[PATCH 5.10 033/199] drm/i915: Check for rq->hwsp validity after acquiring RCU lock

Greg Kroah-Hartman
2021-01-25T19:08:03Z

From: Chris Wilson <chris@chris-wilson.co.uk>

commit 45db630e5f7ec83817c57c8ae387fe219bd42adf upstream.

Since we allow removing the timeline map at runtime, there is a risk
that rq->hwsp points into a stale page. To control that risk, we hold
the RCU read lock while reading *rq->hwsp, but we missed a couple of
important barriers. First, the unpinning / removal of the timeline map
must be after all RCU readers into that map are complete, i.e. after an
rcu barrier (in this case courtesy of call_rcu()). Secondly, we must
make sure that the rq->hwsp we are about to dereference under the RCU
lock is valid. In this case, we make the rq->hwsp pointer safe during
i915_request_retire() and so we know that rq->hwsp may become invalid
only after the request has been signaled. Therefore is the request is
not yet signaled when we acquire rq->hwsp under the RCU, we know that
rq->hwsp will remain valid for the duration of the RCU read lock.

This is a very small window that may lead to either considering the
request not completed (causing a delay until the request is checked
again, any wait for the request is not affected) or dereferencing an
invalid pointer.

Fixes: 3adac4689f58 ("drm/i915: Introduce concept of per-timeline (context) HWSP")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: <stable@vger.kernel.org> # v5.1+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201218122421.18344-1-chris@chris-wilson.co.uk
(cherry picked from commit 9bb36cf66091ddf2d8840e5aa705ad3c93a6279b)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210118101755.476744-1-chris@chris-wilson.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 9 +-----
drivers/gpu/drm/i915/gt/intel_timeline.c | 10 +++----
drivers/gpu/drm/i915/i915_request.h | 37 ++++++++++++++++++++++++----
3 files changed, 38 insertions(+), 18 deletions(-)

--- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
@@ -134,11 +134,6 @@ static bool remove_signaling_context(str
return true;
}

-static inline bool __request_completed(const struct i915_request *rq)
-{
- return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno);
-}
-
__maybe_unused static bool
check_signal_order(struct intel_context *ce, struct i915_request *rq)
{
@@ -257,7 +252,7 @@ static void signal_irq_work(struct irq_w
list_for_each_entry_rcu(rq, &ce->signals, signal_link) {
bool release;

- if (!__request_completed(rq))
+ if (!__i915_request_is_complete(rq))
break;

if (!test_and_clear_bit(I915_FENCE_FLAG_SIGNAL,
@@ -379,7 +374,7 @@ static void insert_breadcrumb(struct i91
* straight onto a signaled list, and queue the irq worker for
* its signal completion.
*/
- if (__request_completed(rq)) {
+ if (__i915_request_is_complete(rq)) {
if (__signal_request(rq) &&
llist_add(&rq->signal_node, &b->signaled_requests))
irq_work_queue(&b->irq_work);
--- a/drivers/gpu/drm/i915/gt/intel_timeline.c
+++ b/drivers/gpu/drm/i915/gt/intel_timeline.c
@@ -126,6 +126,10 @@ static void __rcu_cacheline_free(struct
struct intel_timeline_cacheline *cl =
container_of(rcu, typeof(*cl), rcu);

+ /* Must wait until after all *rq->hwsp are complete before removing */
+ i915_gem_object_unpin_map(cl->hwsp->vma->obj);
+ __idle_hwsp_free(cl->hwsp, ptr_unmask_bits(cl->vaddr, CACHELINE_BITS));
+
i915_active_fini(&cl->active);
kfree(cl);
}
@@ -133,11 +137,6 @@ static void __rcu_cacheline_free(struct
static void __idle_cacheline_free(struct intel_timeline_cacheline *cl)
{
GEM_BUG_ON(!i915_active_is_idle(&cl->active));
-
- i915_gem_object_unpin_map(cl->hwsp->vma->obj);
- i915_vma_put(cl->hwsp->vma);
- __idle_hwsp_free(cl->hwsp, ptr_unmask_bits(cl->vaddr, CACHELINE_BITS));
-
call_rcu(&cl->rcu, __rcu_cacheline_free);
}

@@ -179,7 +178,6 @@ cacheline_alloc(struct intel_timeline_hw
return ERR_CAST(vaddr);
}

- i915_vma_get(hwsp->vma);
cl->hwsp = hwsp;
cl->vaddr = page_pack_bits(vaddr, cacheline);

--- a/drivers/gpu/drm/i915/i915_request.h
+++ b/drivers/gpu/drm/i915/i915_request.h
@@ -434,7 +434,7 @@ static inline u32 hwsp_seqno(const struc

static inline bool __i915_request_has_started(const struct i915_request *rq)
{
- return i915_seqno_passed(hwsp_seqno(rq), rq->fence.seqno - 1);
+ return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno - 1);
}

/**
@@ -465,11 +465,19 @@ static inline bool __i915_request_has_st
*/
static inline bool i915_request_started(const struct i915_request *rq)
{
+ bool result;
+
if (i915_request_signaled(rq))
return true;

- /* Remember: started but may have since been preempted! */
- return __i915_request_has_started(rq);
+ result = true;
+ rcu_read_lock(); /* the HWSP may be freed at runtime */
+ if (likely(!i915_request_signaled(rq)))
+ /* Remember: started but may have since been preempted! */
+ result = __i915_request_has_started(rq);
+ rcu_read_unlock();
+
+ return result;
}

/**
@@ -482,10 +490,16 @@ static inline bool i915_request_started(
*/
static inline bool i915_request_is_running(const struct i915_request *rq)
{
+ bool result;
+
if (!i915_request_is_active(rq))
return false;

- return __i915_request_has_started(rq);
+ rcu_read_lock();
+ result = __i915_request_has_started(rq) && i915_request_is_active(rq);
+ rcu_read_unlock();
+
+ return result;
}

/**
@@ -509,12 +523,25 @@ static inline bool i915_request_is_ready
return !list_empty(&rq->sched.link);
}

+static inline bool __i915_request_is_complete(const struct i915_request *rq)
+{
+ return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno);
+}
+
static inline bool i915_request_completed(const struct i915_request *rq)
{
+ bool result;
+
if (i915_request_signaled(rq))
return true;

- return i915_seqno_passed(hwsp_seqno(rq), rq->fence.seqno);
+ result = true;
+ rcu_read_lock(); /* the HWSP may be freed at runtime */
+ if (likely(!i915_request_signaled(rq)))
+ result = __i915_request_is_complete(rq);
+ rcu_read_unlock();
+
+ return result;
}

static inline void i915_request_mark_complete(struct i915_request *rq)