[PATCH 6.18 322/752] 9p/xen: protect xen_9pfs_front_free against concurrent calls

Sasha Levin
2026-02-28T17:52:38Z

From: Stefano Stabellini <stefano.stabellini@amd.com>

[ Upstream commit ce8ded2e61f47747e31eeefb44dc24a2160a7e32 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/9p/trans_xen.c | 85 ++++++++++++++++++++++++----------------------
1 file changed, 44 insertions(+), 41 deletions(-)

diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
index b9ff69c7522a1..068d57515dd58 100644
--- a/net/9p/trans_xen.c
+++ b/net/9p/trans_xen.c
@@ -274,45 +274,52 @@ 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++) {
- struct xen_9pfs_dataring *ring = &priv->rings[i];
-
- cancel_work_sync(&ring->work);
-
- if (!priv->rings[i].intf)
- break;
- if (priv->rings[i].irq > 0)
- unbind_from_irqhandler(priv->rings[i].irq, ring);
- if (priv->rings[i].data.in) {
- for (j = 0;
- j < (1 << priv->rings[i].intf->ring_order);
- j++) {
- grant_ref_t ref;
-
- ref = priv->rings[i].intf->ref[j];
- gnttab_end_foreign_access(ref, NULL);
- }
- free_pages_exact(priv->rings[i].data.in,
+ if (priv->rings) {
+ for (i = 0; i < XEN_9PFS_NUM_RINGS; i++) {
+ struct xen_9pfs_dataring *ring = &priv->rings[i];
+
+ cancel_work_sync(&ring->work);
+
+ if (!priv->rings[i].intf)
+ break;
+ if (priv->rings[i].irq > 0)
+ unbind_from_irqhandler(priv->rings[i].irq, ring);
+ if (priv->rings[i].data.in) {
+ for (j = 0;
+ j < (1 << priv->rings[i].intf->ring_order);
+ j++) {
+ grant_ref_t ref;
+
+ ref = priv->rings[i].intf->ref[j];
+ gnttab_end_foreign_access(ref, NULL);
+ }
+ free_pages_exact(priv->rings[i].data.in,
1UL << (priv->rings[i].intf->ring_order +
XEN_PAGE_SHIFT));
+ }
+ gnttab_end_foreign_access(priv->rings[i].ref, NULL);
+ free_page((unsigned long)priv->rings[i].intf);
}
- gnttab_end_foreign_access(priv->rings[i].ref, NULL);
- free_page((unsigned long)priv->rings[i].intf);
+ kfree(priv->rings);
}
- 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);
}

@@ -379,7 +386,7 @@ 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;

@@ -407,6 +414,10 @@ 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) {
@@ -465,6 +476,11 @@ 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;

@@ -479,19 +495,6 @@ 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;
}

--
2.51.0