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Message-ID: <20250506210935.5607-1-jason.andryuk@amd.com>
Date: Tue, 6 May 2025 17:09:33 -0400
From: Jason Andryuk <jason.andryuk@....com>
To: Juergen Gross <jgross@...e.com>, Stefano Stabellini
<sstabellini@...nel.org>, Oleksandr Tyshchenko
<oleksandr_tyshchenko@...m.com>, Boris Ostrovsky <boris.ostrovsky@...cle.com>
CC: Jason Andryuk <jason.andryuk@....com>,
Marek Marczykowski-Górecki
<marmarek@...isiblethingslab.com>, <stable@...r.kernel.org>,
<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH] xenbus: Use kref to track req lifetime
Marek reported seeing a NULL pointer fault in the xenbus_thread
callstack:
BUG: kernel NULL pointer dereference, address: 0000000000000000
RIP: e030:__wake_up_common+0x4c/0x180
Call Trace:
<TASK>
__wake_up_common_lock+0x82/0xd0
process_msg+0x18e/0x2f0
xenbus_thread+0x165/0x1c0
process_msg+0x18e is req->cb(req). req->cb is set to xs_wake_up(), a
thin wrapper around wake_up(), or xenbus_dev_queue_reply(). It seems
like it was xs_wake_up() in this case.
It seems like req may have woken up the xs_wait_for_reply(), which
kfree()ed the req. When xenbus_thread resumes, it faults on the zero-ed
data.
Linux Device Drivers 2nd edition states:
"Normally, a wake_up call can cause an immediate reschedule to happen,
meaning that other processes might run before wake_up returns."
... which would match the behaviour observed.
Change to keeping two krefs on each request. One for the caller, and
one for xenbus_thread. Each will kref_put() when finished, and the last
will free it.
This use of kref matches the description in
Documentation/core-api/kref.rst
Link: https://lore.kernel.org/xen-devel/ZO0WrR5J0xuwDIxW@mail-itl/
Reported-by: "Marek Marczykowski-Górecki" <marmarek@...isiblethingslab.com>
Fixes: fd8aa9095a95 ("xen: optimize xenbus driver for multiple concurrent xenstore accesses")
Cc: stable@...r.kernel.org
Signed-off-by: Jason Andryuk <jason.andryuk@....com>
---
Kinda RFC-ish as I don't know if it fixes Marek's issue. This does seem
like the correct approach if we are seeing req free()ed out from under
xenbus_thread.
drivers/xen/xenbus/xenbus.h | 2 ++
drivers/xen/xenbus/xenbus_comms.c | 9 ++++-----
drivers/xen/xenbus/xenbus_dev_frontend.c | 2 +-
drivers/xen/xenbus/xenbus_xs.c | 18 ++++++++++++++++--
4 files changed, 23 insertions(+), 8 deletions(-)
diff --git a/drivers/xen/xenbus/xenbus.h b/drivers/xen/xenbus/xenbus.h
index 13821e7e825e..9ac0427724a3 100644
--- a/drivers/xen/xenbus/xenbus.h
+++ b/drivers/xen/xenbus/xenbus.h
@@ -77,6 +77,7 @@ enum xb_req_state {
struct xb_req_data {
struct list_head list;
wait_queue_head_t wq;
+ struct kref kref;
struct xsd_sockmsg msg;
uint32_t caller_req_id;
enum xsd_sockmsg_type type;
@@ -103,6 +104,7 @@ int xb_init_comms(void);
void xb_deinit_comms(void);
int xs_watch_msg(struct xs_watch_event *event);
void xs_request_exit(struct xb_req_data *req);
+void xs_free_req(struct kref *kref);
int xenbus_match(struct device *_dev, const struct device_driver *_drv);
int xenbus_dev_probe(struct device *_dev);
diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
index e5fda0256feb..82df2da1b880 100644
--- a/drivers/xen/xenbus/xenbus_comms.c
+++ b/drivers/xen/xenbus/xenbus_comms.c
@@ -309,8 +309,8 @@ static int process_msg(void)
virt_wmb();
req->state = xb_req_state_got_reply;
req->cb(req);
- } else
- kfree(req);
+ }
+ kref_put(&req->kref, xs_free_req);
}
mutex_unlock(&xs_response_mutex);
@@ -386,14 +386,13 @@ static int process_writes(void)
state.req->msg.type = XS_ERROR;
state.req->err = err;
list_del(&state.req->list);
- if (state.req->state == xb_req_state_aborted)
- kfree(state.req);
- else {
+ if (state.req->state != xb_req_state_aborted) {
/* write err, then update state */
virt_wmb();
state.req->state = xb_req_state_got_reply;
wake_up(&state.req->wq);
}
+ kref_put(&state.req->kref, xs_free_req);
mutex_unlock(&xb_write_mutex);
diff --git a/drivers/xen/xenbus/xenbus_dev_frontend.c b/drivers/xen/xenbus/xenbus_dev_frontend.c
index 46f8916597e5..f5c21ba64df5 100644
--- a/drivers/xen/xenbus/xenbus_dev_frontend.c
+++ b/drivers/xen/xenbus/xenbus_dev_frontend.c
@@ -406,7 +406,7 @@ void xenbus_dev_queue_reply(struct xb_req_data *req)
mutex_unlock(&u->reply_mutex);
kfree(req->body);
- kfree(req);
+ kref_put(&req->kref, xs_free_req);
kref_put(&u->kref, xenbus_file_free);
diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
index d32c726f7a12..dcf9182c8451 100644
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -112,6 +112,12 @@ static void xs_suspend_exit(void)
wake_up_all(&xs_state_enter_wq);
}
+void xs_free_req(struct kref *kref)
+{
+ struct xb_req_data *req = container_of(kref, struct xb_req_data, kref);
+ kfree(req);
+}
+
static uint32_t xs_request_enter(struct xb_req_data *req)
{
uint32_t rq_id;
@@ -237,6 +243,12 @@ static void xs_send(struct xb_req_data *req, struct xsd_sockmsg *msg)
req->caller_req_id = req->msg.req_id;
req->msg.req_id = xs_request_enter(req);
+ /*
+ * Take 2nd ref. One for this thread, and the second for the
+ * xenbus_thread.
+ */
+ kref_get(&req->kref);
+
mutex_lock(&xb_write_mutex);
list_add_tail(&req->list, &xb_write_list);
notify = list_is_singular(&xb_write_list);
@@ -261,8 +273,8 @@ static void *xs_wait_for_reply(struct xb_req_data *req, struct xsd_sockmsg *msg)
if (req->state == xb_req_state_queued ||
req->state == xb_req_state_wait_reply)
req->state = xb_req_state_aborted;
- else
- kfree(req);
+
+ kref_put(&req->kref, xs_free_req);
mutex_unlock(&xb_write_mutex);
return ret;
@@ -291,6 +303,7 @@ int xenbus_dev_request_and_reply(struct xsd_sockmsg *msg, void *par)
req->cb = xenbus_dev_queue_reply;
req->par = par;
req->user_req = true;
+ kref_init(&req->kref);
xs_send(req, msg);
@@ -319,6 +332,7 @@ static void *xs_talkv(struct xenbus_transaction t,
req->num_vecs = num_vecs;
req->cb = xs_wake_up;
req->user_req = false;
+ kref_init(&req->kref);
msg.req_id = 0;
msg.tx_id = t.id;
--
2.34.1
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