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From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	stable@vger.kernel.org, Miklos Szeredi <mszeredi@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: [PATCH 4.19 05/17] af_unix: fix garbage collect vs MSG_PEEK
Date: Thu, 29 Jul 2021 15:54:06 +0200	[thread overview]
Message-ID: <20210729135137.432633743@linuxfoundation.org> (raw)
In-Reply-To: <20210729135137.260993951@linuxfoundation.org>

From: Miklos Szeredi <mszeredi@redhat.com>

commit cbcf01128d0a92e131bd09f1688fe032480b65ca upstream.

unix_gc() assumes that candidate sockets can never gain an external
reference (i.e.  be installed into an fd) while the unix_gc_lock is
held.  Except for MSG_PEEK this is guaranteed by modifying inflight
count under the unix_gc_lock.

MSG_PEEK does not touch any variable protected by unix_gc_lock (file
count is not), yet it needs to be serialized with garbage collection.
Do this by locking/unlocking unix_gc_lock:

 1) increment file count

 2) lock/unlock barrier to make sure incremented file count is visible
    to garbage collection

 3) install file into fd

This is a lock barrier (unlike smp_mb()) that ensures that garbage
collection is run completely before or completely after the barrier.

Cc: <stable@vger.kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/unix/af_unix.c |   51 +++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 49 insertions(+), 2 deletions(-)

--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1517,6 +1517,53 @@ out:
 	return err;
 }
 
+static void unix_peek_fds(struct scm_cookie *scm, struct sk_buff *skb)
+{
+	scm->fp = scm_fp_dup(UNIXCB(skb).fp);
+
+	/*
+	 * Garbage collection of unix sockets starts by selecting a set of
+	 * candidate sockets which have reference only from being in flight
+	 * (total_refs == inflight_refs).  This condition is checked once during
+	 * the candidate collection phase, and candidates are marked as such, so
+	 * that non-candidates can later be ignored.  While inflight_refs is
+	 * protected by unix_gc_lock, total_refs (file count) is not, hence this
+	 * is an instantaneous decision.
+	 *
+	 * Once a candidate, however, the socket must not be reinstalled into a
+	 * file descriptor while the garbage collection is in progress.
+	 *
+	 * If the above conditions are met, then the directed graph of
+	 * candidates (*) does not change while unix_gc_lock is held.
+	 *
+	 * Any operations that changes the file count through file descriptors
+	 * (dup, close, sendmsg) does not change the graph since candidates are
+	 * not installed in fds.
+	 *
+	 * Dequeing a candidate via recvmsg would install it into an fd, but
+	 * that takes unix_gc_lock to decrement the inflight count, so it's
+	 * serialized with garbage collection.
+	 *
+	 * MSG_PEEK is special in that it does not change the inflight count,
+	 * yet does install the socket into an fd.  The following lock/unlock
+	 * pair is to ensure serialization with garbage collection.  It must be
+	 * done between incrementing the file count and installing the file into
+	 * an fd.
+	 *
+	 * If garbage collection starts after the barrier provided by the
+	 * lock/unlock, then it will see the elevated refcount and not mark this
+	 * as a candidate.  If a garbage collection is already in progress
+	 * before the file count was incremented, then the lock/unlock pair will
+	 * ensure that garbage collection is finished before progressing to
+	 * installing the fd.
+	 *
+	 * (*) A -> B where B is on the queue of A or B is on the queue of C
+	 * which is on the queue of listening socket A.
+	 */
+	spin_lock(&unix_gc_lock);
+	spin_unlock(&unix_gc_lock);
+}
+
 static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool send_fds)
 {
 	int err = 0;
@@ -2142,7 +2189,7 @@ static int unix_dgram_recvmsg(struct soc
 		sk_peek_offset_fwd(sk, size);
 
 		if (UNIXCB(skb).fp)
-			scm.fp = scm_fp_dup(UNIXCB(skb).fp);
+			unix_peek_fds(&scm, skb);
 	}
 	err = (flags & MSG_TRUNC) ? skb->len - skip : size;
 
@@ -2383,7 +2430,7 @@ unlock:
 			/* It is questionable, see note in unix_dgram_recvmsg.
 			 */
 			if (UNIXCB(skb).fp)
-				scm.fp = scm_fp_dup(UNIXCB(skb).fp);
+				unix_peek_fds(&scm, skb);
 
 			sk_peek_offset_fwd(sk, chunk);
 



  parent reply	other threads:[~2021-07-29 13:56 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-29 13:54 [PATCH 4.19 00/17] 4.19.200-rc1 review Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 01/17] selftest: fix build error in tools/testing/selftests/vm/userfaultfd.c Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 02/17] iio: dac: ds4422/ds4424 drop of_node check Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 03/17] KVM: x86: determine if an exception has an error code only when injecting it Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 04/17] net: split out functions related to registering inflight socket files Greg Kroah-Hartman
2021-07-29 13:54 ` Greg Kroah-Hartman [this message]
2021-07-29 13:54 ` [PATCH 4.19 06/17] workqueue: fix UAF in pwq_unbound_release_workfn() Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 07/17] net/802/mrp: fix memleak in mrp_request_join() Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 08/17] net/802/garp: fix memleak in garp_request_join() Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 09/17] net: annotate data race around sk_ll_usec Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 10/17] sctp: move 198 addresses from unusable to private scope Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 11/17] hfs: add missing clean-up in hfs_fill_super Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 12/17] hfs: fix high memory mapping in hfs_bnode_read Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 13/17] hfs: add lock nesting notation to hfs_find_init Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 14/17] firmware: arm_scmi: Fix possible scmi_linux_errmap buffer overflow Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 15/17] firmware: arm_scmi: Fix range check for the maximum number of pending messages Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 16/17] cifs: fix the out of range assignment to bit fields in parse_server_interfaces Greg Kroah-Hartman
2021-07-29 13:54 ` [PATCH 4.19 17/17] ARM: dts: versatile: Fix up interrupt controller node names Greg Kroah-Hartman
2021-07-29 19:47 ` [PATCH 4.19 00/17] 4.19.200-rc1 review Pavel Machek
2021-07-29 22:50 ` Shuah Khan
2021-07-30 10:20 ` Sudip Mukherjee
2021-07-30 11:27 ` Naresh Kamboju
2021-07-31  4:42 ` Guenter Roeck

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