[PATCH v5 0/2] fix a NULL pointer bug and simplify the code

v5: Adjust the title and add “Suggested-by”.
v4: Share exception handling code for if branches and 
	move put_nbd adjustment to a separate patch.
v3: Do not use unlock and add put_nbd.
v2: Use jump target unlock.

Sun Ke (2):
  nbd: Fix NULL pointer in flush_workqueue
  nbd: share nbd_put and return by goto put_nbd

 drivers/block/nbd.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

-- 
2.25.4

Replace the following two statements by the statement “goto put_nbd;”

	nbd_put(nbd);
	return 0;

Suggested-by: Markus Elfring <redacted>
Signed-off-by: Sun Ke <redacted>
---
 drivers/block/nbd.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 3c9b3bf3f4c2..ecae81e43122 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c

@@ -2029,12 +2029,11 @@ static int nbd_genl_disconnect(struct sk_buff *skb, struct genl_info *info)
 	}
 	mutex_unlock(&nbd->config_lock);
 	mutex_unlock(&nbd_index_mutex);
-	if (!refcount_inc_not_zero(&nbd->config_refs)) {
-		nbd_put(nbd);
-		return 0;
-	}
+	if (!refcount_inc_not_zero(&nbd->config_refs))
+		goto put_nbd;
 	nbd_disconnect_and_put(nbd);
 	nbd_config_put(nbd);
+put_nbd:
 	nbd_put(nbd);
 	return 0;
 }

-- 
2.25.4

Open /dev/nbdX first, the config_refs will be 1 and
the pointers in nbd_device are still null. Disconnect
/dev/nbdX, then reference a null recv_workq. The
protection by config_refs in nbd_genl_disconnect is useless.

[  656.366194] BUG: kernel NULL pointer dereference, address: 0000000000000020
[  656.368943] #PF: supervisor write access in kernel mode
[  656.369844] #PF: error_code(0x0002) - not-present page
[  656.370717] PGD 10cc87067 P4D 10cc87067 PUD 1074b4067 PMD 0
[  656.371693] Oops: 0002 [#1] SMP
[  656.372242] CPU: 5 PID: 7977 Comm: nbd-client Not tainted 5.11.0-rc5-00040-g76c057c84d28 #1
[  656.373661] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014
[  656.375904] RIP: 0010:mutex_lock+0x29/0x60
[  656.376627] Code: 00 0f 1f 44 00 00 55 48 89 fd 48 83 05 6f d7 fe 08 01 e8 7a c3 ff ff 48 83 05 6a d7 fe 08 01 31 c0 65 48 8b 14 25 00 6d 01 00 <f0> 48 0f b1 55 d
[  656.378934] RSP: 0018:ffffc900005eb9b0 EFLAGS: 00010246
[  656.379350] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[  656.379915] RDX: ffff888104cf2600 RSI: ffffffffaae8f452 RDI: 0000000000000020
[  656.380473] RBP: 0000000000000020 R08: 0000000000000000 R09: ffff88813bd6b318
[  656.381039] R10: 00000000000000c7 R11: fefefefefefefeff R12: ffff888102710b40
[  656.381599] R13: ffffc900005eb9e0 R14: ffffffffb2930680 R15: ffff88810770ef00
[  656.382166] FS:  00007fdf117ebb40(0000) GS:ffff88813bd40000(0000) knlGS:0000000000000000
[  656.382806] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  656.383261] CR2: 0000000000000020 CR3: 0000000100c84000 CR4: 00000000000006e0
[  656.383819] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  656.384370] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  656.384927] Call Trace:
[  656.385111]  flush_workqueue+0x92/0x6c0
[  656.385395]  nbd_disconnect_and_put+0x81/0xd0
[  656.385716]  nbd_genl_disconnect+0x125/0x2a0
[  656.386034]  genl_family_rcv_msg_doit.isra.0+0x102/0x1b0
[  656.386422]  genl_rcv_msg+0xfc/0x2b0
[  656.386685]  ? nbd_ioctl+0x490/0x490
[  656.386954]  ? genl_family_rcv_msg_doit.isra.0+0x1b0/0x1b0
[  656.387354]  netlink_rcv_skb+0x62/0x180
[  656.387638]  genl_rcv+0x34/0x60
[  656.387874]  netlink_unicast+0x26d/0x590
[  656.388162]  netlink_sendmsg+0x398/0x6c0
[  656.388451]  ? netlink_rcv_skb+0x180/0x180
[  656.388750]  ____sys_sendmsg+0x1da/0x320
[  656.389038]  ? ____sys_recvmsg+0x130/0x220
[  656.389334]  ___sys_sendmsg+0x8e/0xf0
[  656.389605]  ? ___sys_recvmsg+0xa2/0xf0
[  656.389889]  ? handle_mm_fault+0x1671/0x21d0
[  656.390201]  __sys_sendmsg+0x6d/0xe0
[  656.390464]  __x64_sys_sendmsg+0x23/0x30
[  656.390751]  do_syscall_64+0x45/0x70
[  656.391017]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: e9e006f5fcf2 ("nbd: fix max number of supported devs")
Suggested-by: Markus Elfring <redacted>
Signed-off-by: Sun Ke <redacted>
---
v4: Share exception handling code for if branches
v3: Do not use unlock and add put_nbd.
v2: Use jump target unlock.
---
 drivers/block/nbd.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index e6ea5d344f87..3c9b3bf3f4c2 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c

@@ -2014,17 +2014,20 @@ static int nbd_genl_disconnect(struct sk_buff *skb, struct genl_info *info)
 	mutex_lock(&nbd_index_mutex);
 	nbd = idr_find(&nbd_index_idr, index);
 	if (!nbd) {
-		mutex_unlock(&nbd_index_mutex);
 		printk(KERN_ERR "nbd: couldn't find device at index %d\n",
 		       index);
-		return -EINVAL;
+		goto unlock_index;
 	}
-	if (!refcount_inc_not_zero(&nbd->refs)) {
-		mutex_unlock(&nbd_index_mutex);
+	mutex_lock(&nbd->config_lock);
+	if (!refcount_inc_not_zero(&nbd->refs) || !nbd->recv_workq) {
+		mutex_unlock(&nbd->config_lock);
 		printk(KERN_ERR "nbd: device at index %d is going down\n",
 		       index);
+unlock_index:
+		mutex_unlock(&nbd_index_mutex);
 		return -EINVAL;
 	}
+	mutex_unlock(&nbd->config_lock);
 	mutex_unlock(&nbd_index_mutex);
 	if (!refcount_inc_not_zero(&nbd->config_refs)) {
 		nbd_put(nbd);

-- 
2.25.4

On 2/5/21 1:56 AM, Sun Ke wrote:

We can't safely take the ->config_log if we don't have a ref.  Just add

if (nbd->recv_workq)
	flush_workqueue(nbd->recv_workq);

to nbd_disconnect_and_put().  Problem solved, we can't drop it until we drop our last config ref, and we're holding a config ref here.  We could probably add it in the meantime, but at this point we've disconnected all of our sockets so it doesn't matter.  Thanks,

Josef