From 4b848f20eda5974020f043ca14bacf7a7e634fc8 Mon Sep 17 00:00:00 2001 From: Daniel Vetter Date: Sun, 2 Feb 2020 14:21:33 +0100 Subject: [PATCH] drm/vgem: Close use-after-free race in vgem_gem_create There's two references floating around here (for the object reference, not the handle_count reference, that's a different thing): - The temporary reference held by vgem_gem_create, acquired by creating the object and released by calling drm_gem_object_put_unlocked. - The reference held by the object handle, created by drm_gem_handle_create. This one generally outlives the function, except if a 2nd thread races with a GEM_CLOSE ioctl call. So usually everything is correct, except in that race case, where the access to gem_object->size could be looking at freed data already. Which again isn't a real problem (userspace shot its feet off already with the race, we could return garbage), but maybe someone can exploit this as an information leak. Cc: Dan Carpenter Cc: Hillf Danton Reported-by: syzbot+0dc4444774d419e916c8@syzkaller.appspotmail.com Cc: stable@vger.kernel.org Cc: Emil Velikov Cc: Daniel Vetter Cc: Sean Paul Cc: Chris Wilson Cc: Eric Anholt Cc: Sam Ravnborg Cc: Rob Clark Reviewed-by: Chris Wilson Signed-off-by: Daniel Vetter Link: https://patchwork.freedesktop.org/patch/msgid/20200202132133.1891846-1-daniel.vetter@ffwll.ch --- drivers/gpu/drm/vgem/vgem_drv.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c index 5bd60ded3d81..909eba43664a 100644 --- a/drivers/gpu/drm/vgem/vgem_drv.c +++ b/drivers/gpu/drm/vgem/vgem_drv.c @@ -196,9 +196,10 @@ static struct drm_gem_object *vgem_gem_create(struct drm_device *dev, return ERR_CAST(obj); ret = drm_gem_handle_create(file, &obj->base, handle); - drm_gem_object_put_unlocked(&obj->base); - if (ret) + if (ret) { + drm_gem_object_put_unlocked(&obj->base); return ERR_PTR(ret); + } return &obj->base; } @@ -221,7 +222,9 @@ static int vgem_gem_dumb_create(struct drm_file *file, struct drm_device *dev, args->size = gem_object->size; args->pitch = pitch; - DRM_DEBUG("Created object of size %lld\n", size); + drm_gem_object_put_unlocked(gem_object); + + DRM_DEBUG("Created object of size %llu\n", args->size); return 0; }