[PATCH v2 0/2] nbd: fix use-after-freed and double lock bugs

[PATCH v2 0/2] nbd: fix use-after-freed and double lock bugs

[xiubli]

From: Xiubo Li <xiubli@redhat.com>

Changed in V2:
- fixed possible double lock issue in recv_work().
- adding comments instead to explain why there won't have double lock
issue.

Xiubo Li (2):
  nbd: fix use-after-freed crash for nbd->recv_workq
  nbd: add comments about double lock for config_lock confusion

 drivers/block/nbd.c | 36 +++++++++++++++++++++++++++++++-----
 1 file changed, 31 insertions(+), 5 deletions(-)

-- 
2.18.4

[PATCH v2 1/2] nbd: fix use-after-freed crash for nbd->recv_workq

[xiubli]

From: Xiubo Li <xiubli@redhat.com>

The crash call trace:

<6>[ 1012.319386] block nbd1: NBD_DISCONNECT
<1>[ 1012.319437] BUG: kernel NULL pointer dereference, address: 0000000000000020
<1>[ 1012.319439] #PF: supervisor write access in kernel mode
<1>[ 1012.319441] #PF: error_code(0x0002) - not-present page
<6>[ 1012.319442] PGD 0 P4D 0
<4>[ 1012.319448] Oops: 0002 [#1] SMP NOPTI
<4>[ 1012.319454] CPU: 9 PID: 25111 Comm: rbd-nbd Tainted: G            E     5.9.0+ #6
   [...]
<4>[ 1012.319505] PKRU: 55555554
<4>[ 1012.319506] Call Trace:
<4>[ 1012.319560]  flush_workqueue+0x81/0x440
<4>[ 1012.319598]  nbd_disconnect_and_put+0x50/0x70 [nbd]
<4>[ 1012.319607]  nbd_genl_disconnect+0xc7/0x170 [nbd]
<4>[ 1012.319627]  genl_rcv_msg+0x1dd/0x2f9
<4>[ 1012.319642]  ? genl_start+0x140/0x140
<4>[ 1012.319644]  netlink_rcv_skb+0x49/0x110
<4>[ 1012.319649]  genl_rcv+0x24/0x40
<4>[ 1012.319651]  netlink_unicast+0x1a5/0x280
<4>[ 1012.319653]  netlink_sendmsg+0x23d/0x470
<4>[ 1012.319667]  sock_sendmsg+0x5b/0x60
<4>[ 1012.319676]  ____sys_sendmsg+0x1ef/0x260
<4>[ 1012.319679]  ? copy_msghdr_from_user+0x5c/0x90
<4>[ 1012.319680]  ? ____sys_recvmsg+0xa5/0x180
<4>[ 1012.319682]  ___sys_sendmsg+0x7c/0xc0
<4>[ 1012.319683]  ? copy_msghdr_from_user+0x5c/0x90
<4>[ 1012.319685]  ? ___sys_recvmsg+0x89/0xc0
<4>[ 1012.319692]  ? __wake_up_common_lock+0x87/0xc0
<4>[ 1012.319715]  ? __check_object_size+0x46/0x173
<4>[ 1012.319727]  ? _copy_to_user+0x22/0x30
<4>[ 1012.319729]  ? move_addr_to_user+0xc3/0x100
<4>[ 1012.319731]  __sys_sendmsg+0x57/0xa0
<4>[ 1012.319744]  do_syscall_64+0x33/0x40
<4>[ 1012.319760]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
<4>[ 1012.319780] RIP: 0033:0x7f5baa8e3ad5

In case the reference of nbd->config has reached zero and the
related resource has been released, if another process tries to
send the DISCONENCT cmd by using the netlink, it will potentially
crash like this.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
---
 drivers/block/nbd.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index f46e26c9d9b3..3bb8281bb753 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -2003,16 +2003,31 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
 
 static void nbd_disconnect_and_put(struct nbd_device *nbd)
 {
+	bool flush = true;
+
 	mutex_lock(&nbd->config_lock);
 	nbd_disconnect(nbd);
 	nbd_clear_sock(nbd);
-	mutex_unlock(&nbd->config_lock);
 	/*
-	 * Make sure recv thread has finished, so it does not drop the last
-	 * config ref and try to destroy the workqueue from inside the work
-	 * queue.
+	 * In very rare case that the nbd->recv_workq may already have been
+	 * released by the recv_work().
 	 */
-	flush_workqueue(nbd->recv_workq);
+	if (likely(!nbd->recv_workq))
+		refcount_inc(&nbd->config_refs);
+	else
+		flush = false;
+	mutex_unlock(&nbd->config_lock);
+
+	if (flush) {
+		/*
+		 * Make sure recv thread has finished, so it does not drop
+		 * the last config ref and try to destroy the workqueue
+		 * from inside the work queue.
+		 */
+		flush_workqueue(nbd->recv_workq);
+		nbd_config_put(nbd);
+	}
+
 	if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF,
 			       &nbd->config->runtime_flags))
 		nbd_config_put(nbd);
-- 
2.18.4

[PATCH v2 2/2] nbd: add comments about double lock for config_lock confusion

[xiubli]

From: Xiubo Li <xiubli@redhat.com>

When calling the ioctl(), fget() will be called on this fd, and
nbd_release() is only called when the fd's refcount drops to zero.
With this we can make sure that the nbd_release() won't be called
before the ioctl() finished.

So there won't have the double lock issue for the "config_lock",
which has already been held by nbd_ioctl().

Signed-off-by: Xiubo Li <xiubli@redhat.com>
---
 drivers/block/nbd.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 3bb8281bb753..48f36b003bf5 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1345,6 +1345,17 @@ static void nbd_clear_sock_ioctl(struct nbd_device *nbd,
 	sock_shutdown(nbd);
 	__invalidate_device(bdev, true);
 	nbd_bdev_reset(bdev);
+
+	/*
+	 * When calling the ioctl(), fget() will be called on this
+	 * fd, and nbd_release() is only called when the fd's refcount
+	 * drops to zero. With this we can make sure that the
+	 * nbd_release() won't be called before the ioctl() finished.
+	 *
+	 * So there won't have the double lock issue if it will
+	 * call the nbd_config_put() here for the "config_lock", which
+	 * has already been held by nbd_ioctl().
+	 */
 	if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF,
 			       &nbd->config->runtime_flags))
 		nbd_config_put(nbd);
-- 
2.18.4

Re: [PATCH v2 1/2] nbd: fix use-after-freed crash for nbd->recv_workq

[Ming Lei]

On Mon, Nov 02, 2020 at 10:07:57PM -0500, xiubli@redhat.com wrote:
> From: Xiubo Li <xiubli@redhat.com>
> 
> The crash call trace:
> 
> <6>[ 1012.319386] block nbd1: NBD_DISCONNECT
> <1>[ 1012.319437] BUG: kernel NULL pointer dereference, address: 0000000000000020
> <1>[ 1012.319439] #PF: supervisor write access in kernel mode
> <1>[ 1012.319441] #PF: error_code(0x0002) - not-present page
> <6>[ 1012.319442] PGD 0 P4D 0
> <4>[ 1012.319448] Oops: 0002 [#1] SMP NOPTI
> <4>[ 1012.319454] CPU: 9 PID: 25111 Comm: rbd-nbd Tainted: G            E     5.9.0+ #6
>    [...]
> <4>[ 1012.319505] PKRU: 55555554
> <4>[ 1012.319506] Call Trace:
> <4>[ 1012.319560]  flush_workqueue+0x81/0x440
> <4>[ 1012.319598]  nbd_disconnect_and_put+0x50/0x70 [nbd]
> <4>[ 1012.319607]  nbd_genl_disconnect+0xc7/0x170 [nbd]
> <4>[ 1012.319627]  genl_rcv_msg+0x1dd/0x2f9
> <4>[ 1012.319642]  ? genl_start+0x140/0x140
> <4>[ 1012.319644]  netlink_rcv_skb+0x49/0x110
> <4>[ 1012.319649]  genl_rcv+0x24/0x40
> <4>[ 1012.319651]  netlink_unicast+0x1a5/0x280
> <4>[ 1012.319653]  netlink_sendmsg+0x23d/0x470
> <4>[ 1012.319667]  sock_sendmsg+0x5b/0x60
> <4>[ 1012.319676]  ____sys_sendmsg+0x1ef/0x260
> <4>[ 1012.319679]  ? copy_msghdr_from_user+0x5c/0x90
> <4>[ 1012.319680]  ? ____sys_recvmsg+0xa5/0x180
> <4>[ 1012.319682]  ___sys_sendmsg+0x7c/0xc0
> <4>[ 1012.319683]  ? copy_msghdr_from_user+0x5c/0x90
> <4>[ 1012.319685]  ? ___sys_recvmsg+0x89/0xc0
> <4>[ 1012.319692]  ? __wake_up_common_lock+0x87/0xc0
> <4>[ 1012.319715]  ? __check_object_size+0x46/0x173
> <4>[ 1012.319727]  ? _copy_to_user+0x22/0x30
> <4>[ 1012.319729]  ? move_addr_to_user+0xc3/0x100
> <4>[ 1012.319731]  __sys_sendmsg+0x57/0xa0
> <4>[ 1012.319744]  do_syscall_64+0x33/0x40
> <4>[ 1012.319760]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> <4>[ 1012.319780] RIP: 0033:0x7f5baa8e3ad5
> 
> In case the reference of nbd->config has reached zero and the
> related resource has been released, if another process tries to
> send the DISCONENCT cmd by using the netlink, it will potentially
> crash like this.
> 
> Signed-off-by: Xiubo Li <xiubli@redhat.com>
> ---
>  drivers/block/nbd.c | 25 ++++++++++++++++++++-----
>  1 file changed, 20 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index f46e26c9d9b3..3bb8281bb753 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -2003,16 +2003,31 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
>  
>  static void nbd_disconnect_and_put(struct nbd_device *nbd)
>  {
> +	bool flush = true;
> +
>  	mutex_lock(&nbd->config_lock);
>  	nbd_disconnect(nbd);
>  	nbd_clear_sock(nbd);
> -	mutex_unlock(&nbd->config_lock);
>  	/*
> -	 * Make sure recv thread has finished, so it does not drop the last
> -	 * config ref and try to destroy the workqueue from inside the work
> -	 * queue.
> +	 * In very rare case that the nbd->recv_workq may already have been
> +	 * released by the recv_work().
>  	 */
> -	flush_workqueue(nbd->recv_workq);
> +	if (likely(!nbd->recv_workq))

It is actually unlikely, but doesn't make any sense to annotate the check
since it isn't fast path, so please remove it.

> +		refcount_inc(&nbd->config_refs);
> +	else
> +		flush = false;
> +	mutex_unlock(&nbd->config_lock);
> +
> +	if (flush) {
> +		/*
> +		 * Make sure recv thread has finished, so it does not drop
> +		 * the last config ref and try to destroy the workqueue
> +		 * from inside the work queue.
> +		 */
> +		flush_workqueue(nbd->recv_workq);
> +		nbd_config_put(nbd);
> +	}
> +
>  	if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF,
>  			       &nbd->config->runtime_flags))
>  		nbd_config_put(nbd);

The approach is fine, and the reason is that nbd_disconnect_and_put() still
can be called via netlink when the nbd device has been closed:

Once the above likely() is removed, feel free to add:

Reviewed-by: Ming Lei <ming.lei@redhat.com>


Thanks,
Ming

Re: [PATCH v2 1/2] nbd: fix use-after-freed crash for nbd->recv_workq

[Xiubo Li]

On 2020/11/3 12:12, Ming Lei wrote:
> On Mon, Nov 02, 2020 at 10:07:57PM -0500, xiubli@redhat.com wrote:
>> From: Xiubo Li <xiubli@redhat.com>
>>
>> The crash call trace:
>>
>> <6>[ 1012.319386] block nbd1: NBD_DISCONNECT
>> <1>[ 1012.319437] BUG: kernel NULL pointer dereference, address: 0000000000000020
>> <1>[ 1012.319439] #PF: supervisor write access in kernel mode
>> <1>[ 1012.319441] #PF: error_code(0x0002) - not-present page
>> <6>[ 1012.319442] PGD 0 P4D 0
>> <4>[ 1012.319448] Oops: 0002 [#1] SMP NOPTI
>> <4>[ 1012.319454] CPU: 9 PID: 25111 Comm: rbd-nbd Tainted: G            E     5.9.0+ #6
>>     [...]
>> <4>[ 1012.319505] PKRU: 55555554
>> <4>[ 1012.319506] Call Trace:
>> <4>[ 1012.319560]  flush_workqueue+0x81/0x440
>> <4>[ 1012.319598]  nbd_disconnect_and_put+0x50/0x70 [nbd]
>> <4>[ 1012.319607]  nbd_genl_disconnect+0xc7/0x170 [nbd]
>> <4>[ 1012.319627]  genl_rcv_msg+0x1dd/0x2f9
>> <4>[ 1012.319642]  ? genl_start+0x140/0x140
>> <4>[ 1012.319644]  netlink_rcv_skb+0x49/0x110
>> <4>[ 1012.319649]  genl_rcv+0x24/0x40
>> <4>[ 1012.319651]  netlink_unicast+0x1a5/0x280
>> <4>[ 1012.319653]  netlink_sendmsg+0x23d/0x470
>> <4>[ 1012.319667]  sock_sendmsg+0x5b/0x60
>> <4>[ 1012.319676]  ____sys_sendmsg+0x1ef/0x260
>> <4>[ 1012.319679]  ? copy_msghdr_from_user+0x5c/0x90
>> <4>[ 1012.319680]  ? ____sys_recvmsg+0xa5/0x180
>> <4>[ 1012.319682]  ___sys_sendmsg+0x7c/0xc0
>> <4>[ 1012.319683]  ? copy_msghdr_from_user+0x5c/0x90
>> <4>[ 1012.319685]  ? ___sys_recvmsg+0x89/0xc0
>> <4>[ 1012.319692]  ? __wake_up_common_lock+0x87/0xc0
>> <4>[ 1012.319715]  ? __check_object_size+0x46/0x173
>> <4>[ 1012.319727]  ? _copy_to_user+0x22/0x30
>> <4>[ 1012.319729]  ? move_addr_to_user+0xc3/0x100
>> <4>[ 1012.319731]  __sys_sendmsg+0x57/0xa0
>> <4>[ 1012.319744]  do_syscall_64+0x33/0x40
>> <4>[ 1012.319760]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> <4>[ 1012.319780] RIP: 0033:0x7f5baa8e3ad5
>>
>> In case the reference of nbd->config has reached zero and the
>> related resource has been released, if another process tries to
>> send the DISCONENCT cmd by using the netlink, it will potentially
>> crash like this.
>>
>> Signed-off-by: Xiubo Li <xiubli@redhat.com>
>> ---
>>   drivers/block/nbd.c | 25 ++++++++++++++++++++-----
>>   1 file changed, 20 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>> index f46e26c9d9b3..3bb8281bb753 100644
>> --- a/drivers/block/nbd.c
>> +++ b/drivers/block/nbd.c
>> @@ -2003,16 +2003,31 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
>>   
>>   static void nbd_disconnect_and_put(struct nbd_device *nbd)
>>   {
>> +	bool flush = true;
>> +
>>   	mutex_lock(&nbd->config_lock);
>>   	nbd_disconnect(nbd);
>>   	nbd_clear_sock(nbd);
>> -	mutex_unlock(&nbd->config_lock);
>>   	/*
>> -	 * Make sure recv thread has finished, so it does not drop the last
>> -	 * config ref and try to destroy the workqueue from inside the work
>> -	 * queue.
>> +	 * In very rare case that the nbd->recv_workq may already have been
>> +	 * released by the recv_work().
>>   	 */
>> -	flush_workqueue(nbd->recv_workq);
>> +	if (likely(!nbd->recv_workq))
> It is actually unlikely, but doesn't make any sense to annotate the check
> since it isn't fast path, so please remove it.

Sure, will do it.

Thanks.


>
>> +		refcount_inc(&nbd->config_refs);
>> +	else
>> +		flush = false;
>> +	mutex_unlock(&nbd->config_lock);
>> +
>> +	if (flush) {
>> +		/*
>> +		 * Make sure recv thread has finished, so it does not drop
>> +		 * the last config ref and try to destroy the workqueue
>> +		 * from inside the work queue.
>> +		 */
>> +		flush_workqueue(nbd->recv_workq);
>> +		nbd_config_put(nbd);
>> +	}
>> +
>>   	if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF,
>>   			       &nbd->config->runtime_flags))
>>   		nbd_config_put(nbd);
> The approach is fine, and the reason is that nbd_disconnect_and_put() still
> can be called via netlink when the nbd device has been closed:
>
> Once the above likely() is removed, feel free to add:
>
> Reviewed-by: Ming Lei <ming.lei@redhat.com>
>
>
> Thanks,
> Ming
>