diff --git a/kernel-open/common/inc/nvkms-kapi.h b/kernel-open/common/inc/nvkms-kapi.h
index 8e838eafa9..9b292c183a 100644
--- a/kernel-open/common/inc/nvkms-kapi.h
+++ b/kernel-open/common/inc/nvkms-kapi.h
@@ -1357,6 +1357,9 @@ struct NvKmsKapiFunctionsTable {
      * \param [out] pMaxSubmittedMap Returns a CPU mapping of the semaphore
      *                               surface's semaphore memory to the client.
      *
+     * \param [out] pSurfaceSize     Returns the size of the imported semaphore
+     *                               surface in bytes. May be NULL.
+     *
      * \return struct NvKmsKapiSemaphoreSurface* on success, NULL on failure.
      */
     struct NvKmsKapiSemaphoreSurface* (*importSemaphoreSurface)
@@ -1365,7 +1368,8 @@ struct NvKmsKapiFunctionsTable {
          NvU64 nvKmsParamsUser,
          NvU64 nvKmsParamsSize,
          void **pSemaphoreMap,
-         void **pMaxSubmittedMap
+         void **pMaxSubmittedMap,
+         NvU64 *pSurfaceSize
     );
 
     /*!
diff --git a/kernel-open/nvidia-drm/nvidia-drm-fence.c b/kernel-open/nvidia-drm/nvidia-drm-fence.c
index 7af1ed7f13..7483de1ef7 100644
--- a/kernel-open/nvidia-drm/nvidia-drm-fence.c
+++ b/kernel-open/nvidia-drm/nvidia-drm-fence.c
@@ -1231,13 +1231,15 @@ __nv_drm_semsurf_fence_ctx_new(
     struct NvKmsKapiSemaphoreSurface *pSemSurface;
     uint8_t *semMapping;
     uint8_t *maxSubmittedMapping;
+    NvU64 surfaceSize = 0;
     char worker_name[20+16+1]; /* strlen(nvidia-drm/timeline-) + 16 for %llx + NUL */
 
     pSemSurface = nvKms->importSemaphoreSurface(nv_dev->pDevice,
                                                 p->nvkms_params_ptr,
                                                 p->nvkms_params_size,
                                                 (void **)&semMapping,
-                                                (void **)&maxSubmittedMapping);
+                                                (void **)&maxSubmittedMapping,
+                                                &surfaceSize);
     if (!pSemSurface) {
         NV_DRM_DEV_LOG_ERR(
             nv_dev,
@@ -1246,6 +1248,27 @@ __nv_drm_semsurf_fence_ctx_new(
         goto failed;
     }
 
+    /*
+     * The index is provided by userspace as a 64-bit value. Reject values
+     * outside the imported surface before shifting the CPU mappings by them,
+     * and before truncating to the 32-bit RM semaphore index below. The
+     * max-submitted value must fit within one stride so that indexing the
+     * semaphore mapping also bounds the max-submitted mapping.
+     */
+    if (nv_dev->semsurf_stride == 0 ||
+        (nv_dev->semsurf_max_submitted_offset + sizeof(NvU64)) >
+            nv_dev->semsurf_stride ||
+        p->index >= (surfaceSize / nv_dev->semsurf_stride) ||
+        p->index > NV_U32_MAX) {
+        NV_DRM_DEV_LOG_ERR(
+            nv_dev,
+            "Invalid semaphore index %" NvU64_fmtu " for semaphore surface of %"
+            NvU64_fmtu " bytes",
+            p->index, surfaceSize);
+
+        goto failed_alloc_fence_context;
+    }
+
     /*
      * Allocate a fence context object and initialize it.
      */
diff --git a/src/nvidia-modeset/kapi/include/nvkms-kapi-internal.h b/src/nvidia-modeset/kapi/include/nvkms-kapi-internal.h
index 1757f13953..0fcc89328f 100644
--- a/src/nvidia-modeset/kapi/include/nvkms-kapi-internal.h
+++ b/src/nvidia-modeset/kapi/include/nvkms-kapi-internal.h
@@ -237,7 +237,8 @@ nvKmsKapiImportSemaphoreSurface(struct NvKmsKapiDevice *device,
                                 NvU64 nvKmsParamsUser,
                                 NvU64 nvKmsParamsSize,
                                 void **pSemaphoreMap,
-                                void **pMaxSubmittedMap);
+                                void **pMaxSubmittedMap,
+                                NvU64 *pSurfaceSize);
 
 void
 nvKmsKapiFreeSemaphoreSurface(struct NvKmsKapiDevice *device,
diff --git a/src/nvidia-modeset/kapi/interface/nvkms-kapi.h b/src/nvidia-modeset/kapi/interface/nvkms-kapi.h
index 8e838eafa9..9b292c183a 100644
--- a/src/nvidia-modeset/kapi/interface/nvkms-kapi.h
+++ b/src/nvidia-modeset/kapi/interface/nvkms-kapi.h
@@ -1357,6 +1357,9 @@ struct NvKmsKapiFunctionsTable {
      * \param [out] pMaxSubmittedMap Returns a CPU mapping of the semaphore
      *                               surface's semaphore memory to the client.
      *
+     * \param [out] pSurfaceSize     Returns the size of the imported semaphore
+     *                               surface in bytes. May be NULL.
+     *
      * \return struct NvKmsKapiSemaphoreSurface* on success, NULL on failure.
      */
     struct NvKmsKapiSemaphoreSurface* (*importSemaphoreSurface)
@@ -1365,7 +1368,8 @@ struct NvKmsKapiFunctionsTable {
          NvU64 nvKmsParamsUser,
          NvU64 nvKmsParamsSize,
          void **pSemaphoreMap,
-         void **pMaxSubmittedMap
+         void **pMaxSubmittedMap,
+         NvU64 *pSurfaceSize
     );
 
     /*!
diff --git a/src/nvidia-modeset/kapi/src/nvkms-kapi-sync.c b/src/nvidia-modeset/kapi/src/nvkms-kapi-sync.c
index 1b243d64e6..09fa698b10 100644
--- a/src/nvidia-modeset/kapi/src/nvkms-kapi-sync.c
+++ b/src/nvidia-modeset/kapi/src/nvkms-kapi-sync.c
@@ -185,7 +185,8 @@ nvKmsKapiImportSemaphoreSurface
     NvU64 nvKmsParamsUser,
     NvU64 nvKmsParamsSize,
     void **pSemaphoreMap,
-    void **pMaxSubmittedMap
+    void **pMaxSubmittedMap,
+    NvU64 *pSurfaceSize
 )
 {
     struct NvKmsKapiSemaphoreSurface *ss = NULL;
@@ -311,6 +312,10 @@ nvKmsKapiImportSemaphoreSurface
         *pMaxSubmittedMap = NULL;
     }
 
+    if (pSurfaceSize != NULL) {
+        *pSurfaceSize = p.semaphoreSurfaceSize;
+    }
+
     return ss;
 
 fail: