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Message-ID: <CAEivzxcAeBPJYTVTN7cRik7AXo3y-Ox1yffPG2bvwxXsH2WWDg@mail.gmail.com>
Date: Mon, 17 Apr 2023 18:01:16 +0200
From: Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To: Christian Brauner <brauner@...nel.org>
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 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.
>
> 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
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