On 11/23/23 12:08, Boris Brezillon wrote: > 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? Will add the comment. The reason why I added this patch was unrelated to the sgt, that's what I'm talking about. -- Best regards, Dmitry
