[PATCH] 9p/xen: protect xen_9pfs_front_free against concurrent calls

[Stefano Stabellini <stefano.stabellini@xxxxxxx>]

The xenwatch thread can race with other back-end change notifications
and call xen_9pfs_front_free() twice, hitting the observed general
protection fault due to a double-free. Guard the teardown path so only
one caller can release the front-end state at a time, preventing the
crash.

This is a fix for the following double-free:

[   27.052347] Oops: general protection fault, probably for non-canonical 
address 0x6b6b6b6b6b6b6b6b: 0000 [#1] SMP DEBUG_PAGEALLOC NOPTI
[   27.052357] CPU: 0 UID: 0 PID: 32 Comm: xenwatch Not tainted 
6.18.0-02087-g51ab33fc0a8b-dirty #60 PREEMPT(none)
[   27.052363] RIP: e030:xen_9pfs_front_free+0x1d/0x150
[   27.052368] Code: 90 90 90 90 90 90 90 90 90 90 90 90 90 41 55 41 54 55 48 
89 fd 48 c7 c7 48 d0 92 85 53 e8 cb cb 05 00 48 8b 45 08 48 8b 55 00 <48> 3b 28 
0f 85 f9 28 35 fe 48 3b 6a 08 0f 85 ef 28 35 fe 48 89 42
[   27.052377] RSP: e02b:ffffc9004016fdd0 EFLAGS: 00010246
[   27.052381] RAX: 6b6b6b6b6b6b6b6b RBX: ffff88800d66e400 RCX: 0000000000000000
[   27.052385] RDX: 6b6b6b6b6b6b6b6b RSI: 0000000000000000 RDI: 0000000000000000
[   27.052389] RBP: ffff88800a887040 R08: 0000000000000000 R09: 0000000000000000
[   27.052393] R10: 0000000000000000 R11: 0000000000000000 R12: ffff888009e46b68
[   27.052397] R13: 0000000000000200 R14: 0000000000000000 R15: ffff88800a887040
[   27.052404] FS:  0000000000000000(0000) GS:ffff88808ca57000(0000) 
knlGS:0000000000000000
[   27.052408] CS:  e030 DS: 0000 ES: 0000 CR0: 0000000080050033
[   27.052412] CR2: 00007f9714004360 CR3: 0000000004834000 CR4: 0000000000050660
[   27.052418] Call Trace:
[   27.052420]  <TASK>
[   27.052422]  xen_9pfs_front_changed+0x5d5/0x720
[   27.052426]  ? xenbus_otherend_changed+0x72/0x140
[   27.052430]  ? __pfx_xenwatch_thread+0x10/0x10
[   27.052434]  xenwatch_thread+0x94/0x1c0
[   27.052438]  ? __pfx_autoremove_wake_function+0x10/0x10
[   27.052442]  kthread+0xf8/0x240
[   27.052445]  ? __pfx_kthread+0x10/0x10
[   27.052449]  ? __pfx_kthread+0x10/0x10
[   27.052452]  ret_from_fork+0x16b/0x1a0
[   27.052456]  ? __pfx_kthread+0x10/0x10
[   27.052459]  ret_from_fork_asm+0x1a/0x30
[   27.052463]  </TASK>
[   27.052465] Modules linked in:
[   27.052471] ---[ end trace 0000000000000000 ]---

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
---
 net/9p/trans_xen.c | 41 +++++++++++++++++++++--------------------
 1 file changed, 21 insertions(+), 20 deletions(-)

diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
index 280f686f60fbb..8809f3c06ec95 100644
--- a/net/9p/trans_xen.c
+++ b/net/9p/trans_xen.c
@@ -274,11 +274,7 @@ static void xen_9pfs_front_free(struct xen_9pfs_front_priv 
*priv)
 {
        int i, j;
 
-       write_lock(&xen_9pfs_lock);
-       list_del(&priv->list);
-       write_unlock(&xen_9pfs_lock);
-
-       for (i = 0; i < XEN_9PFS_NUM_RINGS; i++) {
+       for (i = 0; priv->rings != NULL && i < XEN_9PFS_NUM_RINGS; i++) {
                struct xen_9pfs_dataring *ring = &priv->rings[i];
 
                cancel_work_sync(&ring->work);
@@ -310,9 +306,18 @@ static void xen_9pfs_front_free(struct xen_9pfs_front_priv 
*priv)
 
 static void xen_9pfs_front_remove(struct xenbus_device *dev)
 {
-       struct xen_9pfs_front_priv *priv = dev_get_drvdata(&dev->dev);
+       struct xen_9pfs_front_priv *priv;
 
+       write_lock(&xen_9pfs_lock);
+       priv = dev_get_drvdata(&dev->dev);
+       if (priv == NULL) {
+               write_unlock(&xen_9pfs_lock);
+               return;
+       }
        dev_set_drvdata(&dev->dev, NULL);
+       list_del_init(&priv->list);
+       write_unlock(&xen_9pfs_lock);
+
        xen_9pfs_front_free(priv);
 }
 
@@ -387,7 +392,7 @@ static int xen_9pfs_front_init(struct xenbus_device *dev)
 {
        int ret, i;
        struct xenbus_transaction xbt;
-       struct xen_9pfs_front_priv *priv = dev_get_drvdata(&dev->dev);
+       struct xen_9pfs_front_priv *priv;
        char *versions, *v;
        unsigned int max_rings, max_ring_order, len = 0;
 
@@ -415,6 +420,10 @@ static int xen_9pfs_front_init(struct xenbus_device *dev)
        if (p9_xen_trans.maxsize > XEN_FLEX_RING_SIZE(max_ring_order))
                p9_xen_trans.maxsize = XEN_FLEX_RING_SIZE(max_ring_order) / 2;
 
+       priv = kzalloc(sizeof(*priv), GFP_KERNEL);
+       if (!priv)
+               return -ENOMEM;
+       priv->dev = dev;
        priv->rings = kcalloc(XEN_9PFS_NUM_RINGS, sizeof(*priv->rings),
                              GFP_KERNEL);
        if (!priv->rings) {
@@ -473,6 +482,11 @@ static int xen_9pfs_front_init(struct xenbus_device *dev)
                goto error;
        }
 
+       write_lock(&xen_9pfs_lock);
+       dev_set_drvdata(&dev->dev, priv);
+       list_add_tail(&priv->list, &xen_9pfs_devs);
+       write_unlock(&xen_9pfs_lock);
+
        xenbus_switch_state(dev, XenbusStateInitialised);
        return 0;
 
@@ -487,19 +501,6 @@ static int xen_9pfs_front_init(struct xenbus_device *dev)
 static int xen_9pfs_front_probe(struct xenbus_device *dev,
                                const struct xenbus_device_id *id)
 {
-       struct xen_9pfs_front_priv *priv = NULL;
-
-       priv = kzalloc(sizeof(*priv), GFP_KERNEL);
-       if (!priv)
-               return -ENOMEM;
-
-       priv->dev = dev;
-       dev_set_drvdata(&dev->dev, priv);
-
-       write_lock(&xen_9pfs_lock);
-       list_add_tail(&priv->list, &xen_9pfs_devs);
-       write_unlock(&xen_9pfs_lock);
-
        return 0;
 }
 
-- 
2.25.1