netdev - [PATCH net 3/4] net-sysfs: take the rtnl lock when storing xps

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Message-Id: <20201217162521.1134496-4-atenart@kernel.org>
Date:   Thu, 17 Dec 2020 17:25:20 +0100
From:   Antoine Tenart <atenart@...nel.org>
To:     davem@...emloft.net, kuba@...nel.org
Cc:     Antoine Tenart <atenart@...nel.org>, netdev@...r.kernel.org,
        pabeni@...hat.com
Subject: [PATCH net 3/4] net-sysfs: take the rtnl lock when storing xps_rxqs

Callers to __netif_set_xps_queue should take the rtnl lock. Failing to
do so can lead to race conditions between netdev_set_num_tc and
__netif_set_xps_queue, triggering various oops:

- __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.

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_rxqs in a concurrent thread. With the right timing an oops is
triggered.

Fixes: 8af2c06ff4b1 ("net-sysfs: Add interface for Rx queue(s) map per Tx queue")
Signed-off-by: Antoine Tenart <atenart@...nel.org>
---
 net/core/net-sysfs.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 65886bfbf822..62ca2f2c0ee6 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -1499,10 +1499,17 @@ static ssize_t xps_rxqs_store(struct netdev_queue *queue, const char *buf,
 		return err;
 	}

+	if (!rtnl_trylock()) {
+		bitmap_free(mask);
+		return restart_syscall();
+	}
+
 	cpus_read_lock();
 	err = __netif_set_xps_queue(dev, mask, index, true);
 	cpus_read_unlock();

+	rtnl_unlock();
+
 	bitmap_free(mask);
 	return err ? : len;
 }
-- 
2.29.2