Commit ce8ded2e authored Jan 29, 2026 by Stefano Stabellini Committed by Dominique Martinet Jan 29, 2026

9p/xen: protect xen_9pfs_front_free against concurrent calls



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@amd.com>
Message-ID: <20260129230348.2390470-1-stefano.stabellini@amd.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

parent 51ffeabe

net/9p/trans_xen.c

+44 −41

Original line number	Diff line number	Diff line
		@@ -277,10 +277,7 @@ 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);

		if (priv->rings) {
		for (i = 0; i < XEN_9PFS_NUM_RINGS; i++) {
		struct xen_9pfs_dataring *ring = &priv->rings[i];

		@@ -307,15 +304,25 @@ static void xen_9pfs_front_free(struct xen_9pfs_front_priv *priv)
		free_page((unsigned long)priv->rings[i].intf);
		}
		kfree(priv->rings);
		}
		kfree(priv->tag);
		kfree(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(&priv->list);
		write_unlock(&xen_9pfs_lock);

		xen_9pfs_front_free(priv);
		}

		@@ -382,7 +389,7 @@ 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;

		@@ -410,6 +417,10 @@ 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) {
		@@ -468,6 +479,11 @@ 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;

		@@ -482,19 +498,6 @@ 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;
		}