On Thu, 23 Nov 2023 01:30:24 +0300 Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote: > 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. Sure, I'm just asking that we add a comment to explain why we leak memory here. Is that too much to ask?
