On 11/13/23 12:54, Boris Brezillon wrote: > On Mon, 30 Oct 2023 02:02:01 +0300 > Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote: > >> Don't free refcounted shmem object to prevent use-after-free bug that >> is worse than a memory leak. >> >> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> >> --- >> drivers/gpu/drm/drm_gem_shmem_helper.c | 7 ++++--- >> 1 file changed, 4 insertions(+), 3 deletions(-) >> >> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c >> index 6dd087f19ea3..4253c367dc07 100644 >> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c >> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c >> @@ -203,9 +203,10 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem) >> if (obj->import_attach) >> drm_prime_gem_destroy(obj, shmem->sgt); >> >> - drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count)); >> - drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count)); >> - drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count)); >> + if (drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count)) || >> + drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count)) || >> + drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count))) >> + return; > > I guess you're worried about ->sgt being referenced by the driver after > the GEM is destroyed. If we assume drivers don't cache the sgt and > always call get_pages_sgt() when they need it that shouldn't be an > issue. What we really don't want to release is the pages themselves, > but the GPU MMU might still have active mappings pointing to these > pages. > > In any case, I'm not against leaking the GEM object when any of these > counters are not zero, but can we at least have a comment in the > code explaining why we're doing that, so people don't have to go look > at the git history to figure it out. This patch is a minor improvement, it doesn't address any specific issue. This should be a common pattern in kernel. If you're giving a warning and know about the inevitable catastrophe, then avoid it if you can. Actually, there are other similar cases in drm-shmem that can be improved. -- Best regards, Dmitry
