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Date: Tue, 31 Oct 2017 11:40:47 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: Jan Kara <jack@...e.cz>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
"amir73il@...il.com" <amir73il@...il.com>,
"peterz@...radead.org" <peterz@...radead.org>,
"keescook@...omium.org" <keescook@...omium.org>
Subject: RE: [PATCH 2/2] fsnotify: convert fsnotify_mark.refcnt from
atomic_t to refcount_t
> On Fri 20-10-17 13:26:02, Elena Reshetova wrote:
> > atomic_t variables are currently used to implement reference
> > counters with the following properties:
> > - counter is initialized to 1 using atomic_set()
> > - a resource is freed upon counter reaching zero
> > - once counter reaches zero, its further
> > increments aren't allowed
> > - counter schema uses basic atomic operations
> > (set, inc, inc_not_zero, dec_and_test, etc.)
> >
> > Such atomic variables should be converted to a newly provided
> > refcount_t type and API that prevents accidental counter overflows
> > and underflows. This is important since overflows and underflows
> > can lead to use-after-free situation and be exploitable.
> >
> > The variable fsnotify_mark.refcnt is used as pure reference counter.
> > Convert it to refcount_t and fix up the operations.
> >
> > Suggested-by: Kees Cook <keescook@...omium.org>
> > Reviewed-by: David Windsor <dwindsor@...il.com>
> > Reviewed-by: Hans Liljestrand <ishkamiel@...il.com>
> > Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
>
> Thanks for the patch! I will add it to my tree in a few days once other
> pending work in this area is done (since this patch is going to conflict
> with it and it's easier to resolve the conflict when applying this patch
> later).
Thank you very much for both!
Best Regards,
Elena.
>
>
> Honza
> > ---
> > fs/notify/inotify/inotify_user.c | 4 ++--
> > fs/notify/mark.c | 14 +++++++-------
> > include/linux/fsnotify_backend.h | 2 +-
> > kernel/audit_tree.c | 2 +-
> > 4 files changed, 11 insertions(+), 11 deletions(-)
> >
> > diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
> > index 7cc7d3f..d3c20e0 100644
> > --- a/fs/notify/inotify/inotify_user.c
> > +++ b/fs/notify/inotify/inotify_user.c
> > @@ -376,7 +376,7 @@ static struct inotify_inode_mark
> *inotify_idr_find_locked(struct fsnotify_group
> >
> > fsnotify_get_mark(fsn_mark);
> > /* One ref for being in the idr, one ref we just took */
> > - BUG_ON(atomic_read(&fsn_mark->refcnt) < 2);
> > + BUG_ON(refcount_read(&fsn_mark->refcnt) < 2);
> > }
> >
> > return i_mark;
> > @@ -446,7 +446,7 @@ static void inotify_remove_from_idr(struct fsnotify_group
> *group,
> > * One ref for being in the idr
> > * one ref grabbed by inotify_idr_find
> > */
> > - if (unlikely(atomic_read(&i_mark->fsn_mark.refcnt) < 2)) {
> > + if (unlikely(refcount_read(&i_mark->fsn_mark.refcnt) < 2)) {
> > printk(KERN_ERR "%s: i_mark=%p i_mark->wd=%d
> i_mark->group=%p\n",
> > __func__, i_mark, i_mark->wd, i_mark-
> >fsn_mark.group);
> > /* we can't really recover with bad ref cnting.. */
> > diff --git a/fs/notify/mark.c b/fs/notify/mark.c
> > index 9991f88..45f1141 100644
> > --- a/fs/notify/mark.c
> > +++ b/fs/notify/mark.c
> > @@ -105,8 +105,8 @@ static DECLARE_WORK(connector_reaper_work,
> fsnotify_connector_destroy_workfn);
> >
> > void fsnotify_get_mark(struct fsnotify_mark *mark)
> > {
> > - WARN_ON_ONCE(!atomic_read(&mark->refcnt));
> > - atomic_inc(&mark->refcnt);
> > + WARN_ON_ONCE(!refcount_read(&mark->refcnt));
> > + refcount_inc(&mark->refcnt);
> > }
> >
> > /*
> > @@ -116,7 +116,7 @@ void fsnotify_get_mark(struct fsnotify_mark *mark)
> > */
> > static bool fsnotify_get_mark_safe(struct fsnotify_mark *mark)
> > {
> > - return atomic_inc_not_zero(&mark->refcnt);
> > + return refcount_inc_not_zero(&mark->refcnt);
> > }
> >
> > static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
> > @@ -211,7 +211,7 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
> >
> > /* Catch marks that were actually never attached to object */
> > if (!mark->connector) {
> > - if (atomic_dec_and_test(&mark->refcnt))
> > + if (refcount_dec_and_test(&mark->refcnt))
> > fsnotify_final_mark_destroy(mark);
> > return;
> > }
> > @@ -220,7 +220,7 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
> > * We have to be careful so that traversals of obj_list under lock can
> > * safely grab mark reference.
> > */
> > - if (!atomic_dec_and_lock(&mark->refcnt, &mark->connector->lock))
> > + if (!refcount_dec_and_lock(&mark->refcnt, &mark->connector-
> >lock))
> > return;
> >
> > conn = mark->connector;
> > @@ -338,7 +338,7 @@ void fsnotify_detach_mark(struct fsnotify_mark *mark)
> >
> > WARN_ON_ONCE(!mutex_is_locked(&group->mark_mutex));
> > WARN_ON_ONCE(!srcu_read_lock_held(&fsnotify_mark_srcu) &&
> > - atomic_read(&mark->refcnt) < 1 +
> > + refcount_read(&mark->refcnt) < 1 +
> > !!(mark->flags &
> FSNOTIFY_MARK_FLAG_ATTACHED));
> >
> > spin_lock(&mark->lock);
> > @@ -738,7 +738,7 @@ void fsnotify_init_mark(struct fsnotify_mark *mark,
> > {
> > memset(mark, 0, sizeof(*mark));
> > spin_lock_init(&mark->lock);
> > - atomic_set(&mark->refcnt, 1);
> > + refcount_set(&mark->refcnt, 1);
> > fsnotify_get_group(group);
> > mark->group = group;
> > }
> > diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
> > index 20a57ba..26485b1 100644
> > --- a/include/linux/fsnotify_backend.h
> > +++ b/include/linux/fsnotify_backend.h
> > @@ -244,7 +244,7 @@ struct fsnotify_mark {
> > __u32 mask;
> > /* We hold one for presence in g_list. Also one ref for each 'thing'
> > * in kernel that found and may be using this mark. */
> > - atomic_t refcnt;
> > + refcount_t refcnt;
> > /* Group this mark is for. Set on mark creation, stable until last ref
> > * is dropped */
> > struct fsnotify_group *group;
> > diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
> > index 011d46e..45ec960 100644
> > --- a/kernel/audit_tree.c
> > +++ b/kernel/audit_tree.c
> > @@ -1007,7 +1007,7 @@ static void audit_tree_freeing_mark(struct
> fsnotify_mark *entry, struct fsnotify
> > * We are guaranteed to have at least one reference to the mark
> from
> > * either the inode or the caller of fsnotify_destroy_mark().
> > */
> > - BUG_ON(atomic_read(&entry->refcnt) < 1);
> > + BUG_ON(refcount_read(&entry->refcnt) < 1);
> > }
> >
> > static const struct fsnotify_ops audit_tree_ops = {
> > --
> > 2.7.4
> >
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
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