netdev - [PATCH net v3 1/4] net-sysfs: take the rtnl lock when storing xps

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Date:   Wed, 23 Dec 2020 22:23:20 +0100
From:   Antoine Tenart <atenart@...nel.org>
To:     davem@...emloft.net, kuba@...nel.org, alexander.duyck@...il.com
Cc:     Antoine Tenart <atenart@...nel.org>, netdev@...r.kernel.org,
        pabeni@...hat.com
Subject: [PATCH net v3 1/4] net-sysfs: take the rtnl lock when storing xps_cpus

Two race conditions can be triggered when storing xps cpus, resulting in
various oops and invalid memory accesses:

1. Calling netdev_set_num_tc while netif_set_xps_queue:

   - netif_set_xps_queue uses dev->tc_num as one of the parameters to
     compute the size of new_dev_maps when allocating it. dev->tc_num is
     also used to access the map, and the compiler may generate code to
     retrieve this field multiple times in the function.

   - netdev_set_num_tc sets dev->tc_num.

   If new_dev_maps is allocated using dev->tc_num and then dev->tc_num
   is set to a higher value through netdev_set_num_tc, later accesses to
   new_dev_maps in netif_set_xps_queue could lead to accessing memory
   outside of new_dev_maps; triggering an oops.

2. Calling netif_set_xps_queue while netdev_set_num_tc is running:

   2.1. netdev_set_num_tc starts by resetting the xps queues,
        dev->tc_num isn't updated yet.

   2.2. netif_set_xps_queue is called, setting up the map with the
        *old* dev->num_tc.

   2.3. netdev_set_num_tc updates dev->tc_num.

   2.4. Later accesses to the map lead to out of bound accesses and
        oops.

   A similar issue can be found with netdev_reset_tc.

One way of triggering this is to set an iface up (for which the driver
uses netdev_set_num_tc in the open path, such as bnx2x) and writing to
xps_cpus in a concurrent thread. With the right timing an oops is
triggered.

Both issues have the same fix: netif_set_xps_queue, netdev_set_num_tc
and netdev_reset_tc should be mutually exclusive. We do that by taking
the rtnl lock in xps_cpus_store.

Fixes: 184c449f91fe ("net: Add support for XPS with QoS via traffic classes")
Signed-off-by: Antoine Tenart <atenart@...nel.org>
---
 net/core/net-sysfs.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 999b70c59761..7cc15dec1717 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -1396,7 +1396,13 @@ static ssize_t xps_cpus_store(struct netdev_queue *queue,
 		return err;
 	}

+	if (!rtnl_trylock()) {
+		free_cpumask_var(mask);
+		return restart_syscall();
+	}
+
 	err = netif_set_xps_queue(dev, mask, index);
+	rtnl_unlock();

 	free_cpumask_var(mask);

-- 
2.29.2