Messages in this thread | | | From | 锦江屠 <> | Date | Thu, 28 Jul 2022 10:37:26 +0800 | Subject | Re: [External] Re: [PATCH] vmscan: fix potential arbitrary pointer passed to kfree in unregister_shrinker |
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On Wed, Jul 27, 2022 at 11:50 PM Yang Shi <shy828301@gmail.com> wrote: > > On Wed, Jul 27, 2022 at 7:43 AM Michal Hocko <mhocko@suse.com> wrote: > > > > [Cc Yang Shi] > > Thanks, Michal. > > > On Wed 27-07-22 17:07:00, tujinjiang@bytedance.com wrote: > > > From: Jinjiang Tu <tujinjiang@bytedance.com> > > > > > > when shrinker is registered with SHRINKER_MEMCG_AWARE flag, > > > register_shrinker will not initialize shrinker->nr_deferred, > > > but the pointer will be passed to kfree in unregister_shrinker > > > when the shrinker is unregistered. This leads to kernel crash > > > when the shrinker object is dynamically allocated. > > > > Is this a real life problem? I thought shrinkers were pre-zeroed > > already. Not that we should be relying on that but it would be good to > > mention whether this is a code fortification or something that we should > > be really worried about. > > Yes, all memcg aware shrinkers are actually pre-zeroed. The fs > shrinkers (embedded in super_block) are allocated by kzalloc, all > other shrinkers are static declared. So I don't think it will cause > any crash in real life. >
Yes, the shrinkers in the current kernel will not cause crash, but a new memcg aware shrinker may be added in the future, and I think we should not assume the shrinker is pre-zeroed.
Function free_prealloced_shrinker does not assume the shrinker is pre-zeroed, and does not call kfree(shrinker->nr_deferred) if the shrinker is memcg aware. So I think it is better for unregister_shrinker to call kfree only when the shrinker is not memcg aware.
> > > > > To fix it, this patch initialize shrinker->nr_deferred at the > > > beginning of prealloc_shrinker. > > > > It would be great to add > > Fixes: 476b30a0949a ("mm: vmscan: don't need allocate shrinker->nr_deferred for memcg aware shrinkers") > > > > > Signed-off-by: Jinjiang Tu <tujinjiang@bytedance.com> > > > --- > > > mm/vmscan.c | 1 + > > > 1 file changed, 1 insertion(+) > > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > > index f7d9a683e3a7..06ab5a398971 100644 > > > --- a/mm/vmscan.c > > > +++ b/mm/vmscan.c > > > @@ -613,6 +613,7 @@ int prealloc_shrinker(struct shrinker *shrinker) > > > unsigned int size; > > > int err; > > > > > > + shrinker->nr_deferred = NULL; > > > if (shrinker->flags & SHRINKER_MEMCG_AWARE) { > > > err = prealloc_memcg_shrinker(shrinker); > > > if (err != -ENOSYS) > > > > You should be able to move it under SHRINKER_MEMCG_AWARE branch, no? > > > > -- > > Michal Hocko > > SUSE Labs
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