net: fix kernel deadlock with interface rename and netdev name retrieval.

When the kernel (compiled with CONFIG_PREEMPT=n) is performing the
rename of a network interface, it can end up waiting for a workqueue
to complete. If userland is able to invoke a SIOCGIFNAME ioctl or a
SO_BINDTODEVICE getsockopt in between, the kernel will deadlock due to
the fact that read_secklock_begin() will spin forever waiting for the
writer process (the one doing the interface rename) to update the
devnet_rename_seq sequence.

This patch fixes the problem by adding a helper (netdev_get_name())
and using it in the code handling the SIOCGIFNAME ioctl and
SO_BINDTODEVICE setsockopt.

The netdev_get_name() helper uses raw_seqcount_begin() to avoid
spinning forever, waiting for devnet_rename_seq->sequence to become
even. cond_resched() is used in the contended case, before retrying
the access to give the writer process a chance to finish.

The use of raw_seqcount_begin() will incur some unneeded work in the
reader process in the contended case, but this is better than
deadlocking the system.

Signed-off-by: Nicolas Schichan <nschichan@freebox.fr>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 34 insertions, 0 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index fc1e289397f..faebb398fb4 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -792,6 +792,40 @@ struct net_device *dev_get_by_index(struct net *net, int ifindex)
 EXPORT_SYMBOL(dev_get_by_index);
 
 /**
+ *	netdev_get_name - get a netdevice name, knowing its ifindex.
+ *	@net: network namespace
+ *	@name: a pointer to the buffer where the name will be stored.
+ *	@ifindex: the ifindex of the interface to get the name from.
+ *
+ *	The use of raw_seqcount_begin() and cond_resched() before
+ *	retrying is required as we want to give the writers a chance
+ *	to complete when CONFIG_PREEMPT is not set.
+ */
+int netdev_get_name(struct net *net, char *name, int ifindex)
+{
+	struct net_device *dev;
+	unsigned int seq;
+
+retry:
+	seq = raw_seqcount_begin(&devnet_rename_seq);
+	rcu_read_lock();
+	dev = dev_get_by_index_rcu(net, ifindex);
+	if (!dev) {
+		rcu_read_unlock();
+		return -ENODEV;
+	}
+
+	strcpy(name, dev->name);
+	rcu_read_unlock();
+	if (read_seqcount_retry(&devnet_rename_seq, seq)) {
+		cond_resched();
+		goto retry;
+	}
+
+	return 0;
+}
+
+/**
  *	dev_getbyhwaddr_rcu - find a device by its hardware address
  *	@net: the applicable net namespace
  *	@type: media type of device