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Message-ID: <20230417-irren-performanz-3c18de02e6e2@brauner>
Date: Mon, 17 Apr 2023 19:16:10 +0200
From: Christian Brauner <brauner@...nel.org>
To: Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
Cc: davem@...emloft.net, linux-kernel@...r.kernel.org,
netdev@...r.kernel.org, daniel@...earbox.net,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Leon Romanovsky <leon@...nel.org>,
David Ahern <dsahern@...nel.org>,
Arnd Bergmann <arnd@...db.de>,
Kees Cook <keescook@...omium.org>,
Kuniyuki Iwashima <kuniyu@...zon.com>,
Lennart Poettering <mzxreary@...inter.de>,
Luca Boccassi <bluca@...ian.org>, linux-arch@...r.kernel.org
Subject: Re: [PATCH net-next v4 1/4] scm: add SO_PASSPIDFD and SCM_PIDFD
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@...nel.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@...emloft.net>
> > > Cc: Eric Dumazet <edumazet@...gle.com>
> > > Cc: Jakub Kicinski <kuba@...nel.org>
> > > Cc: Paolo Abeni <pabeni@...hat.com>
> > > Cc: Leon Romanovsky <leon@...nel.org>
> > > Cc: David Ahern <dsahern@...nel.org>
> > > Cc: Arnd Bergmann <arnd@...db.de>
> > > Cc: Kees Cook <keescook@...omium.org>
> > > Cc: Christian Brauner <brauner@...nel.org>
> > > Cc: Kuniyuki Iwashima <kuniyu@...zon.com>
> > > Cc: Lennart Poettering <mzxreary@...inter.de>
> > > Cc: Luca Boccassi <bluca@...ian.org>
> > > Cc: linux-kernel@...r.kernel.org
> > > Cc: netdev@...r.kernel.org
> > > Cc: linux-arch@...r.kernel.org
> > > Tested-by: Luca Boccassi <bluca@...ian.org>
> > > Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.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.
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