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From:   "Andrea Parri (Microsoft)" <parri.andrea@gmail.com>
To:     linux-kernel@vger.kernel.org
Cc:     "K . Y . Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, linux-hyperv@vger.kernel.org,
        Michael Kelley <mikelley@microsoft.com>,
        Juan Vazquez <juvazq@microsoft.com>,
        Saruhan Karademir <skarade@microsoft.com>,
        "Andrea Parri (Microsoft)" <parri.andrea@gmail.com>
Subject: [PATCH 5/6] Drivers: hv: vmbus: Resolve race condition in vmbus_onoffer_rescind()
Date:   Wed, 18 Nov 2020 15:36:48 +0100
Message-Id: <20201118143649.108465-6-parri.andrea@gmail.com>
In-Reply-To: <20201118143649.108465-1-parri.andrea@gmail.com>
References: <20201118143649.108465-1-parri.andrea@gmail.com>
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An erroneous or malicious host could send multiple rescind messages for
a same channel.  In vmbus_onoffer_rescind(), the guest maps the channel
ID to obtain a pointer to the channel object and it eventually releases
such object and associated data.  The host could time rescind messages
and lead to an use-after-free.  Add a new flag to the channel structure
to make sure that only one instance of vmbus_onoffer_rescind() can get
the reference to the channel object.

Reported-by: Juan Vazquez <juvazq@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
---
 drivers/hv/channel_mgmt.c | 12 ++++++++++++
 include/linux/hyperv.h    |  1 +
 2 files changed, 13 insertions(+)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index 4072fd1f22146..68950a1e4b638 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -1063,6 +1063,18 @@ static void vmbus_onoffer_rescind(struct vmbus_channel_message_header *hdr)
 
 	mutex_lock(&vmbus_connection.channel_mutex);
 	channel = relid2channel(rescind->child_relid);
+	if (channel != NULL) {
+		/*
+		 * Guarantee that no other instance of vmbus_onoffer_rescind()
+		 * has got a reference to the channel object.  Synchronize on
+		 * &vmbus_connection.channel_mutex.
+		 */
+		if (channel->rescind_ref) {
+			mutex_unlock(&vmbus_connection.channel_mutex);
+			return;
+		}
+		channel->rescind_ref = true;
+	}
 	mutex_unlock(&vmbus_connection.channel_mutex);
 
 	if (channel == NULL) {
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 2ea967bc17adf..f0d48a368f131 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -809,6 +809,7 @@ struct vmbus_channel {
 	u8 monitor_bit;
 
 	bool rescind; /* got rescind msg */
+	bool rescind_ref; /* got rescind msg, got channel reference */
 	struct completion rescind_event;
 
 	u32 ringbuffer_gpadlhandle;
-- 
2.25.1