Re: [PATCH v5] ext4: fix potential use after free in system zone via remount with noblock_validity

Jan Kara
2019-08-26T15:03:52Z

On Mon 26-08-19 16:31:41, zhangyi (F) wrote:
> On 2019/8/26 10:56, Theodore Y. Ts'o Wrote:
> > I added a missing rcu_read_lock() to prevent a suspicious RCU
> > warning when CONFIG_PROVE_RCU is enabled:
> >
> > diff --git a/fs/ext4/block_validity.c b/fs/ext4/block_validity.c
> > index 003dc1dc2da3..f7bc914a74df 100644
> > --- a/fs/ext4/block_validity.c
> > +++ b/fs/ext4/block_validity.c
> > @@ -330,11 +330,13 @@ void ext4_release_system_zone(struct super_block *sb)
> > {
> > struct ext4_system_blocks *system_blks;
> >
> > + rcu_read_lock();
> > system_blks = rcu_dereference(EXT4_SB(sb)->system_blks);
> > rcu_assign_pointer(EXT4_SB(sb)->system_blks, NULL);
> >
> > if (system_blks)
> > call_rcu(&system_blks->rcu, ext4_destroy_system_zone);
> > + rcu_read_unlock();
> > }
> >
> > int ext4_data_block_valid(struct ext4_sb_info *sbi, ext4_fsblk_t start_blk,
> >
>
> Hi Ted，
> Sorry about missing this warning, I think switch to use:
> system_blks = rcu_dereference_raw(EXT4_SB(sb)->system_blks);
> or
> system_blks = rcu_dereference_protected(EXT4_SB(sb)->system_blks, true);
> is enough to fix this waring, am I missing something?

Proper fix for this is actually using:

system_blks = rcu_dereference_protected(EXT4_SB(sb)->system_blks,
lockdep_is_held(&sb->s_umount));

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR