netdev - Re: [PATCH v3 net-next 8/9] af_unix: Introduce SO

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Message-ID: <68263fc569b91_25ebe529448@willemb.c.googlers.com.notmuch>
Date: Thu, 15 May 2025 15:25:57 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Willem de Bruijn <willemb@...gle.com>
Cc: Simon Horman <horms@...nel.org>, 
 Christian Brauner <brauner@...nel.org>, 
 Kuniyuki Iwashima <kuniyu@...zon.com>, 
 Kuniyuki Iwashima <kuni1840@...il.com>, 
 netdev@...r.kernel.org
Subject: Re: [PATCH v3 net-next 8/9] af_unix: Introduce SO_PASSRIGHTS.

Kuniyuki Iwashima wrote:
> As long as recvmsg() or recvmmsg() is used with cmsg, it is not
> possible to avoid receiving file descriptors via SCM_RIGHTS.
> 
> This behaviour has occasionally been flagged as problematic, as
> it can be (ab)used to trigger DoS during close(), for example, by
> passing a FUSE-controlled fd or a hung NFS fd.
> 
> For instance, as noted on the uAPI Group page [0], an untrusted peer
> could send a file descriptor pointing to a hung NFS mount and then
> close it.  Once the receiver calls recvmsg() with msg_control, the
> descriptor is automatically installed, and then the responsibility
> for the final close() now falls on the receiver, which may result
> in blocking the process for a long time.
> 
> Regarding this, 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 at all, because the final
> fput() may still occur on the receiver's 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 refuse SCM_RIGHTS at sendmsg().
> 
> Let's introduce SO_PASSRIGHTS to disable SCM_RIGHTS.
> 
> Note that this option is enabled by default for backward
> compatibility.
> 
> Link: https://uapi-group.org/kernel-features/#disabling-reception-of-scm_rights-for-af_unix-sockets #[0]
> Link: https://github.com/systemd/systemd/blob/v257.5/src/basic/fd-util.c#L612-L628 #[1]
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> ---
> v3: Return -EOPNOTSUPP for getsockopt()
> ---
>  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/net/sock.h                      |  4 +++-
>  include/uapi/asm-generic/socket.h       |  2 ++
>  net/core/sock.c                         | 14 ++++++++++++++
>  net/unix/af_unix.c                      | 22 ++++++++++++++++++++--
>  tools/include/uapi/asm-generic/socket.h |  2 ++
>  9 files changed, 49 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> index 3df5f2dd4c0f..8f1f18adcdb5 100644
> --- a/arch/alpha/include/uapi/asm/socket.h
> +++ b/arch/alpha/include/uapi/asm/socket.h
> @@ -150,6 +150,8 @@
>  
>  #define SO_RCVPRIORITY		82
>  
> +#define SO_PASSRIGHTS		83
> +
>  #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 22fa8f19924a..31ac655b7837 100644
> --- a/arch/mips/include/uapi/asm/socket.h
> +++ b/arch/mips/include/uapi/asm/socket.h
> @@ -161,6 +161,8 @@
>  
>  #define SO_RCVPRIORITY		82
>  
> +#define SO_PASSRIGHTS		83
> +
>  #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 96831c988606..1f2d5b7a7f5d 100644
> --- a/arch/parisc/include/uapi/asm/socket.h
> +++ b/arch/parisc/include/uapi/asm/socket.h
> @@ -142,6 +142,8 @@
>  #define SCM_DEVMEM_DMABUF	SO_DEVMEM_DMABUF
>  #define SO_DEVMEM_DONTNEED	0x4050
>  
> +#define SO_PASSRIGHTS		0x4051
> +
>  #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 5b464a568664..adcba7329386 100644
> --- a/arch/sparc/include/uapi/asm/socket.h
> +++ b/arch/sparc/include/uapi/asm/socket.h
> @@ -143,6 +143,8 @@
>  
>  #define SO_RCVPRIORITY           0x005b
>  
> +#define SO_PASSRIGHTS            0x005c
> +
>  #if !defined(__KERNEL__)
>  
>  
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 77232a098934..17fb6b8c4b6e 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -341,6 +341,7 @@ struct sk_filter;
>    *	@sk_scm_credentials: flagged by SO_PASSCRED to recv SCM_CREDENTIALS
>    *	@sk_scm_security: flagged by SO_PASSSEC to recv SCM_SECURITY
>    *	@sk_scm_pidfd: flagged by SO_PASSPIDFD to recv SCM_PIDFD
> +  *	@sk_scm_rights: flagged by SO_PASSRIGHTS to recv SCM_RIGHTS
>    *	@sk_scm_unused: unused flags for scm_recv()
>    *	@ns_tracker: tracker for netns reference
>    *	@sk_user_frags: xarray of pages the user is holding a reference on.
> @@ -534,7 +535,8 @@ struct sock {
>  		u8		sk_scm_credentials : 1,
>  				sk_scm_security : 1,
>  				sk_scm_pidfd : 1,
> -				sk_scm_unused : 5;
> +				sk_scm_rights : 1,
> +				sk_scm_unused : 4;
>  	};
>  	u8			sk_clockid;
>  	u8			sk_txtime_deadline_mode : 1,
> diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
> index aa5016ff3d91..f333a0ac4ee4 100644
> --- a/include/uapi/asm-generic/socket.h
> +++ b/include/uapi/asm-generic/socket.h
> @@ -145,6 +145,8 @@
>  
>  #define SO_RCVPRIORITY		82
>  
> +#define SO_PASSRIGHTS		83
> +
>  #if !defined(__KERNEL__)
>  
>  #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 381abf8f25b7..0cb52e590094 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1571,6 +1571,13 @@ int sk_setsockopt(struct sock *sk, int level, int optname,
>  			ret = -EOPNOTSUPP;
>  		break;
>  
> +	case SO_PASSRIGHTS:
> +		if (sk_is_unix(sk))
> +			sk->sk_scm_rights = valbool;
> +		else
> +			ret = -EOPNOTSUPP;
> +		break;
> +
>  	case SO_INCOMING_CPU:
>  		reuseport_update_incoming_cpu(sk, val);
>  		break;
> @@ -1879,6 +1886,13 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
>  		v.val = sk->sk_scm_pidfd;
>  		break;
>  
> +	case SO_PASSRIGHTS:
> +		if (!sk_is_unix(sk))
> +			return -EOPNOTSUPP;
> +
> +		v.val = sk->sk_scm_rights;

Same question about lockless reading of the field.