Message ID | 20220727090700.3238-1-tujinjiang@bytedance.com (mailing list archive) |
---|---|
State | New |
Headers | show |
Series | vmscan: fix potential arbitrary pointer passed to kfree in unregister_shrinker | expand |
[Cc Yang Shi] 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. > 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?
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. > > > 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
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
On Thu 28-07-22 10:37:26, 锦江屠 wrote: > 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. Agreed. Especially when that is not documented anywhere. > 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. It would be really great to mention this intention in the changelog. Your initial wording might make an impression this is a fix for an existing problem. > > > > 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") Do not use Fixes tag as this is not a real problem currently.
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)