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[PATCH 4.6 12/31] neigh: Explicitly declare RCU-bh read side critical section in neigh_xmit()

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4.6-stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Barroso <dbarroso@fastly.com>

[ Upstream commit b560f03ddfb072bca65e9440ff0dc4f9b1d1f056 ]

neigh_xmit() expects to be called inside an RCU-bh read side critical
section, and while one of its two current callers gets this right, the
other one doesn't.

More specifically, neigh_xmit() has two callers, mpls_forward() and
mpls_output(), and while both callers call neigh_xmit() under
rcu_read_lock(), this provides sufficient protection for neigh_xmit()
only in the case of mpls_forward(), as that is always called from
softirq context and therefore doesn't need explicit BH protection,
while mpls_output() can be called from process context with softirqs
enabled.

When mpls_output() is called from process context, with softirqs
enabled, we can be preempted by a softirq at any time, and RCU-bh
considers the completion of a softirq as signaling the end of any
pending read-side critical sections, so if we do get a softirq
while we are in the part of neigh_xmit() that expects to be run inside
an RCU-bh read side critical section, we can end up with an unexpected
RCU grace period running right in the middle of that critical section,
making things go boom.

This patch fixes this impedance mismatch in the callee, by making
neigh_xmit() always take rcu_read_{,un}lock_bh() around the code that
expects to be treated as an RCU-bh read side critical section, as this
seems a safer option than fixing it in the callers.

Fixes: 4fd3d7d9e868f ("neigh: Add helper function neigh_xmit")
Signed-off-by: David Barroso <dbarroso@fastly.com>
Signed-off-by: Lennert Buytenhek <lbuytenhek@fastly.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Acked-by: Robert Shearman <rshearma@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/core/neighbour.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -2467,13 +2467,17 @@ int neigh_xmit(int index, struct net_dev
 		tbl = neigh_tables[index];
 		if (!tbl)
 			goto out;
+		rcu_read_lock_bh();
 		neigh = __neigh_lookup_noref(tbl, addr, dev);
 		if (!neigh)
 			neigh = __neigh_create(tbl, addr, dev, false);
 		err = PTR_ERR(neigh);
-		if (IS_ERR(neigh))
+		if (IS_ERR(neigh)) {
+			rcu_read_unlock_bh();
 			goto out_kfree_skb;
+		}
 		err = neigh->output(neigh, skb);
+		rcu_read_unlock_bh();
 	}
 	else if (index == NEIGH_LINK_TABLE) {
 		err = dev_hard_header(skb, dev, ntohs(skb->protocol),

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[PATCH 4.6 00/31] 4.6.4-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:20 +0200
  [PATCH 4.6 11/31] bpf, perf: delay release of BPF prog after grace period Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:20 +0200
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  [PATCH 4.6 23/31] usb: musb: host: correct cppi dma channel for isoch transfer Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
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  [PATCH 4.6 18/31] usb: quirks: Fix sorting Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
  [PATCH 4.6 27/31] xhci: Fix handling timeouted commands on hosts in weird states. Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
  [PATCH 4.6 16/31] crypto: user - re-add size check for CRYPTO_MSG_GETALG Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
  [PATCH 4.6 30/31] usb: host: ehci-tegra: Grab the correct UTMI pads reset Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
  [PATCH 4.6 19/31] usb: quirks: Add no-lpm quirk for Acer C120 LED Projector Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
  [PATCH 4.6 21/31] usb: musb: Stop bulk endpoint while queue is rotated Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
  [PATCH 4.6 14/31] crypto: vmx - Increase priority of aes-cbc cipher Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
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  [PATCH 4.6 31/31] usb: dwc3: exynos: Fix deferred probing storm. Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
  [PATCH 4.6 17/31] USB: uas: Fix slave queue_depth not being set Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
  [PATCH 4.6 13/31] AX.25: Close socket connection on session completion Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
  [PATCH 4.6 12/31] neigh: Explicitly declare RCU-bh read side critical section in neigh_xmit() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
  [PATCH 4.6 28/31] USB: mos7720: delete parport Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
  [PATCH 4.6 15/31] crypto: ux500 - memmove the right size Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 03:40 +0200
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  Re: [PATCH 4.6 00/31] 4.6.4-stable review Heinz Diehl <htd+ml@fritha.org> - 2016-07-07 14:50 +0200
    Re: [PATCH 4.6 00/31] 4.6.4-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-07 21:20 +0200
  Re: [PATCH 4.6 00/31] 4.6.4-stable review Guenter Roeck <linux@roeck-us.net> - 2016-07-07 15:40 +0200
  Re: [PATCH 4.6 00/31] 4.6.4-stable review Shuah Khan <shuahkh@osg.samsung.com> - 2016-07-08 05:50 +0200
    Re: [PATCH 4.6 00/31] 4.6.4-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-09 07:20 +0200

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