Am 29.08.24 um 19:12 schrieb Boris Brezillon:
dma_fence objects created by an entity might outlive the
drm_gpu_scheduler this entity was bound to if those fences are retained
by other other objects, like a dma_buf resv. This means that
drm_sched_fence::sched might be invalid when the resv is walked, which
in turn leads to a UAF when dma_fence_ops::get_timeline_name() is called.
This probably went unnoticed so far, because the drm_gpu_scheduler had
the lifetime of the drm_device, so, unless you were removing the device,
there were no reasons for the scheduler to be gone before its fences.
Nope, that is intentional design. get_timeline_name() is not safe to be
called after the fence signaled because that would causes circular
dependency problems.
E.g. when you have hardware fences it can happen that fences reference a
driver module (for the function printing the name) and the module in
turn keeps fences around.
So you easily end up with a module you can never unload.
With the introduction of a new model where each entity has its own
drm_gpu_scheduler instance, this situation is likely to happen every time
a GPU context is destroyed and some of its fences remain attached to
dma_buf objects still owned by other drivers/processes.
In order to make drm_sched_fence_get_timeline_name() safe, we need to
copy the scheduler name into our own refcounted object that's only
destroyed when both the scheduler and all its fences are gone.
The fact drm_sched_fence might have a reference to the drm_gpu_scheduler
even after it's been released is worrisome though, but I'd rather
discuss that with everyone than come up with a solution that's likely
to end up being rejected.
Note that the bug was found while repeatedly reading dma_buf's debugfs
file, which, at some point, calls dma_resv_describe() on a resv that
contains signalled fences coming from a destroyed GPU context.
AFAIK, there's nothing invalid there.
Yeah but reading debugfs is not guaranteed to crash the kernel.
On the other hand the approach with a kref'ed string looks rather sane
to me. One comment on this below.
Cc: Luben Tuikov <ltuikov89@xxxxxxxxx>
Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
Cc: "Christian König" <christian.koenig@xxxxxxx>
Cc: Danilo Krummrich <dakr@xxxxxxxxxx>
Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
---
drivers/gpu/drm/scheduler/sched_fence.c | 8 +++-
drivers/gpu/drm/scheduler/sched_main.c | 18 ++++++++-
include/drm/gpu_scheduler.h | 52 +++++++++++++++++++++++++
3 files changed, 75 insertions(+), 3 deletions(-)
diff --git a/drivers/gpu/drm/scheduler/sched_fence.c b/drivers/gpu/drm/scheduler/sched_fence.c
index 0f35f009b9d3..efa2a71d98eb 100644
--- a/drivers/gpu/drm/scheduler/sched_fence.c
+++ b/drivers/gpu/drm/scheduler/sched_fence.c
@@ -90,7 +90,7 @@ static const char *drm_sched_fence_get_driver_name(struct dma_fence *fence)
static const char *drm_sched_fence_get_timeline_name(struct dma_fence *f)
{
struct drm_sched_fence *fence = to_drm_sched_fence(f);
- return (const char *)fence->sched->name;
+ return (const char *)fence->timeline->name;
}
static void drm_sched_fence_free_rcu(struct rcu_head *rcu)
@@ -112,8 +112,10 @@ static void drm_sched_fence_free_rcu(struct rcu_head *rcu)
*/
void drm_sched_fence_free(struct drm_sched_fence *fence)
{
+ drm_sched_fence_timeline_put(fence->timeline);
+
/* This function should not be called if the fence has been initialized. */
- if (!WARN_ON_ONCE(fence->sched))
+ if (!WARN_ON_ONCE(fence->timeline))
kmem_cache_free(sched_fence_slab, fence);
}
@@ -224,6 +226,8 @@ void drm_sched_fence_init(struct drm_sched_fence *fence,
unsigned seq;
fence->sched = entity->rq->sched;
+ fence->timeline = fence->sched->fence_timeline;
+ drm_sched_fence_timeline_get(fence->timeline);
seq = atomic_inc_return(&entity->fence_seq);
dma_fence_init(&fence->scheduled, &drm_sched_fence_ops_scheduled,
&fence->lock, entity->fence_context, seq);
diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
index 7e90c9f95611..1e31a9c8ce15 100644
--- a/drivers/gpu/drm/scheduler/sched_main.c
+++ b/drivers/gpu/drm/scheduler/sched_main.c
@@ -1288,10 +1288,21 @@ int drm_sched_init(struct drm_gpu_scheduler *sched,
sched->own_submit_wq = true;
}
+ sched->fence_timeline = kzalloc(sizeof(*sched->fence_timeline), GFP_KERNEL);
+ if (!sched->fence_timeline)
+ goto Out_check_own;
+
+ sched->fence_timeline->name = kasprintf(GFP_KERNEL, "%s", sched->name);
+ if (!sched->fence_timeline->name)
+ goto Out_free_fence_timeline;
+
+ kref_init(&sched->fence_timeline->kref);
+
sched->sched_rq = kmalloc_array(num_rqs, sizeof(*sched->sched_rq),
GFP_KERNEL | __GFP_ZERO);
if (!sched->sched_rq)
- goto Out_check_own;
+ goto Out_free_fence_timeline;
+
sched->num_rqs = num_rqs;
for (i = DRM_SCHED_PRIORITY_KERNEL; i < sched->num_rqs; i++) {
sched->sched_rq[i] = kzalloc(sizeof(*sched->sched_rq[i]), GFP_KERNEL);
@@ -1319,6 +1330,10 @@ int drm_sched_init(struct drm_gpu_scheduler *sched,
kfree(sched->sched_rq);
sched->sched_rq = NULL;
+Out_free_fence_timeline:
+ if (sched->fence_timeline)
+ kfree(sched->fence_timeline->name);
+ kfree(sched->fence_timeline);
Out_check_own:
if (sched->own_submit_wq)
destroy_workqueue(sched->submit_wq);
@@ -1367,6 +1382,7 @@ void drm_sched_fini(struct drm_gpu_scheduler *sched)
sched->ready = false;
kfree(sched->sched_rq);
sched->sched_rq = NULL;
+ drm_sched_fence_timeline_put(sched->fence_timeline);
}
EXPORT_SYMBOL(drm_sched_fini);
diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
index 5acc64954a88..615ca89f77dc 100644
--- a/include/drm/gpu_scheduler.h
+++ b/include/drm/gpu_scheduler.h
@@ -261,6 +261,52 @@ struct drm_sched_rq {
struct rb_root_cached rb_tree_root;
};
+/**
+ * struct drm_sched_fence_timeline - Wrapped around the timeline name
+ *
+ * This is needed to cope with the fact dma_fence objects created by
+ * an entity might outlive the drm_gpu_scheduler this entity was bound
+ * to, making drm_sched_fence::sched invalid and leading to a UAF when
+ * dma_fence_ops::get_timeline_name() is called.
+ */
+struct drm_sched_fence_timeline {
+ /** @kref: Reference count of this timeline object. */
+ struct kref kref;
+
+ /**
+ * @name: Name of the timeline.
+ *
+ * This is currently a copy of drm_gpu_scheduler::name.
+ */
+ const char *name;
Make that a char name[] and embed the name into the structure. The macro
struct_size() can be used to calculate the size.
+};
+
+static inline void
+drm_sched_fence_timeline_release(struct kref *kref)
+{
+ struct drm_sched_fence_timeline *tl =
+ container_of(kref, struct drm_sched_fence_timeline, kref);
+
+ kfree(tl->name);
+ kfree(tl);
This avoids having two allocations for the timeline name.
Regards,
Christian.
+}
+
+static inline void
+drm_sched_fence_timeline_put(struct drm_sched_fence_timeline *tl)
+{
+ if (tl)
+ kref_put(&tl->kref, drm_sched_fence_timeline_release);
+}
+
+static inline struct drm_sched_fence_timeline *
+drm_sched_fence_timeline_get(struct drm_sched_fence_timeline *tl)
+{
+ if (tl)
+ kref_get(&tl->kref);
+
+ return tl;
+}
+
/**
* struct drm_sched_fence - fences corresponding to the scheduling of a job.
*/
@@ -289,6 +335,9 @@ struct drm_sched_fence {
*/
ktime_t deadline;
+ /** @timeline: the timeline this fence belongs to. */
+ struct drm_sched_fence_timeline *timeline;
+
/**
* @parent: the fence returned by &drm_sched_backend_ops.run_job
* when scheduling the job on hardware. We signal the
@@ -488,6 +537,8 @@ struct drm_sched_backend_ops {
* @credit_count: the current credit count of this scheduler
* @timeout: the time after which a job is removed from the scheduler.
* @name: name of the ring for which this scheduler is being used.
+ * @fence_timeline: fence timeline that will be passed to fences created by
+ * and entity that's bound to this scheduler.
* @num_rqs: Number of run-queues. This is at most DRM_SCHED_PRIORITY_COUNT,
* as there's usually one run-queue per priority, but could be less.
* @sched_rq: An allocated array of run-queues of size @num_rqs;
@@ -521,6 +572,7 @@ struct drm_gpu_scheduler {
atomic_t credit_count;
long timeout;
const char *name;
+ struct drm_sched_fence_timeline *fence_timeline;
u32 num_rqs;
struct drm_sched_rq **sched_rq;
wait_queue_head_t job_scheduled;
