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, 4 insertions, 15 deletions

diff --git a/net/core/dev_ioctl.c b/net/core/dev_ioctl.c
index 6cc0481faade..5b7d0e1d0664 100644
--- a/net/core/dev_ioctl.c
+++ b/net/core/dev_ioctl.c
@@ -19,9 +19,8 @@
 
 static int dev_ifname(struct net *net, struct ifreq __user *arg)
 {
-	struct net_device *dev;
 	struct ifreq ifr;
-	unsigned seq;
+	int error;
 
 	/*
 	 *	Fetch the caller's info block.
@@ -30,19 +29,9 @@ static int dev_ifname(struct net *net, struct ifreq __user *arg)
 	if (copy_from_user(&ifr, arg, sizeof(struct ifreq)))
 		return -EFAULT;
 
-retry:
-	seq = read_seqcount_begin(&devnet_rename_seq);
-	rcu_read_lock();
-	dev = dev_get_by_index_rcu(net, ifr.ifr_ifindex);
-	if (!dev) {
-		rcu_read_unlock();
-		return -ENODEV;
-	}
-
-	strcpy(ifr.ifr_name, dev->name);
-	rcu_read_unlock();
-	if (read_seqcount_retry(&devnet_rename_seq, seq))
-		goto retry;
+	error = netdev_get_name(net, ifr.ifr_name, ifr.ifr_ifindex);
+	if (error)
+		return error;
 
 	if (copy_to_user(arg, &ifr, sizeof(struct ifreq)))
 		return -EFAULT;