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Subject[PATCH v4] drm/i915: Synchronize active and retire callbacks
Date
From: Sultan Alsawaf <sultan@kerneltoast.com>

Active and retire callbacks can run simultaneously, causing panics and
mayhem. The most notable case is with the intel_context_pin/unpin race
that causes ring and page table corruption. In 5.4, this race is more
noticeable because intel_ring_unpin() sets ring->vaddr to NULL and
causes a clean NULL-pointer-dereference panic, but in newer kernels this
race goes unnoticed.

Here is an example of a crash caused by this race on 5.4:
BUG: unable to handle page fault for address: 0000000000003448
RIP: 0010:gen8_emit_flush_render+0x163/0x190
Call Trace:
execlists_request_alloc+0x25/0x40
__i915_request_create+0x1f4/0x2c0
i915_request_create+0x71/0xc0
i915_gem_do_execbuffer+0xb98/0x1a80
? preempt_count_add+0x68/0xa0
? _raw_spin_lock+0x13/0x30
? _raw_spin_unlock+0x16/0x30
i915_gem_execbuffer2_ioctl+0x1de/0x3c0
? i915_gem_busy_ioctl+0x7f/0x1d0
? i915_gem_execbuffer_ioctl+0x2d0/0x2d0
drm_ioctl_kernel+0xb2/0x100
drm_ioctl+0x209/0x360
? i915_gem_execbuffer_ioctl+0x2d0/0x2d0
ksys_ioctl+0x87/0xc0
__x64_sys_ioctl+0x16/0x20
do_syscall_64+0x4e/0x150
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Protect the active and retire callbacks with their own lock to prevent
them from running at the same time as one another.

Fixes: 12c255b5dad1 ("drm/i915: Provide an i915_active.acquire callback")
Cc: <stable@vger.kernel.org>
Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
---
drivers/gpu/drm/i915/i915_active.c | 52 ++++++++++++++++++++----
drivers/gpu/drm/i915/i915_active.h | 10 ++---
drivers/gpu/drm/i915/i915_active_types.h | 2 +
3 files changed, 50 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index b0a499753526..5802233f71ec 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -147,8 +147,22 @@ __active_retire(struct i915_active *ref)
spin_unlock_irqrestore(&ref->tree_lock, flags);

/* After the final retire, the entire struct may be freed */
- if (ref->retire)
- ref->retire(ref);
+ if (ref->retire) {
+ if (ref->active) {
+ bool freed = false;
+
+ /* Don't race with the active callback, and avoid UaF */
+ down_write(&ref->rwsem);
+ ref->freed = &freed;
+ ref->retire(ref);
+ if (!freed) {
+ ref->freed = NULL;
+ up_write(&ref->rwsem);
+ }
+ } else {
+ ref->retire(ref);
+ }
+ }

/* ... except if you wait on it, you must manage your own references! */
wake_up_var(ref);
@@ -278,7 +292,8 @@ void __i915_active_init(struct i915_active *ref,
int (*active)(struct i915_active *ref),
void (*retire)(struct i915_active *ref),
struct lock_class_key *mkey,
- struct lock_class_key *wkey)
+ struct lock_class_key *wkey,
+ struct lock_class_key *rkey)
{
unsigned long bits;

@@ -287,8 +302,13 @@ void __i915_active_init(struct i915_active *ref,
ref->flags = 0;
ref->active = active;
ref->retire = ptr_unpack_bits(retire, &bits, 2);
- if (bits & I915_ACTIVE_MAY_SLEEP)
+ ref->freed = NULL;
+ if (ref->active && ref->retire) {
+ __init_rwsem(&ref->rwsem, "i915_active.rwsem", rkey);
ref->flags |= I915_ACTIVE_RETIRE_SLEEPS;
+ } else if (bits & I915_ACTIVE_MAY_SLEEP) {
+ ref->flags |= I915_ACTIVE_RETIRE_SLEEPS;
+ }

spin_lock_init(&ref->tree_lock);
ref->tree = RB_ROOT;
@@ -417,8 +437,20 @@ int i915_active_acquire(struct i915_active *ref)
return err;

if (likely(!i915_active_acquire_if_busy(ref))) {
- if (ref->active)
- err = ref->active(ref);
+ if (ref->active) {
+ if (ref->retire) {
+ /*
+ * This can be a recursive call, and the mutex
+ * above already protects from concurrent active
+ * callbacks, so a read lock fits best.
+ */
+ down_read(&ref->rwsem);
+ err = ref->active(ref);
+ up_read(&ref->rwsem);
+ } else {
+ err = ref->active(ref);
+ }
+ }
if (!err) {
spin_lock_irq(&ref->tree_lock); /* __active_retire() */
debug_active_activate(ref);
@@ -502,16 +534,20 @@ int i915_request_await_active(struct i915_request *rq, struct i915_active *ref)
return err;
}

-#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)
void i915_active_fini(struct i915_active *ref)
{
+ if (ref->freed) {
+ *ref->freed = true;
+ up_write(&ref->rwsem);
+ }
+#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)
debug_active_fini(ref);
GEM_BUG_ON(atomic_read(&ref->count));
GEM_BUG_ON(work_pending(&ref->work));
GEM_BUG_ON(!RB_EMPTY_ROOT(&ref->tree));
mutex_destroy(&ref->mutex);
-}
#endif
+}

static inline bool is_idle_barrier(struct active_node *node, u64 idx)
{
diff --git a/drivers/gpu/drm/i915/i915_active.h b/drivers/gpu/drm/i915/i915_active.h
index 51e1e854ca55..b684b1fdcc02 100644
--- a/drivers/gpu/drm/i915/i915_active.h
+++ b/drivers/gpu/drm/i915/i915_active.h
@@ -153,14 +153,16 @@ void __i915_active_init(struct i915_active *ref,
int (*active)(struct i915_active *ref),
void (*retire)(struct i915_active *ref),
struct lock_class_key *mkey,
- struct lock_class_key *wkey);
+ struct lock_class_key *wkey,
+ struct lock_class_key *rkey);

/* Specialise each class of i915_active to avoid impossible lockdep cycles. */
#define i915_active_init(ref, active, retire) do { \
static struct lock_class_key __mkey; \
static struct lock_class_key __wkey; \
+ static struct lock_class_key __rkey; \
\
- __i915_active_init(ref, active, retire, &__mkey, &__wkey); \
+ __i915_active_init(ref, active, retire, &__mkey, &__wkey, &__rkey); \
} while (0)

int i915_active_ref(struct i915_active *ref,
@@ -200,11 +202,7 @@ i915_active_is_idle(const struct i915_active *ref)
return !atomic_read(&ref->count);
}

-#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)
void i915_active_fini(struct i915_active *ref);
-#else
-static inline void i915_active_fini(struct i915_active *ref) { }
-#endif

int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
struct intel_engine_cs *engine);
diff --git a/drivers/gpu/drm/i915/i915_active_types.h b/drivers/gpu/drm/i915/i915_active_types.h
index 6360c3e4b765..aaee2548cb19 100644
--- a/drivers/gpu/drm/i915/i915_active_types.h
+++ b/drivers/gpu/drm/i915/i915_active_types.h
@@ -32,6 +32,8 @@ struct active_node;
struct i915_active {
atomic_t count;
struct mutex mutex;
+ struct rw_semaphore rwsem;
+ bool *freed;

spinlock_t tree_lock;
struct active_node *cache;
--
2.26.0
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