[PATCH v1 bpf-next 4/5] bpf: Add kfunc to scrub SCM_RIGHTS at security_unix_may_send().

[PATCH v1 bpf-next 4/5] bpf: Add kfunc to scrub SCM_RIGHTS at security_unix_may_send().

[Kuniyuki Iwashima]

As Christian Brauner said [0], systemd calls cmsg_close_all() [1] after
each recvmsg() to close() unwanted file descriptors sent via SCM_RIGHTS.

However, this cannot work around the issue that close() for unwanted file
descriptors could block longer because the last fput() could occur on
the receiver side once sendmsg() with SCM_RIGHTS succeeds.

Also, even filtering by LSM at recvmsg() does not work for the same reason.

Thus, we need a better way to filter SCM_RIGHTS on the sender side.

Let's add a new kfunc to scrub all file descriptors from skb in
sendmsg().

This allows the receiver to keep recv()ing the bare data and disallows
the sender to impose the potential slowness of the last fput().

If necessary, we can add more granular filtering per file descriptor
after refactoring GC code and adding some fd-to-file helpers for BPF.

Sample:

SEC("lsm/unix_may_send")
int BPF_PROG(unix_scrub_scm_rights,
             struct socket *sock, struct socket *other, struct sk_buff *skb)
{
        struct unix_skb_parms *cb;

        if (skb && bpf_unix_scrub_fds(skb))
                return -EPERM;

        return 0;
}

Link: https://lore.kernel.org/netdev/20250502-fanden-unbeschadet-89973225255f@brauner/ #[0]
Link: https://github.com/systemd/systemd/blob/v257.5/src/basic/fd-util.c#L612-L628 #[1]
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
---
 include/net/af_unix.h |  1 +
 net/core/filter.c     | 19 +++++++++++++++++--
 net/unix/af_unix.c    | 15 +++++++++++++++
 3 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 1af1841b7601..109f92df2de2 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -58,4 +58,5 @@ struct unix_sock {
 #define unix_state_lock(s)	spin_lock(&unix_sk(s)->lock)
 #define unix_state_unlock(s)	spin_unlock(&unix_sk(s)->lock)
 
+int unix_scrub_fds(struct sk_buff *skb);
 #endif
diff --git a/net/core/filter.c b/net/core/filter.c
index 79cab4d78dc3..a9c46584da10 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -82,7 +82,7 @@
 #include <net/mptcp.h>
 #include <net/netfilter/nf_conntrack_bpf.h>
 #include <net/netkit.h>
-#include <linux/un.h>
+#include <net/af_unix.h>
 #include <net/xdp_sock_drv.h>
 #include <net/inet_dscp.h>
 
@@ -12153,6 +12153,11 @@ __bpf_kfunc int bpf_sock_ops_enable_tx_tstamp(struct bpf_sock_ops_kern *skops,
 	return 0;
 }
 
+__bpf_kfunc int bpf_unix_scrub_fds(struct sk_buff *skb)
+{
+	return unix_scrub_fds(skb);
+}
+
 __bpf_kfunc_end_defs();
 
 int bpf_dynptr_from_skb_rdonly(struct __sk_buff *skb, u64 flags,
@@ -12190,6 +12195,10 @@ BTF_KFUNCS_START(bpf_kfunc_check_set_sock_ops)
 BTF_ID_FLAGS(func, bpf_sock_ops_enable_tx_tstamp, KF_TRUSTED_ARGS)
 BTF_KFUNCS_END(bpf_kfunc_check_set_sock_ops)
 
+BTF_KFUNCS_START(bpf_kfunc_check_set_scm_rights)
+BTF_ID_FLAGS(func, bpf_unix_scrub_fds, KF_TRUSTED_ARGS)
+BTF_KFUNCS_END(bpf_kfunc_check_set_scm_rights)
+
 static const struct btf_kfunc_id_set bpf_kfunc_set_skb = {
 	.owner = THIS_MODULE,
 	.set = &bpf_kfunc_check_set_skb,
@@ -12215,6 +12224,11 @@ static const struct btf_kfunc_id_set bpf_kfunc_set_sock_ops = {
 	.set = &bpf_kfunc_check_set_sock_ops,
 };
 
+static const struct btf_kfunc_id_set bpf_kfunc_set_scm_rights = {
+	.owner = THIS_MODULE,
+	.set = &bpf_kfunc_check_set_scm_rights,
+};
+
 static int __init bpf_kfunc_init(void)
 {
 	int ret;
@@ -12234,7 +12248,8 @@ static int __init bpf_kfunc_init(void)
 	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_CGROUP_SOCK_ADDR,
 					       &bpf_kfunc_set_sock_addr);
 	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &bpf_kfunc_set_tcp_reqsk);
-	return ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SOCK_OPS, &bpf_kfunc_set_sock_ops);
+	ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SOCK_OPS, &bpf_kfunc_set_sock_ops);
+	return ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_LSM, &bpf_kfunc_set_scm_rights);
 }
 late_initcall(bpf_kfunc_init);
 
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 692cce579c89..4c088316dfb7 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1885,6 +1885,21 @@ static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool sen
 	return err;
 }
 
+int unix_scrub_fds(struct sk_buff *skb)
+{
+	struct scm_cookie scm = {};
+
+	if (skb->destructor != unix_destruct_scm)
+		return -EINVAL;
+
+	if (UNIXCB(skb).fp) {
+		unix_detach_fds(&scm, skb);
+		scm_fp_destroy(&scm);
+	}
+
+	return 0;
+}
+
 static bool unix_passcred_enabled(const struct socket *sock,
 				  const struct sock *other)
 {
-- 
2.49.0

Re: [PATCH v1 bpf-next 4/5] bpf: Add kfunc to scrub SCM_RIGHTS at security_unix_may_send().

[Alexei Starovoitov]

On Mon, May 5, 2025 at 3:00 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
>
> As Christian Brauner said [0], systemd calls cmsg_close_all() [1] after
> each recvmsg() to close() unwanted file descriptors sent via SCM_RIGHTS.
>
> However, this cannot work around the issue that close() for unwanted file
> descriptors could block longer because the last fput() could occur on
> the receiver side once sendmsg() with SCM_RIGHTS succeeds.
>
> Also, even filtering by LSM at recvmsg() does not work for the same reason.
>
> Thus, we need a better way to filter SCM_RIGHTS on the sender side.
>
> Let's add a new kfunc to scrub all file descriptors from skb in
> sendmsg().
>
> This allows the receiver to keep recv()ing the bare data and disallows
> the sender to impose the potential slowness of the last fput().
>
> If necessary, we can add more granular filtering per file descriptor
> after refactoring GC code and adding some fd-to-file helpers for BPF.
>
> Sample:
>
> SEC("lsm/unix_may_send")
> int BPF_PROG(unix_scrub_scm_rights,
>              struct socket *sock, struct socket *other, struct sk_buff *skb)
> {
>         struct unix_skb_parms *cb;
>
>         if (skb && bpf_unix_scrub_fds(skb))
>                 return -EPERM;
>
>         return 0;
> }

Any other programmability do you need there?

If not and above is all that is needed then what Jann proposed
sounds like better path to me:
"
I think the thorough fix would probably be to introduce a socket
option (controlled via setsockopt()) that already blocks the peer's
sendmsg().
"

Easier to operate and upriv process can use such setsockopt() too.

Re: [PATCH v1 bpf-next 4/5] bpf: Add kfunc to scrub SCM_RIGHTS at security_unix_may_send().

[Kuniyuki Iwashima]

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Mon, 5 May 2025 17:13:32 -0700
> On Mon, May 5, 2025 at 3:00 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> >
> > As Christian Brauner said [0], systemd calls cmsg_close_all() [1] after
> > each recvmsg() to close() unwanted file descriptors sent via SCM_RIGHTS.
> >
> > However, this cannot work around the issue that close() for unwanted file
> > descriptors could block longer because the last fput() could occur on
> > the receiver side once sendmsg() with SCM_RIGHTS succeeds.
> >
> > Also, even filtering by LSM at recvmsg() does not work for the same reason.
> >
> > Thus, we need a better way to filter SCM_RIGHTS on the sender side.
> >
> > Let's add a new kfunc to scrub all file descriptors from skb in
> > sendmsg().
> >
> > This allows the receiver to keep recv()ing the bare data and disallows
> > the sender to impose the potential slowness of the last fput().
> >
> > If necessary, we can add more granular filtering per file descriptor
> > after refactoring GC code and adding some fd-to-file helpers for BPF.
> >
> > Sample:
> >
> > SEC("lsm/unix_may_send")
> > int BPF_PROG(unix_scrub_scm_rights,
> >              struct socket *sock, struct socket *other, struct sk_buff *skb)
> > {
> >         struct unix_skb_parms *cb;
> >
> >         if (skb && bpf_unix_scrub_fds(skb))
> >                 return -EPERM;
> >
> >         return 0;
> > }
> 
> Any other programmability do you need there?

This is kind of PoC, and as Kumar mentioned, per-fd scrubbing
is ideal to cover the real use cases.

https://lore.kernel.org/netdev/CAP01T77STmncrPt=BsFfEY6SX1+oYNXhPeZ1HC9J=S2jhOwQoQ@mail.gmail.com/

for example:
https://uapi-group.org/kernel-features/#filtering-on-received-file-descriptors

"""
An alternative to the previous item could be if some form of filtering
could be enforced on the file descriptors suitable for enqueuing on the
AF_UNIX socket. i.e. allow filtering by superblock type or similar, so
that policies such as “only memfds are OK to be received” may be
expressed. (BPF?).
"""

I think Christian can add more scenarios if needed.



> 
> If not and above is all that is needed then what Jann proposed
> sounds like better path to me:
> "
> I think the thorough fix would probably be to introduce a socket
> option (controlled via setsockopt()) that already blocks the peer's
> sendmsg().
> "
> 
> Easier to operate and upriv process can use such setsockopt() too.

Re: [PATCH v1 bpf-next 4/5] bpf: Add kfunc to scrub SCM_RIGHTS at security_unix_may_send().

[Alexei Starovoitov]

On Mon, May 5, 2025 at 5:46 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
>
> From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Date: Mon, 5 May 2025 17:13:32 -0700
> > On Mon, May 5, 2025 at 3:00 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> > >
> > > As Christian Brauner said [0], systemd calls cmsg_close_all() [1] after
> > > each recvmsg() to close() unwanted file descriptors sent via SCM_RIGHTS.
> > >
> > > However, this cannot work around the issue that close() for unwanted file
> > > descriptors could block longer because the last fput() could occur on
> > > the receiver side once sendmsg() with SCM_RIGHTS succeeds.
> > >
> > > Also, even filtering by LSM at recvmsg() does not work for the same reason.
> > >
> > > Thus, we need a better way to filter SCM_RIGHTS on the sender side.
> > >
> > > Let's add a new kfunc to scrub all file descriptors from skb in
> > > sendmsg().
> > >
> > > This allows the receiver to keep recv()ing the bare data and disallows
> > > the sender to impose the potential slowness of the last fput().
> > >
> > > If necessary, we can add more granular filtering per file descriptor
> > > after refactoring GC code and adding some fd-to-file helpers for BPF.
> > >
> > > Sample:
> > >
> > > SEC("lsm/unix_may_send")
> > > int BPF_PROG(unix_scrub_scm_rights,
> > >              struct socket *sock, struct socket *other, struct sk_buff *skb)
> > > {
> > >         struct unix_skb_parms *cb;
> > >
> > >         if (skb && bpf_unix_scrub_fds(skb))
> > >                 return -EPERM;
> > >
> > >         return 0;
> > > }
> >
> > Any other programmability do you need there?
>
> This is kind of PoC, and as Kumar mentioned, per-fd scrubbing
> is ideal to cover the real use cases.
>
> https://lore.kernel.org/netdev/CAP01T77STmncrPt=BsFfEY6SX1+oYNXhPeZ1HC9J=S2jhOwQoQ@mail.gmail.com/
>
> for example:
> https://uapi-group.org/kernel-features/#filtering-on-received-file-descriptors

Fair enough.
Would be great to have them as selftests to make sure that advanced
use cases are actually working.

Re: [PATCH v1 bpf-next 4/5] bpf: Add kfunc to scrub SCM_RIGHTS at security_unix_may_send().

[Mickaël Salaün]

On Mon, May 05, 2025 at 05:13:32PM -0700, Alexei Starovoitov wrote:
> On Mon, May 5, 2025 at 3:00 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> >
> > As Christian Brauner said [0], systemd calls cmsg_close_all() [1] after
> > each recvmsg() to close() unwanted file descriptors sent via SCM_RIGHTS.
> >
> > However, this cannot work around the issue that close() for unwanted file
> > descriptors could block longer because the last fput() could occur on
> > the receiver side once sendmsg() with SCM_RIGHTS succeeds.
> >
> > Also, even filtering by LSM at recvmsg() does not work for the same reason.
> >
> > Thus, we need a better way to filter SCM_RIGHTS on the sender side.
> >
> > Let's add a new kfunc to scrub all file descriptors from skb in
> > sendmsg().
> >
> > This allows the receiver to keep recv()ing the bare data and disallows
> > the sender to impose the potential slowness of the last fput().
> >
> > If necessary, we can add more granular filtering per file descriptor
> > after refactoring GC code and adding some fd-to-file helpers for BPF.
> >
> > Sample:
> >
> > SEC("lsm/unix_may_send")
> > int BPF_PROG(unix_scrub_scm_rights,
> >              struct socket *sock, struct socket *other, struct sk_buff *skb)
> > {
> >         struct unix_skb_parms *cb;
> >
> >         if (skb && bpf_unix_scrub_fds(skb))
> >                 return -EPERM;
> >
> >         return 0;
> > }
> 
> Any other programmability do you need there?
> 
> If not and above is all that is needed then what Jann proposed
> sounds like better path to me:
> "
> I think the thorough fix would probably be to introduce a socket
> option (controlled via setsockopt()) that already blocks the peer's
> sendmsg().
> "
> 
> Easier to operate and upriv process can use such setsockopt() too.

Adding a flag with setsockopt() will enable any program to protect
themselves instead of requiring the capability to load an eBPF program.
For the systemd use case, I think a flag would be enough, and it would
benefit more than only/mainly systemd.

Another thing is that we should have a consistent user space error code
if passing file descriptors is denied, to avoid confusing senders, to
not silently ignore dropped file descriptors, and to let the sender know
that it can send again but without passed file descriptors or maybe with
a maximum number of file descriptors.  The ENFILE errno (file table
overflow) looks like a good candidate.

I guess both approaches are valuable, but this series should add this
new flag as well, and specify the expected error handling.

If we want to be able to handle legacy software not using this new flag,
we can leverage seccomp unotify.

Re: [PATCH v1 bpf-next 4/5] bpf: Add kfunc to scrub SCM_RIGHTS at security_unix_may_send().

[Christian Brauner]

On Mon, May 05, 2025 at 05:56:49PM -0700, Alexei Starovoitov wrote:
> On Mon, May 5, 2025 at 5:46 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> >
> > From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> > Date: Mon, 5 May 2025 17:13:32 -0700
> > > On Mon, May 5, 2025 at 3:00 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> > > >
> > > > As Christian Brauner said [0], systemd calls cmsg_close_all() [1] after
> > > > each recvmsg() to close() unwanted file descriptors sent via SCM_RIGHTS.
> > > >
> > > > However, this cannot work around the issue that close() for unwanted file
> > > > descriptors could block longer because the last fput() could occur on
> > > > the receiver side once sendmsg() with SCM_RIGHTS succeeds.
> > > >
> > > > Also, even filtering by LSM at recvmsg() does not work for the same reason.
> > > >
> > > > Thus, we need a better way to filter SCM_RIGHTS on the sender side.
> > > >
> > > > Let's add a new kfunc to scrub all file descriptors from skb in
> > > > sendmsg().
> > > >
> > > > This allows the receiver to keep recv()ing the bare data and disallows
> > > > the sender to impose the potential slowness of the last fput().
> > > >
> > > > If necessary, we can add more granular filtering per file descriptor
> > > > after refactoring GC code and adding some fd-to-file helpers for BPF.
> > > >
> > > > Sample:
> > > >
> > > > SEC("lsm/unix_may_send")
> > > > int BPF_PROG(unix_scrub_scm_rights,
> > > >              struct socket *sock, struct socket *other, struct sk_buff *skb)
> > > > {
> > > >         struct unix_skb_parms *cb;
> > > >
> > > >         if (skb && bpf_unix_scrub_fds(skb))
> > > >                 return -EPERM;
> > > >
> > > >         return 0;
> > > > }
> > >
> > > Any other programmability do you need there?
> >
> > This is kind of PoC, and as Kumar mentioned, per-fd scrubbing
> > is ideal to cover the real use cases.
> >
> > https://lore.kernel.org/netdev/CAP01T77STmncrPt=BsFfEY6SX1+oYNXhPeZ1HC9J=S2jhOwQoQ@mail.gmail.com/
> >
> > for example:
> > https://uapi-group.org/kernel-features/#filtering-on-received-file-descriptors
> 
> Fair enough.
> Would be great to have them as selftests to make sure that advanced
> use cases are actually working.

I think we should do both a socket option and the bpf fd filtering. They
can compliment each other. We should not force the use of bpf for this.
This is a very basic security guarantee we want that shouldn't require
the involvement of any LSM whatsoever.

Re: [PATCH v1 bpf-next 4/5] bpf: Add kfunc to scrub SCM_RIGHTS at security_unix_may_send().

[kernel test robot]

Hi Kuniyuki,

kernel test robot noticed the following build errors:

[auto build test ERROR on bpf-next/master]

url:    https://github.com/intel-lab-lkp/linux/commits/Kuniyuki-Iwashima/af_unix-Call-security_unix_may_send-in-sendmsg-for-all-socket-types/20250506-060219
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
patch link:    https://lore.kernel.org/r/20250505215802.48449-5-kuniyu%40amazon.com
patch subject: [PATCH v1 bpf-next 4/5] bpf: Add kfunc to scrub SCM_RIGHTS at security_unix_may_send().
config: csky-randconfig-001-20250509 (https://download.01.org/0day-ci/archive/20250509/202505092221.8wrWSFI7-lkp@intel.com/config)
compiler: csky-linux-gcc (GCC) 14.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250509/202505092221.8wrWSFI7-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202505092221.8wrWSFI7-lkp@intel.com/

All errors (new ones prefixed by >>):

   csky-linux-ld: net/core/filter.o: in function `bpf_unix_scrub_fds':
>> filter.c:(.text+0xc796): undefined reference to `unix_scrub_fds'
   csky-linux-ld: net/core/filter.o: in function `bpf_sock_destroy':
   filter.c:(.text+0xc7dc): undefined reference to `unix_scrub_fds'

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki