[PATCH] nbd: fix potential NULL pointer fault in connect and disconnect process

[PATCH] nbd: fix potential NULL pointer fault in connect and disconnect process

[Sun Ke]

Connect and disconnect a nbd device repeatedly, will cause
NULL pointer fault.

It will appear by the steps:
1. Connect the nbd device and disconnect it, but now nbd device
   is not disconnected totally.
2. Connect the same nbd device again immediately, it will fail
   in nbd_start_device with a EBUSY return value.
3. Wait a second to make sure the last config_refs is reduced
   and run nbd_config_put to disconnect the nbd device totally.
4. Start another process to open the nbd_device, config_refs
   will increase and at the same time disconnect it.

To fix it, add a NBD_HAS_STARTED flag. Set it in nbd_start_device_ioctl
and nbd_genl_connect if nbd device is started successfully.
Clear it in nbd_config_put. Test it in nbd_genl_disconnect and
nbd_genl_reconfigure.

Signed-off-by: Sun Ke <sunke32@huawei.com>
---
 drivers/block/nbd.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index b4607dd96185..ddd364e208ab 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -83,6 +83,7 @@ struct link_dead_args {
 
 #define NBD_DESTROY_ON_DISCONNECT	0
 #define NBD_DISCONNECT_REQUESTED	1
+#define NBD_HAS_STARTED				2
 
 struct nbd_config {
 	u32 flags;
@@ -1215,6 +1216,7 @@ static void nbd_config_put(struct nbd_device *nbd)
 		nbd->disk->queue->limits.discard_alignment = 0;
 		blk_queue_max_discard_sectors(nbd->disk->queue, UINT_MAX);
 		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, nbd->disk->queue);
+		clear_bit(NBD_HAS_STARTED, &nbd->flags);
 
 		mutex_unlock(&nbd->config_lock);
 		nbd_put(nbd);
@@ -1290,6 +1292,8 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd, struct block_device *b
 	ret = nbd_start_device(nbd);
 	if (ret)
 		return ret;
+	else
+		set_bit(NBD_HAS_STARTED, &nbd->flags);
 
 	if (max_part)
 		bdev->bd_invalidated = 1;
@@ -1961,6 +1965,7 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
 	mutex_unlock(&nbd->config_lock);
 	if (!ret) {
 		set_bit(NBD_RT_HAS_CONFIG_REF, &config->runtime_flags);
+		set_bit(NBD_HAS_STARTED, &nbd->flags);
 		refcount_inc(&nbd->config_refs);
 		nbd_connect_reply(info, nbd->index);
 	}
@@ -2008,6 +2013,14 @@ static int nbd_genl_disconnect(struct sk_buff *skb, struct genl_info *info)
 		       index);
 		return -EINVAL;
 	}
+
+	if (!test_bit(NBD_HAS_STARTED, &nbd->flags)) {
+		mutex_unlock(&nbd_index_mutex);
+		printk(KERN_ERR "nbd: device at index %d failed to start\n",
+		       index);
+		return -EBUSY;
+	}
+
 	if (!refcount_inc_not_zero(&nbd->refs)) {
 		mutex_unlock(&nbd_index_mutex);
 		printk(KERN_ERR "nbd: device at index %d is going down\n",
@@ -2049,6 +2062,14 @@ static int nbd_genl_reconfigure(struct sk_buff *skb, struct genl_info *info)
 		       index);
 		return -EINVAL;
 	}
+
+	if (!test_bit(NBD_HAS_STARTED, &nbd->flags)) {
+		mutex_unlock(&nbd_index_mutex);
+		printk(KERN_ERR "nbd: device at index %d failed to start\n",
+		       index);
+		return -EBUSY;
+	}
+
 	if (!refcount_inc_not_zero(&nbd->refs)) {
 		mutex_unlock(&nbd_index_mutex);
 		printk(KERN_ERR "nbd: device at index %d is going down\n",
-- 
2.17.2

Re: [PATCH] nbd: fix potential NULL pointer fault in connect and disconnect process

[Josef Bacik]

On 1/17/20 6:50 AM, Sun Ke wrote:
> Connect and disconnect a nbd device repeatedly, will cause
> NULL pointer fault.
> 
> It will appear by the steps:
> 1. Connect the nbd device and disconnect it, but now nbd device
>     is not disconnected totally.
> 2. Connect the same nbd device again immediately, it will fail
>     in nbd_start_device with a EBUSY return value.
> 3. Wait a second to make sure the last config_refs is reduced
>     and run nbd_config_put to disconnect the nbd device totally.
> 4. Start another process to open the nbd_device, config_refs
>     will increase and at the same time disconnect it.
> 
> To fix it, add a NBD_HAS_STARTED flag. Set it in nbd_start_device_ioctl
> and nbd_genl_connect if nbd device is started successfully.
> Clear it in nbd_config_put. Test it in nbd_genl_disconnect and
> nbd_genl_reconfigure.

I don't doubt what you are seeing, but what exactly are we NULL pointer 
dereferencing?  I can't quite figure it out from the steps.

> 
> Signed-off-by: Sun Ke <sunke32@huawei.com>
> ---
>   drivers/block/nbd.c | 21 +++++++++++++++++++++
>   1 file changed, 21 insertions(+)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index b4607dd96185..ddd364e208ab 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -83,6 +83,7 @@ struct link_dead_args {
>   
>   #define NBD_DESTROY_ON_DISCONNECT	0
>   #define NBD_DISCONNECT_REQUESTED	1
> +#define NBD_HAS_STARTED				2
>   
>   struct nbd_config {
>   	u32 flags;
> @@ -1215,6 +1216,7 @@ static void nbd_config_put(struct nbd_device *nbd)
>   		nbd->disk->queue->limits.discard_alignment = 0;
>   		blk_queue_max_discard_sectors(nbd->disk->queue, UINT_MAX);
>   		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, nbd->disk->queue);
> +		clear_bit(NBD_HAS_STARTED, &nbd->flags);
>   
>   		mutex_unlock(&nbd->config_lock);
>   		nbd_put(nbd);
> @@ -1290,6 +1292,8 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd, struct block_device *b
>   	ret = nbd_start_device(nbd);
>   	if (ret)
>   		return ret;
> +	else
> +		set_bit(NBD_HAS_STARTED, &nbd->flags);

The else is superfluous here.  Thanks,

Josef

Re: [PATCH] nbd: fix potential NULL pointer fault in connect and disconnect process

[Mike Christie]

On 01/17/2020 05:50 AM, Sun Ke wrote:
> Connect and disconnect a nbd device repeatedly, will cause
> NULL pointer fault.
> 
> It will appear by the steps:
> 1. Connect the nbd device and disconnect it, but now nbd device
>    is not disconnected totally.
> 2. Connect the same nbd device again immediately, it will fail
>    in nbd_start_device with a EBUSY return value.
> 3. Wait a second to make sure the last config_refs is reduced
>    and run nbd_config_put to disconnect the nbd device totally.
> 4. Start another process to open the nbd_device, config_refs
>    will increase and at the same time disconnect it.

Just to make sure I understood this, for step 4 the process is doing:

open(/dev/nbdX);
ioctl(NBD_DISCONNECT, /dev/nbdX) or nbd_genl_disconnect(for /dev/nbdX)

?

There is no successful NBD_DO_IT / nbd_genl_connect between the open and
disconnect calls at step #4, because it would normally be done at #2 and
that failed. nbd_disconnect_and_put could then reference a null
recv_workq. If we are also racing with a close() then that could free
the device/config from under nbd_disconnect_and_put.

> 
> To fix it, add a NBD_HAS_STARTED flag. Set it in nbd_start_device_ioctl

I'm not sure if we need the new bit. We could just add a check for a non
null task_recv in nbd_genl_disconnect like how nbd_start_device and
nbd_genl_disconnect do.

The new bit might be more clear which is nice. If we got this route,
should the new bit be a runtime_flag like other device state bits?


> and nbd_genl_connect if nbd device is started successfully.
> Clear it in nbd_config_put. Test it in nbd_genl_disconnect and
> nbd_genl_reconfigure.
> 
> Signed-off-by: Sun Ke <sunke32@huawei.com>
> ---
>  drivers/block/nbd.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index b4607dd96185..ddd364e208ab 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -83,6 +83,7 @@ struct link_dead_args {
>  
>  #define NBD_DESTROY_ON_DISCONNECT	0
>  #define NBD_DISCONNECT_REQUESTED	1
> +#define NBD_HAS_STARTED				2
>  
>  struct nbd_config {
>  	u32 flags;
> @@ -1215,6 +1216,7 @@ static void nbd_config_put(struct nbd_device *nbd)
>  		nbd->disk->queue->limits.discard_alignment = 0;
>  		blk_queue_max_discard_sectors(nbd->disk->queue, UINT_MAX);
>  		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, nbd->disk->queue);
> +		clear_bit(NBD_HAS_STARTED, &nbd->flags);
>  
>  		mutex_unlock(&nbd->config_lock);
>  		nbd_put(nbd);
> @@ -1290,6 +1292,8 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd, struct block_device *b
>  	ret = nbd_start_device(nbd);
>  	if (ret)
>  		return ret;
> +	else
> +		set_bit(NBD_HAS_STARTED, &nbd->flags);
>  
>  	if (max_part)
>  		bdev->bd_invalidated = 1;
> @@ -1961,6 +1965,7 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
>  	mutex_unlock(&nbd->config_lock);
>  	if (!ret) {
>  		set_bit(NBD_RT_HAS_CONFIG_REF, &config->runtime_flags);
> +		set_bit(NBD_HAS_STARTED, &nbd->flags);
>  		refcount_inc(&nbd->config_refs);
>  		nbd_connect_reply(info, nbd->index);
>  	}
> @@ -2008,6 +2013,14 @@ static int nbd_genl_disconnect(struct sk_buff *skb, struct genl_info *info)
>  		       index);
>  		return -EINVAL;
>  	}
> +
> +	if (!test_bit(NBD_HAS_STARTED, &nbd->flags)) {
> +		mutex_unlock(&nbd_index_mutex);
> +		printk(KERN_ERR "nbd: device at index %d failed to start\n",
> +		       index);
> +		return -EBUSY;
> +	}
> +
>  	if (!refcount_inc_not_zero(&nbd->refs)) {
>  		mutex_unlock(&nbd_index_mutex);
>  		printk(KERN_ERR "nbd: device at index %d is going down\n",
> @@ -2049,6 +2062,14 @@ static int nbd_genl_reconfigure(struct sk_buff *skb, struct genl_info *info)
>  		       index);
>  		return -EINVAL;
>  	}
> +
> +	if (!test_bit(NBD_HAS_STARTED, &nbd->flags)) {
> +		mutex_unlock(&nbd_index_mutex);
> +		printk(KERN_ERR "nbd: device at index %d failed to start\n",
> +		       index);
> +		return -EBUSY;
> +	}
> +
>  	if (!refcount_inc_not_zero(&nbd->refs)) {
>  		mutex_unlock(&nbd_index_mutex);
>  		printk(KERN_ERR "nbd: device at index %d is going down\n",
> 

Re: [PATCH] nbd: fix potential NULL pointer fault in connect and disconnect process

[sunke (E)]

在 2020/1/17 22:18, Josef Bacik 写道:
> On 1/17/20 6:50 AM, Sun Ke wrote:
>> Connect and disconnect a nbd device repeatedly, will cause
>> NULL pointer fault.
>>
>> It will appear by the steps:
>> 1. Connect the nbd device and disconnect it, but now nbd device
>>     is not disconnected totally.
>> 2. Connect the same nbd device again immediately, it will fail
>>     in nbd_start_device with a EBUSY return value.
>> 3. Wait a second to make sure the last config_refs is reduced
>>     and run nbd_config_put to disconnect the nbd device totally.
>> 4. Start another process to open the nbd_device, config_refs
>>     will increase and at the same time disconnect it.
>>
>> To fix it, add a NBD_HAS_STARTED flag. Set it in nbd_start_device_ioctl
>> and nbd_genl_connect if nbd device is started successfully.
>> Clear it in nbd_config_put. Test it in nbd_genl_disconnect and
>> nbd_genl_reconfigure.
> 
> I don't doubt what you are seeing, but what exactly are we NULL pointer 
> dereferencing?  I can't quite figure it out from the steps.
The root case is when do disconnect, pointers in structure nbd_device 
will not be free immeditily, it should wait for the last config_refs to 
be decreased.

I got this kernel NULL pointer dereference report:

[  256.454582] Dev nbd0: unable to read RDB block 0
[  256.455611] Dev nbd0: unable to read RDB block 0
[  256.457528] Dev nbd0: unable to read RDB block 0
[  256.458742] Dev nbd0: unable to read RDB block 0
[  256.516375] Dev nbd0: unable to read RDB block 0
[  257.468970] BUG: kernel NULL pointer dereference, address: 
0000000000000020
[  257.469645] #PF: supervisor write access in kernel mode
[  257.470445] #PF: error_code(0x0002) - not-present page
[  257.470888] PGD 12ecb7067 P4D 12ecb7067 PUD 12f3f2067 PMD 0
[  257.471384] Oops: 0002 [#1] SMP
[  257.471671] CPU: 1 PID: 1651 Comm: nbd-client Not tainted 
5.5.0-rc5-00039-gae6088216ce4 #22
[  257.472501] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), 
BIOS ?-20180531_142017-buildhw-08.phx2.fedoraproject.org-1.fc28 04/01/2014
[  257.473776] RIP: 0010:mutex_lock+0x29/0x60
[  257.474593] Code: 00 0f 1f 44 00 00 53 48 89 fb 48 83 05 cf f8 01 02 
01 e8 ea bd ff ff 48 83 05 ca f8 01 02 01 31 c0 65 48 8b 14 25 00 7d 01 
00 <f0> 48 0f b1 13 74 f
[  257.476221] RSP: 0018:ffffc900004cfa10 EFLAGS: 00010246
[  257.476670] RAX: 0000000000000000 RBX: 0000000000000020 RCX: 
0000000000000000
[  257.477289] RDX: ffff88812f524a00 RSI: ffffffff82e44212 RDI: 
0000000000000020
[  257.477999] RBP: ffffc900004cfab0 R08: ffff88813bc6c110 R09: 
0000000000000000
[  257.478617] R10: 8080808080808080 R11: 0000000000000018 R12: 
ffff88813584b000
[  257.479228] R13: ffffffff838b1f00 R14: ffffc900004cfbb8 R15: 
ffffc900004cfa40
[  257.479871] FS:  00007f0c30d75b40(0000) GS:ffff88813bc40000(0000) 
knlGS:0000000000000000
[  257.480569] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  257.481336] CR2: 0000000000000020 CR3: 000000012f5ea000 CR4: 
00000000000006e0
[  257.481980] Call Trace:
[  257.482262]  flush_workqueue+0x91/0x690
[  257.482627]  ? nbd_size_update+0x180/0x180 [nbd]
[  257.483063]  nbd_disconnect_and_put+0x80/0xd0 [nbd]
[  257.483497]  nbd_genl_disconnect+0x153/0x2d0 [nbd]
[  257.483969]  genl_rcv_msg+0x2ab/0x620
[  257.484302]  ? netlink_unicast+0x3b8/0x5e0
[  257.484663]  ? __nlmsg_put+0x78/0x90
[  257.485009]  ? genl_family_rcv_msg_attrs_parse+0x1a0/0x1a0
[  257.485488]  netlink_rcv_skb+0x5a/0x1a0
[  257.485849]  genl_rcv+0x34/0x60
[  257.486129]  netlink_unicast+0x2a4/0x5e0
[  257.486468]  netlink_sendmsg+0x369/0x6b0
[  257.486854]  ? rw_copy_check_uvector+0x50/0x1d0
[  257.487257]  ____sys_sendmsg+0x1f7/0x370
[  257.487604]  ? copy_msghdr_from_user+0xff/0x1e0
[  257.488016]  ___sys_sendmsg+0x8c/0xe0
[  257.488335]  ? copy_msghdr_from_user+0xff/0x1e0
[  257.488730]  ? ___sys_recvmsg+0xa1/0xe0
[  257.489091]  ? handle_mm_fault+0x199/0x390
[  257.489454]  __sys_sendmsg+0x6b/0xe0
[  257.489766]  __x64_sys_sendmsg+0x23/0x30
[  257.490149]  do_syscall_64+0xab/0x410
[  257.490474]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  257.490942] RIP: 0033:0x7f0c3047cb87
[  257.491288] Code: 64 89 02 48 c7 c0 ff ff ff ff eb b9 0f 1f 80 00 00 
00 00 8b 05 6a 2b 2c 00 48 63 d2 48 63 ff 85 c0 75 18 b8 2e 00 00 00 0f 
05 <48> 3d 00 f0 ff ff 8
[  257.492940] RSP: 002b:00007ffefb59db28 EFLAGS: 00000246 ORIG_RAX: 
000000000000002e
[  257.493627] RAX: ffffffffffffffda RBX: 00000000023b0120 RCX: 
00007f0c3047cb87
[  257.494497] RDX: 0000000000000000 RSI: 00007ffefb59db60 RDI: 
0000000000000003
[  257.495116] RBP: 00000000023b01f0 R08: 0000000000000014 R09: 
0000000000000002
[  257.495731] R10: 0000000000000006 R11: 0000000000000246 R12: 
00000000023b0030
[  257.496356] R13: 00007ffefb59db60 R14: 0000000000000001 R15: 
00000000ffffffff
[  257.496989] Modules linked in: nbd
[  257.497580] CR2: 0000000000000020

Thanks,

Ke

> 
>>
>> Signed-off-by: Sun Ke <sunke32@huawei.com>
>> ---
>>   drivers/block/nbd.c | 21 +++++++++++++++++++++
>>   1 file changed, 21 insertions(+)
>>
>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>> index b4607dd96185..ddd364e208ab 100644
>> --- a/drivers/block/nbd.c
>> +++ b/drivers/block/nbd.c
>> @@ -83,6 +83,7 @@ struct link_dead_args {
>>   #define NBD_DESTROY_ON_DISCONNECT    0
>>   #define NBD_DISCONNECT_REQUESTED    1
>> +#define NBD_HAS_STARTED                2
>>   struct nbd_config {
>>       u32 flags;
>> @@ -1215,6 +1216,7 @@ static void nbd_config_put(struct nbd_device *nbd)
>>           nbd->disk->queue->limits.discard_alignment = 0;
>>           blk_queue_max_discard_sectors(nbd->disk->queue, UINT_MAX);
>>           blk_queue_flag_clear(QUEUE_FLAG_DISCARD, nbd->disk->queue);
>> +        clear_bit(NBD_HAS_STARTED, &nbd->flags);
>>           mutex_unlock(&nbd->config_lock);
>>           nbd_put(nbd);
>> @@ -1290,6 +1292,8 @@ static int nbd_start_device_ioctl(struct 
>> nbd_device *nbd, struct block_device *b
>>       ret = nbd_start_device(nbd);
>>       if (ret)
>>           return ret;
>> +    else
>> +        set_bit(NBD_HAS_STARTED, &nbd->flags);
> 
> The else is superfluous here.  Thanks,
> 
> Josef
> 
> .

Re: [PATCH] nbd: fix potential NULL pointer fault in connect and disconnect process

[sunke (E)]

Thanks for your detailed suggestions.

在 2020/1/18 1:32, Mike Christie 写道:
> On 01/17/2020 05:50 AM, Sun Ke wrote:
>> Connect and disconnect a nbd device repeatedly, will cause
>> NULL pointer fault.
>>
>> It will appear by the steps:
>> 1. Connect the nbd device and disconnect it, but now nbd device
>>     is not disconnected totally.
>> 2. Connect the same nbd device again immediately, it will fail
>>     in nbd_start_device with a EBUSY return value.
>> 3. Wait a second to make sure the last config_refs is reduced
>>     and run nbd_config_put to disconnect the nbd device totally.
>> 4. Start another process to open the nbd_device, config_refs
>>     will increase and at the same time disconnect it.
> 
> Just to make sure I understood this, for step 4 the process is doing:
> 
> open(/dev/nbdX);
> ioctl(NBD_DISCONNECT, /dev/nbdX) or nbd_genl_disconnect(for /dev/nbdX)
> 
> ?
> 
do nbd_genl_disconnect(for /dev/nbdX)；
I tested it. Connect /dev/nbdX
through ioctl interface by nbd-client -L -N export localhost /dev/nbdX and
through netlink interface by nbd-client localhost XXXX /dev/nbdX,
disconnect /dev/nbdX by nbd-client -d /dev/nbdX.
Both call nbd_genl_disconnect(for /dev/nbdX) and both contain the same 
null pointer dereference.
> There is no successful NBD_DO_IT / nbd_genl_connect between the open and
> disconnect calls at step #4, because it would normally be done at #2 and
> that failed. nbd_disconnect_and_put could then reference a null
> recv_workq. If we are also racing with a close() then that could free
> the device/config from under nbd_disconnect_and_put.
> 
Yes, nbd_disconnect_and_put could then reference a null recv_workq.
>>
>> To fix it, add a NBD_HAS_STARTED flag. Set it in nbd_start_device_ioctl
> 
> I'm not sure if we need the new bit. We could just add a check for a non
> null task_recv in nbd_genl_disconnect like how nbd_start_device and
> nbd_genl_disconnect do.
> 
I am also not very sure which is better.
because in nbd_config_put, not only recv_workq is null,
nbd->task_recv and nbd->config the same.
so I doubt that if step 4 do something else will also reference a null 
pointer.
> The new bit might be more clear which is nice. If we got this route,
> should the new bit be a runtime_flag like other device state bits?
> 
> 
Yes, I realize it. Just add a check for a non null task_recv in 
nbd_genl_disconnect is better, right?
>> and nbd_genl_connect if nbd device is started successfully.
>> Clear it in nbd_config_put. Test it in nbd_genl_disconnect and
>> nbd_genl_reconfigure.
>>
>> Signed-off-by: Sun Ke <sunke32@huawei.com>
>> ---
>>   drivers/block/nbd.c | 21 +++++++++++++++++++++
>>   1 file changed, 21 insertions(+)
>>
>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>> index b4607dd96185..ddd364e208ab 100644
>> --- a/drivers/block/nbd.c
>> +++ b/drivers/block/nbd.c
>> @@ -83,6 +83,7 @@ struct link_dead_args {
>>   
>>   #define NBD_DESTROY_ON_DISCONNECT	0
>>   #define NBD_DISCONNECT_REQUESTED	1
>> +#define NBD_HAS_STARTED				2
>>   
>>   struct nbd_config {
>>   	u32 flags;
>> @@ -1215,6 +1216,7 @@ static void nbd_config_put(struct nbd_device *nbd)
>>   		nbd->disk->queue->limits.discard_alignment = 0;
>>   		blk_queue_max_discard_sectors(nbd->disk->queue, UINT_MAX);
>>   		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, nbd->disk->queue);
>> +		clear_bit(NBD_HAS_STARTED, &nbd->flags);
>>   
>>   		mutex_unlock(&nbd->config_lock);
>>   		nbd_put(nbd);
>> @@ -1290,6 +1292,8 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd, struct block_device *b
>>   	ret = nbd_start_device(nbd);
>>   	if (ret)
>>   		return ret;
>> +	else
>> +		set_bit(NBD_HAS_STARTED, &nbd->flags);
>>   
>>   	if (max_part)
>>   		bdev->bd_invalidated = 1;
>> @@ -1961,6 +1965,7 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
>>   	mutex_unlock(&nbd->config_lock);
>>   	if (!ret) {
>>   		set_bit(NBD_RT_HAS_CONFIG_REF, &config->runtime_flags);
>> +		set_bit(NBD_HAS_STARTED, &nbd->flags);
>>   		refcount_inc(&nbd->config_refs);
>>   		nbd_connect_reply(info, nbd->index);
>>   	}
>> @@ -2008,6 +2013,14 @@ static int nbd_genl_disconnect(struct sk_buff *skb, struct genl_info *info)
>>   		       index);
>>   		return -EINVAL;
>>   	}
>> +
>> +	if (!test_bit(NBD_HAS_STARTED, &nbd->flags)) {
>> +		mutex_unlock(&nbd_index_mutex);
>> +		printk(KERN_ERR "nbd: device at index %d failed to start\n",
>> +		       index);
>> +		return -EBUSY;
>> +	}
>> +
>>   	if (!refcount_inc_not_zero(&nbd->refs)) {
>>   		mutex_unlock(&nbd_index_mutex);
>>   		printk(KERN_ERR "nbd: device at index %d is going down\n",
>> @@ -2049,6 +2062,14 @@ static int nbd_genl_reconfigure(struct sk_buff *skb, struct genl_info *info)
>>   		       index);
>>   		return -EINVAL;
>>   	}
>> +
>> +	if (!test_bit(NBD_HAS_STARTED, &nbd->flags)) {
>> +		mutex_unlock(&nbd_index_mutex);
>> +		printk(KERN_ERR "nbd: device at index %d failed to start\n",
>> +		       index);
>> +		return -EBUSY;
>> +	}
>> +
>>   	if (!refcount_inc_not_zero(&nbd->refs)) {
>>   		mutex_unlock(&nbd_index_mutex);
>>   		printk(KERN_ERR "nbd: device at index %d is going down\n",
>>
I thought the changes in nbd_genl_reconfigure is necessary althought my 
test do not call it. but now I think it is superfluous, 
nbd_genl_reconfigure checks for a non null task_recv.

Thanks,

Ke
> 
> 
> .
> 

Re: [PATCH] nbd: fix potential NULL pointer fault in connect and disconnect process

[Mike Christie]

[-- Attachment #1: Type: text/plain, Size: 2110 bytes --]

On 01/19/2020 01:10 AM, sunke (E) wrote:
> 
> Thanks for your detailed suggestions.
> 
> 在 2020/1/18 1:32, Mike Christie 写道:
>> On 01/17/2020 05:50 AM, Sun Ke wrote:
>>> Connect and disconnect a nbd device repeatedly, will cause
>>> NULL pointer fault.
>>>
>>> It will appear by the steps:
>>> 1. Connect the nbd device and disconnect it, but now nbd device
>>>     is not disconnected totally.
>>> 2. Connect the same nbd device again immediately, it will fail
>>>     in nbd_start_device with a EBUSY return value.
>>> 3. Wait a second to make sure the last config_refs is reduced
>>>     and run nbd_config_put to disconnect the nbd device totally.
>>> 4. Start another process to open the nbd_device, config_refs
>>>     will increase and at the same time disconnect it.
>>
>> Just to make sure I understood this, for step 4 the process is doing:
>>
>> open(/dev/nbdX);
>> ioctl(NBD_DISCONNECT, /dev/nbdX) or nbd_genl_disconnect(for /dev/nbdX)
>>
>> ?
>>
> do nbd_genl_disconnect(for /dev/nbdX)；
> I tested it. Connect /dev/nbdX
> through ioctl interface by nbd-client -L -N export localhost /dev/nbdX and
> through netlink interface by nbd-client localhost XXXX /dev/nbdX,
> disconnect /dev/nbdX by nbd-client -d /dev/nbdX.
> Both call nbd_genl_disconnect(for /dev/nbdX) and both contain the same
> null pointer dereference.
>> There is no successful NBD_DO_IT / nbd_genl_connect between the open and
>> disconnect calls at step #4, because it would normally be done at #2 and
>> that failed. nbd_disconnect_and_put could then reference a null
>> recv_workq. If we are also racing with a close() then that could free
>> the device/config from under nbd_disconnect_and_put.
>>
> Yes, nbd_disconnect_and_put could then reference a null recv_workq.

Hey Sunke

How about the attached patch. I am still testing it. The basic idea is
that we need to do a flush whenever we have done a sock_shutdown and are
in the disconnect/connect/clear sock path, so it just adds the flush in
that function. We then do not need to keep adding these flushes everywhere.

[-- Attachment #2: 0001-nbd-fix-crash-during-nbd_genl_disconnect.patch --]
[-- Type: text/x-patch, Size: 5016 bytes --]

From c007f70b73d31a11ea90ae53c748266eec88c0ab Mon Sep 17 00:00:00 2001
From: Mike Christie <mchristi@redhat.com>
Date: Sun, 9 Feb 2020 21:06:00 -0600
Subject: [PATCH] nbd: fix crash during nbd_genl_disconnect

If we open a nbd device, but have not done a nbd_genl_connect we will
crash in nbd_genl_disconnect when we try to flush the workqueue since it
was never allocated.

This patch moves all the flush calls to sock_shutdown and adds a check
for a valid workqueue.

Note that we flush_workqueue will do the right thing if there are no
running works so we did not need the if (i) check in nbd_start_device.
This also fixes a possible bug where you could do nbd_genl_connect, then
do a NBD_DISCONNECT and NBD_CLEAR_SOCK and the workqueue would not get
flushed.
---
 drivers/block/nbd.c | 44 +++++++++++++++++---------------------------
 1 file changed, 17 insertions(+), 27 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 78181908f0df..1afc70ed1f0a 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -341,9 +341,12 @@ static void nbd_complete_rq(struct request *req)
 }
 
 /*
- * Forcibly shutdown the socket causing all listeners to error
+ * Forcibly shutdown the socket causing all listeners to error. If called
+ * from a disconnect/clearing context we must make sure the recv workqueues
+ * have exited so they don't release the last nbd_config reference and try to
+ * destroy the workqueue from inside itself.
  */
-static void sock_shutdown(struct nbd_device *nbd)
+static void sock_shutdown(struct nbd_device *nbd, bool flush_work)
 {
 	struct nbd_config *config = nbd->config;
 	int i;
@@ -351,7 +354,7 @@ static void sock_shutdown(struct nbd_device *nbd)
 	if (config->num_connections == 0)
 		return;
 	if (test_and_set_bit(NBD_RT_DISCONNECTED, &config->runtime_flags))
-		return;
+		goto try_flush;
 
 	for (i = 0; i < config->num_connections; i++) {
 		struct nbd_sock *nsock = config->socks[i];
@@ -360,6 +363,10 @@ static void sock_shutdown(struct nbd_device *nbd)
 		mutex_unlock(&nsock->tx_lock);
 	}
 	dev_warn(disk_to_dev(nbd->disk), "shutting down sockets\n");
+
+try_flush:
+	if (flush_work && nbd->recv_workq)
+		flush_workqueue(nbd->recv_workq);
 }
 
 static u32 req_to_nbd_cmd_type(struct request *req)
@@ -446,7 +453,7 @@ static enum blk_eh_timer_return nbd_xmit_timeout(struct request *req,
 	set_bit(NBD_RT_TIMEDOUT, &config->runtime_flags);
 	cmd->status = BLK_STS_IOERR;
 	mutex_unlock(&cmd->lock);
-	sock_shutdown(nbd);
+	sock_shutdown(nbd, false);
 	nbd_config_put(nbd);
 done:
 	blk_mq_complete_request(req);
@@ -910,7 +917,7 @@ static int nbd_handle_cmd(struct nbd_cmd *cmd, int index)
 			 * for the reconnect timer don't trigger the timer again
 			 * and instead just error out.
 			 */
-			sock_shutdown(nbd);
+			sock_shutdown(nbd, false);
 			nbd_config_put(nbd);
 			blk_mq_start_request(req);
 			return -EIO;
@@ -1178,7 +1185,7 @@ static int nbd_disconnect(struct nbd_device *nbd)
 
 static void nbd_clear_sock(struct nbd_device *nbd)
 {
-	sock_shutdown(nbd);
+	sock_shutdown(nbd, true);
 	nbd_clear_que(nbd);
 	nbd->task_setup = NULL;
 }
@@ -1264,17 +1271,7 @@ static int nbd_start_device(struct nbd_device *nbd)
 
 		args = kzalloc(sizeof(*args), GFP_KERNEL);
 		if (!args) {
-			sock_shutdown(nbd);
-			/*
-			 * If num_connections is m (2 < m),
-			 * and NO.1 ~ NO.n(1 < n < m) kzallocs are successful.
-			 * But NO.(n + 1) failed. We still have n recv threads.
-			 * So, add flush_workqueue here to prevent recv threads
-			 * dropping the last config_refs and trying to destroy
-			 * the workqueue from inside the workqueue.
-			 */
-			if (i)
-				flush_workqueue(nbd->recv_workq);
+			sock_shutdown(nbd, true);
 			return -ENOMEM;
 		}
 		sk_set_memalloc(config->socks[i]->sock->sk);
@@ -1306,9 +1303,7 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd, struct block_device *b
 	mutex_unlock(&nbd->config_lock);
 	ret = wait_event_interruptible(config->recv_wq,
 					 atomic_read(&config->recv_threads) == 0);
-	if (ret)
-		sock_shutdown(nbd);
-	flush_workqueue(nbd->recv_workq);
+	sock_shutdown(nbd, true);
 
 	mutex_lock(&nbd->config_lock);
 	nbd_bdev_reset(bdev);
@@ -1323,7 +1318,7 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd, struct block_device *b
 static void nbd_clear_sock_ioctl(struct nbd_device *nbd,
 				 struct block_device *bdev)
 {
-	sock_shutdown(nbd);
+	sock_shutdown(nbd, true);
 	__invalidate_device(bdev, true);
 	nbd_bdev_reset(bdev);
 	if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF,
@@ -1986,12 +1981,7 @@ static void nbd_disconnect_and_put(struct nbd_device *nbd)
 	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.
-	 */
-	flush_workqueue(nbd->recv_workq);
+
 	if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF,
 			       &nbd->config->runtime_flags))
 		nbd_config_put(nbd);
-- 
2.20.1

Re: [PATCH] nbd: fix potential NULL pointer fault in connect and disconnect process

[sunke (E)]

Hi Mike

Your idea looks good.

Thanks,
Sun Ke

在 2020/2/10 11:16, Mike Christie 写道:
> On 01/19/2020 01:10 AM, sunke (E) wrote:
>>
>> Thanks for your detailed suggestions.
>>
>> 在 2020/1/18 1:32, Mike Christie 写道:
>>> On 01/17/2020 05:50 AM, Sun Ke wrote:
>>>> Connect and disconnect a nbd device repeatedly, will cause
>>>> NULL pointer fault.
>>>>
>>>> It will appear by the steps:
>>>> 1. Connect the nbd device and disconnect it, but now nbd device
>>>>      is not disconnected totally.
>>>> 2. Connect the same nbd device again immediately, it will fail
>>>>      in nbd_start_device with a EBUSY return value.
>>>> 3. Wait a second to make sure the last config_refs is reduced
>>>>      and run nbd_config_put to disconnect the nbd device totally.
>>>> 4. Start another process to open the nbd_device, config_refs
>>>>      will increase and at the same time disconnect it.
>>>
>>> Just to make sure I understood this, for step 4 the process is doing:
>>>
>>> open(/dev/nbdX);
>>> ioctl(NBD_DISCONNECT, /dev/nbdX) or nbd_genl_disconnect(for /dev/nbdX)
>>>
>>> ?
>>>
>> do nbd_genl_disconnect(for /dev/nbdX)；
>> I tested it. Connect /dev/nbdX
>> through ioctl interface by nbd-client -L -N export localhost /dev/nbdX and
>> through netlink interface by nbd-client localhost XXXX /dev/nbdX,
>> disconnect /dev/nbdX by nbd-client -d /dev/nbdX.
>> Both call nbd_genl_disconnect(for /dev/nbdX) and both contain the same
>> null pointer dereference.
>>> There is no successful NBD_DO_IT / nbd_genl_connect between the open and
>>> disconnect calls at step #4, because it would normally be done at #2 and
>>> that failed. nbd_disconnect_and_put could then reference a null
>>> recv_workq. If we are also racing with a close() then that could free
>>> the device/config from under nbd_disconnect_and_put.
>>>
>> Yes, nbd_disconnect_and_put could then reference a null recv_workq.
> 
> Hey Sunke
> 
> How about the attached patch. I am still testing it. The basic idea is
> that we need to do a flush whenever we have done a sock_shutdown and are
> in the disconnect/connect/clear sock path, so it just adds the flush in
> that function. We then do not need to keep adding these flushes everywhere.
>