linux-kernel - [PATCH 2.6.32 26/42] af_unix: fix a fatal race with bit fields

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Message-Id: <20160123141223.114779810@1wt.eu>
Date:	Sat, 23 Jan 2016 15:12:47 +0100
From:	Willy Tarreau <w@....eu>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	Ambrose Feinstein <ambrose@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	"David S. Miller" <davem@...emloft.net>,
	Ben Hutchings <ben@...adent.org.uk>, Willy Tarreau <w@....eu>
Subject: [PATCH 2.6.32 26/42] af_unix: fix a fatal race with bit fields

2.6.32-longterm review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <eric.dumazet@...il.com>

commit 60bc851ae59bfe99be6ee89d6bc50008c85ec75d upstream.

Using bit fields is dangerous on ppc64/sparc64, as the compiler [1]
uses 64bit instructions to manipulate them.
If the 64bit word includes any atomic_t or spinlock_t, we can lose
critical concurrent changes.

This is happening in af_unix, where unix_sk(sk)->gc_candidate/
gc_maybe_cycle/lock share the same 64bit word.

This leads to fatal deadlock, as one/several cpus spin forever
on a spinlock that will never be available again.

A safer way would be to use a long to store flags.
This way we are sure compiler/arch wont do bad things.

As we own unix_gc_lock spinlock when clearing or setting bits,
we can use the non atomic __set_bit()/__clear_bit().

recursion_level can share the same 64bit location with the spinlock,
as it is set only with this spinlock held.

[1] bug fixed in gcc-4.8.0 :
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52080

Reported-by: Ambrose Feinstein <ambrose@...gle.com>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Paul Mackerras <paulus@...ba.org>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
(cherry picked from commit 2ee9cbe7e7bfe2d36374288b818aa31b2c4981db)
[wt: adjusted context]
Signed-off-by: Willy Tarreau <w@....eu>
---
 include/net/af_unix.h |  5 +++--
 net/unix/garbage.c    | 12 ++++++------
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index c364711..faf1d6d 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -55,9 +55,10 @@ struct unix_sock {
 	struct list_head	link;
         atomic_long_t           inflight;
         spinlock_t		lock;
-	unsigned int		gc_candidate : 1;
-	unsigned int		gc_maybe_cycle : 1;
 	unsigned char		recursion_level;
+	unsigned long		gc_flags;
+#define UNIX_GC_CANDIDATE	0
+#define UNIX_GC_MAYBE_CYCLE	1
         wait_queue_head_t       peer_wait;
 	wait_queue_t		peer_wake;
 };
diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index cb72e91..de93193 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -195,7 +195,7 @@ static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
 					 * have been added to the queues after
 					 * starting the garbage collection
 					 */
-					if (u->gc_candidate) {
+					if (test_bit(UNIX_GC_CANDIDATE, &u->gc_flags)) {
 						hit = true;
 						func(u);
 					}
@@ -264,7 +264,7 @@ static void inc_inflight_move_tail(struct unix_sock *u)
 	 * of the list, so that it's checked even if it was already
 	 * passed over
 	 */
-	if (u->gc_maybe_cycle)
+	if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags))
 		list_move_tail(&u->link, &gc_candidates);
 }
 
@@ -325,8 +325,8 @@ void unix_gc(void)
 		BUG_ON(total_refs < inflight_refs);
 		if (total_refs == inflight_refs) {
 			list_move_tail(&u->link, &gc_candidates);
-			u->gc_candidate = 1;
-			u->gc_maybe_cycle = 1;
+			__set_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
+			__set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
 		}
 	}
 
@@ -354,7 +354,7 @@ void unix_gc(void)
 
 		if (atomic_long_read(&u->inflight) > 0) {
 			list_move_tail(&u->link, &not_cycle_list);
-			u->gc_maybe_cycle = 0;
+			__clear_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
 			scan_children(&u->sk, inc_inflight_move_tail, NULL);
 		}
 	}
@@ -366,7 +366,7 @@ void unix_gc(void)
 	 */
 	while (!list_empty(&not_cycle_list)) {
 		u = list_entry(not_cycle_list.next, struct unix_sock, link);
-		u->gc_candidate = 0;
+		__clear_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
 		list_move_tail(&u->link, &gc_inflight_list);
 	}
 
-- 
1.7.12.2.21.g234cd45.dirty