diff mbox

[v3] netpoll: Close race condition between poll_one_napi and napi_disable

Message ID 1443034678-1475-1-git-send-email-nhorman@redhat.com
State Accepted, archived
Delegated to: David Miller
Headers show

Commit Message

Neil Horman Sept. 23, 2015, 6:57 p.m. UTC 
Drivers might call napi_disable while not holding the napi instance poll_lock.
In those instances, its possible for a race condition to exist between
poll_one_napi and napi_disable.  That is to say, poll_one_napi only tests the
NAPI_STATE_SCHED bit to see if there is work to do during a poll, and as such
the following may happen:

CPU0				CPU1
ndo_tx_timeout			napi_poll_dev
 napi_disable			 poll_one_napi
  test_and_set_bit (ret 0)
				  test_bit (ret 1)
   reset adapter		   napi_poll_routine

If the adapter gets a tx timeout without a napi instance scheduled, its possible
for the adapter to think it has exclusive access to the hardware  (as the napi
instance is now scheduled via the napi_disable call), while the netpoll code
thinks there is simply work to do.  The result is parallel hardware access
leading to corrupt data structures in the driver, and a crash.

Additionaly, there is another, more critical race between netpoll and
napi_disable.  The disabled napi state is actually identical to the scheduled
state for a given napi instance.  The implication being that, if a napi instance
is disabled, a netconsole instance would see the napi state of the device as
having been scheduled, and poll it, likely while the driver was dong something
requiring exclusive access.  In the case above, its fairly clear that not having
the rings in a state ready to be polled will cause any number of crashes.

The fix should be pretty easy.  netpoll uses its own bit to indicate that that
the napi instance is in a state of being serviced by netpoll (NAPI_STATE_NPSVC).
We can just gate disabling on that bit as well as the sched bit.  That should
prevent netpoll from conducting a napi poll if we convert its set bit to a
test_and_set_bit operation to provide mutual exclusion

Change notes:
V2)
	Remove a trailing whtiespace
	Resubmit with proper subject prefix

V3)
	Clean up spacing nits

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: "David S. Miller" <davem@davemloft.net>
CC: jmaxwell@redhat.com
Tested-by: jmaxwell@redhat.com
---
 include/linux/netdevice.h |  1 +
 net/core/dev.c            |  2 ++
 net/core/netpoll.c        | 11 +++++++++--
 3 files changed, 12 insertions(+), 2 deletions(-)

Comments

David Miller Sept. 23, 2015, 9:33 p.m. UTC | #1 
From: Neil Horman <nhorman@redhat.com>
Date: Wed, 23 Sep 2015 14:57:58 -0400

> Drivers might call napi_disable while not holding the napi instance poll_lock.
> In those instances, its possible for a race condition to exist between
> poll_one_napi and napi_disable.  That is to say, poll_one_napi only tests the
> NAPI_STATE_SCHED bit to see if there is work to do during a poll, and as such
> the following may happen:
> 
> CPU0				CPU1
> ndo_tx_timeout			napi_poll_dev
>  napi_disable			 poll_one_napi
>   test_and_set_bit (ret 0)
> 				  test_bit (ret 1)
>    reset adapter		   napi_poll_routine
> 
> If the adapter gets a tx timeout without a napi instance scheduled, its possible
> for the adapter to think it has exclusive access to the hardware  (as the napi
> instance is now scheduled via the napi_disable call), while the netpoll code
> thinks there is simply work to do.  The result is parallel hardware access
> leading to corrupt data structures in the driver, and a crash.
> 
> Additionaly, there is another, more critical race between netpoll and
> napi_disable.  The disabled napi state is actually identical to the scheduled
> state for a given napi instance.  The implication being that, if a napi instance
> is disabled, a netconsole instance would see the napi state of the device as
> having been scheduled, and poll it, likely while the driver was dong something
> requiring exclusive access.  In the case above, its fairly clear that not having
> the rings in a state ready to be polled will cause any number of crashes.
> 
> The fix should be pretty easy.  netpoll uses its own bit to indicate that that
> the napi instance is in a state of being serviced by netpoll (NAPI_STATE_NPSVC).
> We can just gate disabling on that bit as well as the sched bit.  That should
> prevent netpoll from conducting a napi poll if we convert its set bit to a
> test_and_set_bit operation to provide mutual exclusion
> 
> Change notes:
> V2)
> 	Remove a trailing whtiespace
> 	Resubmit with proper subject prefix
> 
> V3)
> 	Clean up spacing nits
> 
> Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
> CC: "David S. Miller" <davem@davemloft.net>
> CC: jmaxwell@redhat.com
> Tested-by: jmaxwell@redhat.com

Applied, thanks Neil.
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diff mbox

Patch

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index b791405..d2ffeaf 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -507,6 +507,7 @@  static inline void napi_enable(struct napi_struct *n)
 	BUG_ON(!test_bit(NAPI_STATE_SCHED, &n->state));
 	smp_mb__before_atomic();
 	clear_bit(NAPI_STATE_SCHED, &n->state);
+	clear_bit(NAPI_STATE_NPSVC, &n->state);
 }
 
 #ifdef CONFIG_SMP
diff --git a/net/core/dev.c b/net/core/dev.c
index ee0d628..b6b01bf 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4723,6 +4723,8 @@  void napi_disable(struct napi_struct *n)
 
 	while (test_and_set_bit(NAPI_STATE_SCHED, &n->state))
 		msleep(1);
+	while (test_and_set_bit(NAPI_STATE_NPSVC, &n->state))
+		msleep(1);
 
 	hrtimer_cancel(&n->timer);
 
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 6aa3db8..1734f50 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -142,7 +142,7 @@  static void queue_process(struct work_struct *work)
  */
 static int poll_one_napi(struct napi_struct *napi, int budget)
 {
-	int work;
+	int work = 0;
 
 	/* net_rx_action's ->poll() invocations and our's are
 	 * synchronized by this test which is only made while
@@ -151,7 +151,13 @@  static int poll_one_napi(struct napi_struct *napi, int budget)
 	if (!test_bit(NAPI_STATE_SCHED, &napi->state))
 		return budget;
 
-	set_bit(NAPI_STATE_NPSVC, &napi->state);
+	/* If we set this bit but see that it has already been set,
+	 * that indicates that napi has been disabled and we need
+	 * to abort this operation
+ 	 */
+
+	if (test_and_set_bit(NAPI_STATE_NPSVC, &napi->state))
+		goto out;
 
 	work = napi->poll(napi, budget);
 	WARN_ONCE(work > budget, "%pF exceeded budget in poll\n", napi->poll);
@@ -159,6 +165,7 @@  static int poll_one_napi(struct napi_struct *napi, int budget)
 
 	clear_bit(NAPI_STATE_NPSVC, &napi->state);
 
+out:
 	return budget - work;
 }