linux-kernel - RE: [PATCH] fs, nilfs: convert nilfs_root.count from atomic_t to refcount

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Date:   Mon, 23 Oct 2017 07:24:44 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nilfs@...r.kernel.org" <linux-nilfs@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>
Subject: RE: [PATCH] fs, nilfs: convert nilfs_root.count from atomic_t to
 refcount_t

> On 2017/10/20 19:51, 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 nilfs_root.count 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>
> 
> Applied, thank you.

Thank you very much!

Best Regards,
Elena.

> 
> Ryusuke Konishi
> 
> > ---
> >   fs/nilfs2/the_nilfs.c | 8 ++++----
> >   fs/nilfs2/the_nilfs.h | 5 +++--
> >   2 files changed, 7 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
> > index 2dd75bf..afebb50 100644
> > --- a/fs/nilfs2/the_nilfs.c
> > +++ b/fs/nilfs2/the_nilfs.c
> > @@ -737,7 +737,7 @@ struct nilfs_root *nilfs_lookup_root(struct the_nilfs *nilfs,
> __u64 cno)
> >   		} else if (cno > root->cno) {
> >   			n = n->rb_right;
> >   		} else {
> > -			atomic_inc(&root->count);
> > +			refcount_inc(&root->count);
> >   			spin_unlock(&nilfs->ns_cptree_lock);
> >   			return root;
> >   		}
> > @@ -776,7 +776,7 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64
> cno)
> >   		} else if (cno > root->cno) {
> >   			p = &(*p)->rb_right;
> >   		} else {
> > -			atomic_inc(&root->count);
> > +			refcount_inc(&root->count);
> >   			spin_unlock(&nilfs->ns_cptree_lock);
> >   			kfree(new);
> >   			return root;
> > @@ -786,7 +786,7 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64
> cno)
> >   	new->cno = cno;
> >   	new->ifile = NULL;
> >   	new->nilfs = nilfs;
> > -	atomic_set(&new->count, 1);
> > +	refcount_set(&new->count, 1);
> >   	atomic64_set(&new->inodes_count, 0);
> >   	atomic64_set(&new->blocks_count, 0);
> >
> > @@ -806,7 +806,7 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64
> cno)
> >
> >   void nilfs_put_root(struct nilfs_root *root)
> >   {
> > -	if (atomic_dec_and_test(&root->count)) {
> > +	if (refcount_dec_and_test(&root->count)) {
> >   		struct the_nilfs *nilfs = root->nilfs;
> >
> >   		nilfs_sysfs_delete_snapshot_group(root);
> > diff --git a/fs/nilfs2/the_nilfs.h b/fs/nilfs2/the_nilfs.h
> > index b305c6f..883d732 100644
> > --- a/fs/nilfs2/the_nilfs.h
> > +++ b/fs/nilfs2/the_nilfs.h
> > @@ -27,6 +27,7 @@
> >   #include <linux/blkdev.h>
> >   #include <linux/backing-dev.h>
> >   #include <linux/slab.h>
> > +#include <linux/refcount.h>
> >
> >   struct nilfs_sc_info;
> >   struct nilfs_sysfs_dev_subgroups;
> > @@ -246,7 +247,7 @@ struct nilfs_root {
> >   	__u64 cno;
> >   	struct rb_node rb_node;
> >
> > -	atomic_t count;
> > +	refcount_t count;
> >   	struct the_nilfs *nilfs;
> >   	struct inode *ifile;
> >
> > @@ -299,7 +300,7 @@ void nilfs_swap_super_block(struct the_nilfs *);
> >
> >   static inline void nilfs_get_root(struct nilfs_root *root)
> >   {
> > -	atomic_inc(&root->count);
> > +	refcount_inc(&root->count);
> >   }
> >
> >   static inline int nilfs_valid_fs(struct the_nilfs *nilfs)
> >