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.
