[PATCH net v2 0/2] af_unix: fix inq_len accounting and consumed OOB skb handling in unix_stream_read_skb()

[PATCH net v2 0/2] af_unix: fix inq_len accounting and consumed OOB skb handling in unix_stream_read_skb()

[Hui Peng]

This series addresses two OOB skb handling bugs in AF_UNIX stream sockets:

1. Decrement u->inq_len by 1 when manage_oob() unlinks an unread OOB skb,
   preventing u->inq_len from remaining permanently inflated and causing
   SIOCINQ / FIONREAD to report a stale positive byte count on an empty
   socket.
2. Skip zero-length consumed OOB skbs (after releasing SCM rights via
   unix_orphan_scm()) and pull UNIXCB(skb).consumed bytes via pskb_pull()
   in unix_stream_read_skb(), preventing BPF sockmap from re-delivering
   already consumed OOB bytes or partially consumed skb prefixes.

Changes in v2:
- Split into a 2-patch series as requested by Kuniyuki Iwashima.
- Update Fixes: tag to f4e1fb04c123 ("af_unix: Use cached value for
  SOCK_STREAM in unix_inq_len().").
- Call unix_orphan_scm(sk, skb) before consume_skb(skb) and use
  pskb_pull() instead of skb_pull() to safely handle non-linear paged skbs
  as noted by Sashiko.

Hui Peng (2):
  af_unix: decrement u->inq_len when skipping unread OOB skb in manage_oob()
  af_unix: skip consumed OOB skb and pull consumed bytes in unix_stream_read_skb()

 net/unix/af_unix.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

[PATCH net v2 1/2] af_unix: decrement u->inq_len when skipping unread OOB skb in manage_oob()

[Hui Peng]

When a normal in-band read on an AF_UNIX stream socket (without MSG_PEEK
and without SO_OOBINLINE) encounters an unread u->oob_skb in manage_oob(),
manage_oob() clears u->oob_skb, unlinks the 1-byte skb from
sk->sk_receive_queue, and drops it with SKB_DROP_REASON_UNIX_SKIP_OOB
without decrementing u->inq_len. Because queue_oob() incremented
u->inq_len by 1 when queuing the OOB skb, u->inq_len remains permanently
inflated by 1 byte for each skipped unread OOB skb, causing SIOCINQ /
FIONREAD to report a stale positive byte count on an empty socket.

Decrement u->inq_len by 1 when unlinking the unread OOB skb in
manage_oob().

Tested in QEMU against Linux 7.3.0-rc3 by sending three 1-byte MSG_OOB
packets interleaved with normal stream data on an AF_UNIX SOCK_STREAM
socketpair and draining all in-band data via recv(): on the unfixed
kernel, ioctl(SIOCINQ) reports 3 on the empty socket; with this patch
applied, ioctl(SIOCINQ) reports 0.

Fixes: f4e1fb04c123 ("af_unix: Use cached value for SOCK_STREAM in unix_inq_len().")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v2:
- Split the manage_oob() and unix_stream_read_skb() fixes into a 2-patch
  series as requested by Kuniyuki Iwashima.
- Update Fixes: tag to f4e1fb04c123 ("af_unix: Use cached value for
  SOCK_STREAM in unix_inq_len().") and clarify the commit message as
  noted by Sashiko.

 net/unix/af_unix.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 42cffeafc8c1..1770af3c2684 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2849,6 +2849,7 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sk_buff **last,
 		WRITE_ONCE(u->oob_skb, NULL);
 
 		if (!sock_flag(sk, SOCK_URGINLINE)) {
+			WRITE_ONCE(u->inq_len, u->inq_len - 1);
 			__skb_unlink(skb, &sk->sk_receive_queue);
 			unread_skb = skb;
 			skb = skb_peek(&sk->sk_receive_queue);
-- 
2.55.0.1082.g2b9226bbc0-goog

[PATCH net v2 2/2] af_unix: skip consumed OOB skb and pull consumed bytes in unix_stream_read_skb()

[Hui Peng]

When an OOB byte is consumed via recv(MSG_OOB) in unix_stream_recv_urg(),
u->oob_skb is cleared to NULL and UNIXCB(oob_skb).consumed is incremented
to 1, but oob_skb remains on sk->sk_receive_queue (with unix_skb_len(skb)
== 0) to preserve the OOB mark until normal reads advance past it.
Similarly, a partial recv() in unix_stream_read_generic() advances
UNIXCB(skb).consumed without pulling the skb header and leaves the
partially consumed skb at the head of sk->sk_receive_queue.

If the socket is subsequently read via unix_stream_read_skb() (used by
BPF sockmap), unix_stream_read_skb() only checks skb == u->oob_skb
(which is only true for an unconsumed OOB skb) and ignores
UNIXCB(skb).consumed. As a result, a consumed OOB skb (unix_skb_len(skb)
== 0) is handed to recv_actor() and re-delivers the already consumed OOB
byte, and a partially consumed skb re-delivers its already consumed
prefix.

In unix_stream_read_skb(), skip and free zero-length consumed skbs (after
calling unix_orphan_scm() so SCM_RIGHTS fd accounting remains balanced)
and pull UNIXCB(skb).consumed bytes via pskb_pull() (which safely handles
both linear and non-linear paged skbs) before invoking recv_actor().

Tested in QEMU against Linux 7.3.0-rc3 with BPF_MAP_TYPE_SOCKMAP and a
BPF_SK_SKB_STREAM_VERDICT program:
1. Consuming a 1-byte MSG_OOB packet ("Z") before inserting the socket
   into sockmap and sending "HELLO" re-delivers "ZHELLO" on the unfixed
   kernel, whereas with this patch applied recv() receives "HELLO!".
2. Consuming a 3-byte prefix ("123") of "12345678" before inserting the
   socket into sockmap re-delivers "12345678" on the unfixed kernel,
   whereas with this patch applied recv() receives "45678".

Fixes: 77462de14a43 ("af_unix: Add read_sock for stream socket types")
Fixes: 314001f0bf92 ("af_unix: Add OOB support")
Fixes: 638f32604385 ("af_unix: Disable MSG_OOB handling for sockets in sockmap/sockhash")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v2:
- Split out from the manage_oob() fix into patch 2/2 as requested by
  Kuniyuki Iwashima.
- Call unix_orphan_scm(sk, skb) before consume_skb(skb) when dropping a
  zero-length consumed skb so u->scm_stat.nr_fds is decremented, and use
  pskb_pull() instead of skb_pull() to safely handle non-linear paged
  skbs without hitting BUG() in __skb_pull(), as noted by Sashiko.

 net/unix/af_unix.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 1770af3c2684..f5d64e7b5c1c 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2885,6 +2885,7 @@ static int unix_stream_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 		return err;
 
 	mutex_lock(&u->iolock);
+again:
 	spin_lock(&queue->lock);
 
 	skb = __skb_dequeue(queue);
@@ -2894,6 +2895,13 @@ static int unix_stream_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 		return -EAGAIN;
 	}
 
+	if (!unix_skb_len(skb)) {
+		spin_unlock(&queue->lock);
+		unix_orphan_scm(sk, skb);
+		consume_skb(skb);
+		goto again;
+	}
+
 	WRITE_ONCE(u->inq_len, u->inq_len - unix_skb_len(skb));
 
 #if IS_ENABLED(CONFIG_AF_UNIX_OOB)
@@ -2913,6 +2921,14 @@ static int unix_stream_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 
 	mutex_unlock(&u->iolock);
 
+	if (UNIXCB(skb).consumed) {
+		if (!pskb_pull(skb, UNIXCB(skb).consumed)) {
+			kfree_skb(skb);
+			return -ENOMEM;
+		}
+		UNIXCB(skb).consumed = 0;
+	}
+
 	return recv_actor(sk, skb);
 }
 
-- 
2.55.0.1082.g2b9226bbc0-goog

Re: [PATCH] af_unix: fix u->inq_len accounting in manage_oob() and unix_stream_read_skb()

[Kuniyuki Iwashima]

On Sat, Sep 19, 2026 at 3:17 PM Hui Peng <benquike@gmail.com> wrote:
>
> When an old OOB skb is replaced and discarded in manage_oob(), or when a
> fully consumed OOB skb is skipped in unix_stream_read_skb(), the
> discarded byte count is not subtracted from u->inq_len (or is subtracted
> inconsistently), causing SIOCINQ / FIONREAD to report stale positive
> byte counts on an empty AF_UNIX stream socket. Properly account
> u->inq_len when dropping or skipping consumed OOB skbs.
>
> Fixes: 314001f0bf92 ("af_unix: Add OOB support")
> Assisted-by: LLM

Please tell your LLM not to mix different fixes into one.

btw I'm tempted to remove OOB support entirely.

---
pw-bot: cr

[PATCH net v2 1/2] af_unix: decrement u->inq_len when skipping unread OOB skb in manage_oob()

[Hui Peng]

When a normal in-band read on an AF_UNIX stream socket (without MSG_PEEK
and without SO_OOBINLINE) encounters an unread u->oob_skb in manage_oob(),
manage_oob() clears u->oob_skb, unlinks the 1-byte skb from
sk->sk_receive_queue, and drops it with SKB_DROP_REASON_UNIX_SKIP_OOB
without decrementing u->inq_len. Because queue_oob() incremented
u->inq_len by 1 when queuing the OOB skb, u->inq_len remains permanently
inflated by 1 byte for each skipped unread OOB skb, causing SIOCINQ /
FIONREAD to report a stale positive byte count on an empty socket.

Decrement u->inq_len by 1 when unlinking the unread OOB skb in
manage_oob().

Tested in QEMU against Linux 7.3.0-rc3 by sending three 1-byte MSG_OOB
packets interleaved with normal stream data on an AF_UNIX SOCK_STREAM
socketpair and draining all in-band data via recv(): on the unfixed
kernel, ioctl(SIOCINQ) reports 3 on the empty socket; with this patch
applied, ioctl(SIOCINQ) reports 0.

Fixes: f4e1fb04c123 ("af_unix: Use cached value for SOCK_STREAM in unix_inq_len().")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v2:
- Split the manage_oob() and unix_stream_read_skb() fixes into a 2-patch
  series as requested by Kuniyuki Iwashima.
- Update Fixes: tag to f4e1fb04c123 ("af_unix: Use cached value for
  SOCK_STREAM in unix_inq_len().") and clarify the commit message as
  noted by Sashiko.

 net/unix/af_unix.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 42cffeafc8c1..1770af3c2684 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2849,6 +2849,7 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sk_buff **last,
 		WRITE_ONCE(u->oob_skb, NULL);
 
 		if (!sock_flag(sk, SOCK_URGINLINE)) {
+			WRITE_ONCE(u->inq_len, u->inq_len - 1);
 			__skb_unlink(skb, &sk->sk_receive_queue);
 			unread_skb = skb;
 			skb = skb_peek(&sk->sk_receive_queue);
-- 
2.49.0

[PATCH net v2 2/2] af_unix: skip consumed OOB skb and pull consumed bytes in unix_stream_read_skb()

[Hui Peng]

When an OOB byte is consumed via recv(MSG_OOB) in unix_stream_recv_urg(),
u->oob_skb is cleared to NULL and UNIXCB(oob_skb).consumed is incremented
to 1, but oob_skb remains on sk->sk_receive_queue (with unix_skb_len(skb)
== 0) to preserve the OOB mark until normal reads advance past it.
Similarly, a partial recv() in unix_stream_read_generic() advances
UNIXCB(skb).consumed without pulling the skb header and leaves the
partially consumed skb at the head of sk->sk_receive_queue.

If the socket is subsequently read via unix_stream_read_skb() (used by
BPF sockmap), unix_stream_read_skb() only checks skb == u->oob_skb
(which is only true for an unconsumed OOB skb) and ignores
UNIXCB(skb).consumed. As a result, a consumed OOB skb (unix_skb_len(skb)
== 0) is handed to recv_actor() and re-delivers the already consumed OOB
byte, and a partially consumed skb re-delivers its already consumed
prefix.

In unix_stream_read_skb(), skip and free zero-length consumed skbs (after
calling unix_orphan_scm() so SCM_RIGHTS fd accounting remains balanced)
and pull UNIXCB(skb).consumed bytes via pskb_pull() (which safely handles
both linear and non-linear paged skbs) before invoking recv_actor().

Tested in QEMU against Linux 7.3.0-rc3 with BPF_MAP_TYPE_SOCKMAP and a
BPF_SK_SKB_STREAM_VERDICT program:
1. Consuming a 1-byte MSG_OOB packet ("Z") before inserting the socket
   into sockmap and sending "HELLO" re-delivers "ZHELLO" on the unfixed
   kernel, whereas with this patch applied recv() receives "HELLO!".
2. Consuming a 3-byte prefix ("123") of "12345678" before inserting the
   socket into sockmap re-delivers "12345678" on the unfixed kernel,
   whereas with this patch applied recv() receives "45678".

Fixes: 77462de14a43 ("af_unix: Add read_sock for stream socket types")
Fixes: 314001f0bf92 ("af_unix: Add OOB support")
Fixes: 638f32604385 ("af_unix: Disable MSG_OOB handling for sockets in sockmap/sockhash")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v2:
- Split out from the manage_oob() fix into patch 2/2 as requested by
  Kuniyuki Iwashima.
- Call unix_orphan_scm(sk, skb) before consume_skb(skb) when dropping a
  zero-length consumed skb so u->scm_stat.nr_fds is decremented, and use
  pskb_pull() instead of skb_pull() to safely handle non-linear paged
  skbs without hitting BUG() in __skb_pull(), as noted by Sashiko.

 net/unix/af_unix.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 1770af3c2684..f5d64e7b5c1c 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2885,6 +2885,7 @@ static int unix_stream_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 		return err;
 
 	mutex_lock(&u->iolock);
+again:
 	spin_lock(&queue->lock);
 
 	skb = __skb_dequeue(queue);
@@ -2894,6 +2895,13 @@ static int unix_stream_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 		return -EAGAIN;
 	}
 
+	if (!unix_skb_len(skb)) {
+		spin_unlock(&queue->lock);
+		unix_orphan_scm(sk, skb);
+		consume_skb(skb);
+		goto again;
+	}
+
 	WRITE_ONCE(u->inq_len, u->inq_len - unix_skb_len(skb));
 
 #if IS_ENABLED(CONFIG_AF_UNIX_OOB)
@@ -2913,6 +2921,14 @@ static int unix_stream_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 
 	mutex_unlock(&u->iolock);
 
+	if (UNIXCB(skb).consumed) {
+		if (!pskb_pull(skb, UNIXCB(skb).consumed)) {
+			kfree_skb(skb);
+			return -ENOMEM;
+		}
+		UNIXCB(skb).consumed = 0;
+	}
+
 	return recv_actor(sk, skb);
 }
 
-- 
2.49.0

Re: [PATCH net v2 2/2] af_unix: skip consumed OOB skb and pull consumed bytes in unix_stream_read_skb()

[netdev-bot+sashiko]

Thank you for your contribution! Sashiko AI review found 1 potential
issue(s) to consider.

Critical: 0 · High: 1 · Medium: 0 (1) · Low: 0

If you believe none of the issues are legitimate, please reply to each
issue of Medium severity or higher and explain why the AI is wrong.
If any of them are legitimate and you plan to respin, please let patchwork
know by including "pw-bot: cr" as a separate line at the end of your reply
(one such reply per series is enough).

- [High] net/unix/af_unix.c, unix_stream_read_skb(): the newly added pull
  block frees the whole SKB and returns -ENOMEM when pskb_pull(skb,…

Pre-existing issues:
- [Medium] Unbalanced u->scm_stat.nr_fds accounting on AF_UNIX skb drop
  paths.

--

af_unix: skip consumed OOB skb and pull consumed bytes in unix_stream_read_skb()

This changes unix_stream_read_skb() to drop zero-length skbs left behind
after an OOB byte was consumed, and to pull UNIXCB(skb).consumed bytes off
a partially consumed skb before handing it to recv_actor().  Without this,
sockmap readers re-deliver bytes that a previous recv() already consumed.

> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 0903bd587696..a1c06b9995c7 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -2885,6 +2885,7 @@ static int unix_stream_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
>  		return err;
>  
>  	mutex_lock(&u->iolock);
> +again:
>  	spin_lock(&queue->lock);
>  
>  	skb = __skb_dequeue(queue);
> @@ -2894,6 +2895,13 @@ static int unix_stream_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
>  		return -EAGAIN;
>  	}
>  
> +	if (!unix_skb_len(skb)) {
> +		spin_unlock(&queue->lock);
> +		unix_orphan_scm(sk, skb);
> +		consume_skb(skb);
> +		goto again;
> +	}
> +

[Severity: Medium]
This isn't a bug introduced by this patch, but the new block here uses the
correct pairing (unix_orphan_scm() before consume_skb()) while several
neighbouring free paths for the same class of skb do not, so it highlights
an existing imbalance in u->scm_stat.nr_fds.

scm_stat_del(), reached only from unix_orphan_scm(), is the only place that
does the matching decrement:

net/unix/af_unix.c:scm_stat_del() {
	...
	if (unlikely(fp && fp->count)) {
		atomic_sub(fp->count, &u->scm_stat.nr_fds);
		unix_del_edges(fp);
	}
	...
}

unix_destruct_scm() -> unix_detach_fds() -> unix_destroy_fpl() releases the
files and the GC edges but never touches nr_fds.

The OOB drop branch a few lines below in this same function frees a skb that
may still carry UNIXCB(skb).fp without going through unix_orphan_scm():

	if (skb == u->oob_skb) {
		WRITE_ONCE(u->oob_skb, NULL);
		spin_unlock(&queue->lock);
		mutex_unlock(&u->iolock);

		kfree_skb_reason(skb, SKB_DROP_REASON_UNIX_SKIP_OOB);
		return -EAGAIN;
	}

manage_oob() does the same with a bare consume_skb(read_skb) /
kfree_skb_reason(unread_skb), and unix_stream_recv_urg() frees a zero-length
predecessor with consume_skb(read_skb).

unix_stream_recv_urg() is also what creates the zero-length-with-fds state:
it copies the OOB byte and bumps UNIXCB(oob_skb).consumed without detaching
fds, so the leftover skb can still hold SCM_RIGHTS charged earlier by
scm_stat_add() in queue_oob().

Should those paths also call unix_orphan_scm() so the nr_fds charge reported
as scm_fds: in /proc/<pid>/fdinfo does not stay permanently inflated?

>  	WRITE_ONCE(u->inq_len, u->inq_len - unix_skb_len(skb));
>  
>  #if IS_ENABLED(CONFIG_AF_UNIX_OOB)
> @@ -2913,6 +2921,14 @@ static int unix_stream_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
>  
>  	mutex_unlock(&u->iolock);
>  
> +	if (UNIXCB(skb).consumed) {
> +		if (!pskb_pull(skb, UNIXCB(skb).consumed)) {
> +			kfree_skb(skb);
> +			return -ENOMEM;
> +		}
> +		UNIXCB(skb).consumed = 0;
> +	}
> +

[Severity: High]
Can this kfree_skb() discard unread stream bytes?

At this point in unix_stream_read_skb() the skb has already been unlinked by
__skb_dequeue(), and u->inq_len has already been reduced:

	WRITE_ONCE(u->inq_len, u->inq_len - unix_skb_len(skb));

so once the skb is freed the remaining unix_skb_len(skb) bytes are neither
recoverable nor still accounted for in the queue length.

The commit message describes pskb_pull() as the way to "safely handle both
linear and non-linear paged skbs", but it can still fail.  For a pull past
the linear head it goes pskb_pull() -> pskb_may_pull() ->
__pskb_pull_tail() -> pskb_expand_head():

net/core/skbuff.c:__pskb_pull_tail() {
	...
	if (eat > 0 || skb_cloned(skb)) {
		if (pskb_expand_head(skb, 0, eat > 0 ? eat + 128 : 0,
				     GFP_ATOMIC))
			return NULL;
	}
	...
}

unix_stream_sendmsg() produces exactly the skbs that reach that branch:

net/unix/af_unix.c:unix_stream_sendmsg() {
	...
		data_len = max_t(int, 0, size - SKB_MAX_HEAD(0));
		data_len = min_t(size_t, size, PAGE_ALIGN(data_len));
		skb = sock_alloc_send_pskb(sk, size - data_len, data_len, ...);
	...
}

MSG_SPLICE_PAGES skbs have skb_headlen() == 0 entirely, so any partial
recv() leaves UNIXCB(skb).consumed beyond the linear head and forces the
fallible path with a GFP_ATOMIC expansion of up to the full frag size.

The return value also does not appear to reach anyone who can retry:

net/core/skmsg.c:sk_psock_verdict_data_ready() {
	...
	copied = ops->read_skb(sk, sk_psock_verdict_recv);
	if (copied >= 0) {
		...
		sk_psock_data_ready(sk, psock);
	}
	...
}

The -ENOMEM is swallowed, sk_psock_data_ready() is skipped for data still
queued behind the dropped skb, and the sockmap consumer sees a hole in an
ordered byte stream with no error reported to userspace.

Would it be better to restore the state on failure, for example
__skb_queue_head() plus adding unix_skb_len(skb) back to u->inq_len, and let
the caller come back later, instead of freeing the skb?

>  	return recv_actor(sk, skb);
>  }
>

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260921054823.1822122-1-benquike%40gmail.com