[PATCH] amdkfd: fix doorbell allocation race creating duplicate allocations

[Henry Martin <bsdhenrymartin@gmail.com>]

The check-and-allocate sequence on pdd->qpd.proc_doorbells is
lockless at every call site of kfd_alloc_process_doorbells():
kfd_get_process_doorbells() (mmap of /dev/kfd), kfd_ioctl_create_queue()
and the CRIU restore path each test the pointer under different
locking (or none), so two racing threads can both observe NULL and
each allocate a doorbell BO and bitmap, orphaning one allocation and
handing the two mappings different backing pages.

Move the serialization into kfd_alloc_process_doorbells() itself:
take kfd->doorbell_mutex and re-check under it, so all three call
sites are covered regardless of their own locking.  The existing
doorbell_mutex users (kfd_get_kernel_doorbell() /
kfd_release_kernel_doorbell()) only guard the device-global
kfd->doorbell_bitmap in short non-sleeping sections and never call
into this path, so there is no nesting.

This vulnerability was discovered by Tencent CodeBuddy Security.

Fixes: 428542d91772 ("drm/amd/display: Setup for mmhubbub3_warmup_mcif with big buffer")
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
---
 drivers/gpu/drm/amd/amdkfd/kfd_doorbell.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_doorbell.c b/drivers/gpu/drm/amd/amdkfd/kfd_doorbell.c
index fdcf7f2d1b5b4..07f881573837e 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_doorbell.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_doorbell.c
@@ -257,12 +257,21 @@ int kfd_alloc_process_doorbells(struct kfd_dev *kfd, struct kfd_process_device *
 	int r;
 	struct qcm_process_device *qpd = &pdd->qpd;

+	mutex_lock(&kfd->doorbell_mutex);
+
+	/* Another thread may have won the check-and-allocate race */
+	if (qpd->proc_doorbells) {
+		mutex_unlock(&kfd->doorbell_mutex);
+		return 0;
+	}
+
 	/* Allocate bitmap for dynamic doorbell allocation */
 	qpd->doorbell_bitmap = bitmap_zalloc(KFD_MAX_NUM_OF_QUEUES_PER_PROCESS,
 					     GFP_KERNEL);
 	if (!qpd->doorbell_bitmap) {
 		DRM_ERROR("Failed to allocate process doorbell bitmap\n");
-		return -ENOMEM;
+		r = -ENOMEM;
+		goto unlock;
 	}

 	r = init_doorbell_bitmap(&pdd->qpd, kfd);
@@ -284,11 +293,15 @@ int kfd_alloc_process_doorbells(struct kfd_dev *kfd, struct kfd_process_device *
 		DRM_ERROR("Failed to allocate process doorbells\n");
 		goto err;
 	}
+
+	mutex_unlock(&kfd->doorbell_mutex);
 	return 0;

 err:
 	bitmap_free(qpd->doorbell_bitmap);
 	qpd->doorbell_bitmap = NULL;
+unlock:
+	mutex_unlock(&kfd->doorbell_mutex);
 	return r;
 }

--
2.43.7