[PATCH net] af_unix: order MSG_PEEK references against garbage collection

[Yingjie Wang]

Commit e5b31d988a41 ("af_unix: Give up GC if MSG_PEEK intervened.")
made unix_peek_fpl() invalidate unix_peek_seq when gc_in_progress is
set.

scm_fp_dup() increments the received files' reference counts before
unix_peek_fpl() checks that flag, while unix_gc() sets the flag before
reading the same reference counts. The reference-count operations and
flag accesses are relaxed. spin_lock() provides acquire ordering, but
does not order the earlier flag store against the later file_count()
loads.

On weakly ordered machines, GC can therefore miss the reference acquired
by MSG_PEEK while MSG_PEEK misses gc_in_progress. unix_peek_seq then
remains unchanged, allowing GC to classify an SCC as dead and purge a
live receive queue.

Add smp_mb__after_atomic() after scm_fp_dup()'s reference increments and
smp_mb__after_spinlock() before GC samples the reference counts. This
ensures that either GC observes the new reference or MSG_PEEK observes
the active GC and invalidates the seqcount.

LKMM admits the stale/stale outcome without these barriers and forbids it
with them. An explicit AArch64 model gives the same result. The original
workqueue layout in that commit also lacked the PEEK-side ordering, so the
incomplete handshake dates to e5b31d988a41.

Fixes: e5b31d988a41 ("af_unix: Give up GC if MSG_PEEK intervened.")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Yingjie Wang <1075151112@xxxxxx>
---
 net/unix/garbage.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index da774f56ca648..c3ed00f52b04e 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -329,6 +329,14 @@ void unix_peek_fpl(struct scm_fp_list *fpl)
 	if (!fpl || !fpl->count_unix)
 		return;
 
+	/*
+	 * Pair with smp_mb__after_spinlock() in unix_gc(). get_file() in
+	 * scm_fp_dup() increments an atomic reference count, so this ensures
+	 * that either GC sees the new references or this thread sees
+	 * gc_in_progress and invalidates unix_peek_seq.
+	 */
+	smp_mb__after_atomic();
+
 	if (!READ_ONCE(gc_in_progress))
 		return;
 
@@ -614,6 +622,12 @@ static void unix_gc(struct work_struct *work)
 	WRITE_ONCE(gc_in_progress, true);
 
 	spin_lock(&unix_gc_lock);
+	/*
+	 * Pair with smp_mb__after_atomic() in unix_peek_fpl() to ensure that
+	 * either the file-count reads below see MSG_PEEK's increments or
+	 * MSG_PEEK sees gc_in_progress.
+	 */
+	smp_mb__after_spinlock();
 
 	if (unix_graph_state == UNIX_GRAPH_NOT_CYCLIC) {
 		spin_unlock(&unix_gc_lock);

base-commit: f4377bf85887f57b59abb5479f7660a81ffe4449
-- 
2.43.0