [PATCH net-next v4 1/4] scm: add SO_PASSPIDFD and SCM_PIDFD"
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