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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 06 May 2025 21:09:24.8239 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 1a3ed70f-39bd-49f0-e110-08dd8ce247af X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[SATLEXMB04.amd.com] X-MS-Exchange-CrossTenant-AuthSource: SJ1PEPF00002321.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA1PR12MB8945 X-ZohoMail-DKIM: pass (identity @amd.com) X-ZM-MESSAGEID: 1746565797294019000 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: __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-G=C3=B3recki" Fixes: fd8aa9095a95 ("xen: optimize xenbus driver for multiple concurrent x= enstore accesses") Cc: stable@vger.kernel.org Signed-off-by: Jason Andryuk Reviewed-by: Juergen Gross --- 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); =20 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 =3D xb_req_state_got_reply; req->cb(req); - } else - kfree(req); + } + kref_put(&req->kref, xs_free_req); } =20 mutex_unlock(&xs_response_mutex); @@ -386,14 +386,13 @@ static int process_writes(void) state.req->msg.type =3D XS_ERROR; state.req->err =3D err; list_del(&state.req->list); - if (state.req->state =3D=3D xb_req_state_aborted) - kfree(state.req); - else { + if (state.req->state !=3D xb_req_state_aborted) { /* write err, then update state */ virt_wmb(); state.req->state =3D xb_req_state_got_reply; wake_up(&state.req->wq); } + kref_put(&state.req->kref, xs_free_req); =20 mutex_unlock(&xb_write_mutex); =20 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); =20 kfree(req->body); - kfree(req); + kref_put(&req->kref, xs_free_req); =20 kref_put(&u->kref, xenbus_file_free); =20 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); } =20 +void xs_free_req(struct kref *kref) +{ + struct xb_req_data *req =3D 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 xs= d_sockmsg *msg) req->caller_req_id =3D req->msg.req_id; req->msg.req_id =3D xs_request_enter(req); =20 + /* + * 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 =3D 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 =3D=3D xb_req_state_queued || req->state =3D=3D xb_req_state_wait_reply) req->state =3D xb_req_state_aborted; - else - kfree(req); + + kref_put(&req->kref, xs_free_req); mutex_unlock(&xb_write_mutex); =20 return ret; @@ -291,6 +303,7 @@ int xenbus_dev_request_and_reply(struct xsd_sockmsg *ms= g, void *par) req->cb =3D xenbus_dev_queue_reply; req->par =3D par; req->user_req =3D true; + kref_init(&req->kref); =20 xs_send(req, msg); =20 @@ -319,6 +332,7 @@ static void *xs_talkv(struct xenbus_transaction t, req->num_vecs =3D num_vecs; req->cb =3D xs_wake_up; req->user_req =3D false; + kref_init(&req->kref); =20 msg.req_id =3D 0; msg.tx_id =3D t.id; --=20 2.34.1