Re: [PATCH v7 9/9] ext4: hold s_fc_lock while during fast commit

harshad shirwadkar
2025-04-14T16:59:40Z

On Mon, Jan 13, 2025 at 6:16 AM Baokun Li <libaokun1@huawei.com> wrote:
>
> Hi Harshad,
>
> On 2024/8/18 12:03, Harshad Shirwadkar wrote:
> > Leaving s_fc_lock in between during commit in ext4_fc_perform_commit()
> > function leaves room for subtle concurrency bugs where ext4_fc_del() may
> > delete an inode from the fast commit list, leaving list in an inconsistent
> > state. Also, this patch converts s_fc_lock to mutex type so that it can be
> > held when kmem_cache_* functions are called.
> >
> > Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
> > ---
> > fs/ext4/ext4.h | 2 +-
> > fs/ext4/fast_commit.c | 91 +++++++++++++++++--------------------------
> > fs/ext4/super.c | 2 +-
> > 3 files changed, 38 insertions(+), 57 deletions(-)
> >
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 4ecb63f95..a1acd34ff 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -1748,7 +1748,7 @@ struct ext4_sb_info {
> > * following fields:
> > * ei->i_fc_list, s_fc_dentry_q, s_fc_q, s_fc_bytes, s_fc_bh.
> > */
> > - spinlock_t s_fc_lock;
> > + struct mutex s_fc_lock;
> > struct buffer_head *s_fc_bh;
> > struct ext4_fc_stats s_fc_stats;
> > tid_t s_fc_ineligible_tid;
> > diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
> > index 7525450f1..c3627efd9 100644
> > --- a/fs/ext4/fast_commit.c
> > +++ b/fs/ext4/fast_commit.c
> > @@ -236,9 +236,9 @@ void ext4_fc_del(struct inode *inode)
> > if (ext4_fc_disabled(inode->i_sb))
> > return;
> >
> > - spin_lock(&EXT4_SB(inode->i_sb)->s_fc_lock);
> > + mutex_lock(&EXT4_SB(inode->i_sb)->s_fc_lock);
> > if (list_empty(&ei->i_fc_list) && list_empty(&ei->i_fc_dilist)) {
> > - spin_unlock(&EXT4_SB(inode->i_sb)->s_fc_lock);
> > + mutex_unlock(&EXT4_SB(inode->i_sb)->s_fc_lock);
> > return;
> > }
> >
> > @@ -266,7 +266,7 @@ void ext4_fc_del(struct inode *inode)
> > * dentry create references, since it is not needed to log it anyways.
> > */
> > if (list_empty(&ei->i_fc_dilist)) {
> > - spin_unlock(&sbi->s_fc_lock);
> > + mutex_unlock(&sbi->s_fc_lock);
> > return;
> > }
> >
> > @@ -276,7 +276,7 @@ void ext4_fc_del(struct inode *inode)
> > list_del_init(&fc_dentry->fcd_dilist);
> >
> > WARN_ON(!list_empty(&ei->i_fc_dilist));
> > - spin_unlock(&sbi->s_fc_lock);
> > + mutex_unlock(&sbi->s_fc_lock);
> >
> > if (fc_dentry->fcd_name.name &&
> > fc_dentry->fcd_name.len > DNAME_INLINE_LEN)
> > @@ -306,10 +306,10 @@ void ext4_fc_mark_ineligible(struct super_block *sb, int reason, handle_t *handl
> > sbi->s_journal->j_running_transaction->t_tid : 0;
> > read_unlock(&sbi->s_journal->j_state_lock);
> > }
> > - spin_lock(&sbi->s_fc_lock);
> > + mutex_lock(&sbi->s_fc_lock);
> > if (tid_gt(tid, sbi->s_fc_ineligible_tid))
> > sbi->s_fc_ineligible_tid = tid;
> > - spin_unlock(&sbi->s_fc_lock);
> > + mutex_unlock(&sbi->s_fc_lock);
> > WARN_ON(reason >= EXT4_FC_REASON_MAX);
> > sbi->s_fc_stats.fc_ineligible_reason_count[reason]++;
> > }
> > @@ -349,14 +349,14 @@ static int ext4_fc_track_template(
> > if (!enqueue)
> > return ret;
> >
> > - spin_lock(&sbi->s_fc_lock);
> > + mutex_lock(&sbi->s_fc_lock);
> > if (list_empty(&EXT4_I(inode)->i_fc_list))
> > list_add_tail(&EXT4_I(inode)->i_fc_list,
> > (sbi->s_journal->j_flags & JBD2_FULL_COMMIT_ONGOING ||
> > sbi->s_journal->j_flags & JBD2_FAST_COMMIT_ONGOING) ?
> > &sbi->s_fc_q[FC_Q_STAGING] :
> > &sbi->s_fc_q[FC_Q_MAIN]);
> > - spin_unlock(&sbi->s_fc_lock);
> > + mutex_unlock(&sbi->s_fc_lock);
> >
> > return ret;
> > }
> > @@ -414,7 +414,8 @@ static int __track_dentry_update(struct inode *inode, void *arg, bool update)
> > }
> > node->fcd_name.len = dentry->d_name.len;
> > INIT_LIST_HEAD(&node->fcd_dilist);
> > - spin_lock(&sbi->s_fc_lock);
> > + INIT_LIST_HEAD(&node->fcd_list);
> > + mutex_lock(&sbi->s_fc_lock);
> > if (sbi->s_journal->j_flags & JBD2_FULL_COMMIT_ONGOING ||
> > sbi->s_journal->j_flags & JBD2_FAST_COMMIT_ONGOING)
> > list_add_tail(&node->fcd_list,
> > @@ -435,7 +436,7 @@ static int __track_dentry_update(struct inode *inode, void *arg, bool update)
> > WARN_ON(!list_empty(&ei->i_fc_dilist));
> > list_add_tail(&node->fcd_dilist, &ei->i_fc_dilist);
> > }
> > - spin_unlock(&sbi->s_fc_lock);
> > + mutex_unlock(&sbi->s_fc_lock);
> > spin_lock(&ei->i_fc_lock);
> >
> > return 0;
> > @@ -955,15 +956,15 @@ static int ext4_fc_submit_inode_data_all(journal_t *journal)
> > struct ext4_inode_info *ei;
> > int ret = 0;
> >
> > - spin_lock(&sbi->s_fc_lock);
> > + mutex_lock(&sbi->s_fc_lock);
> > list_for_each_entry(ei, &sbi->s_fc_q[FC_Q_MAIN], i_fc_list) {
> > - spin_unlock(&sbi->s_fc_lock);
> > + mutex_unlock(&sbi->s_fc_lock);
> > ret = jbd2_submit_inode_data(journal, ei->jinode);
> > if (ret)
> > return ret;
> > - spin_lock(&sbi->s_fc_lock);
> > + mutex_lock(&sbi->s_fc_lock);
> > }
> > - spin_unlock(&sbi->s_fc_lock);
> > + mutex_unlock(&sbi->s_fc_lock);
> >
> We're also seeing a similar race condition here. This issue was encountered
> while running `kvm-xfstests -c ext4/adv -C 500 generic/241`:
>
> P1 | P2
> ----------------------------------------------------
> evict
> ext4_evict_inode
> ext4_free_inode
> ext4_clear_inode
> ext4_fc_del(inode)
> ext4_sync_file
> ext4_fsync_journal
> ext4_fc_commit
> ext4_fc_perform_commit
> ext4_fc_submit_inode_data_all
> -- spin_lock(&sbi->s_fc_lock);
> list_for_each_entry(i_fc_list)
> -- spin_unlock(&sbi->s_fc_lock);
> -- spin_lock(&sbi->s_fc_lock)
> if (!list_empty(&ei->i_fc_list))
> list_del_init(&ei->i_fc_list);
> -- spin_unlock(&sbi->s_fc_lock);
> jbd2_free_inode(EXT4_I(inode)->jinode)
> EXT4_I(inode)->jinode = NULL
> jbd2_submit_inode_data
> journal->j_submit_inode_data_buffers
> ext4_journal_submit_inode_data_buffers
> ext4_should_journal_data(jinode->i_vfs_inode)
> // a. jinode may use-after-free !!!
> ext4_inode_journal_mode(inode)
> EXT4_JOURNAL(inode)
> (inode)->i_sb
> // b. inode may null-ptr-deref !!!
> -- spin_lock(&sbi->s_fc_lock);
> -- spin_unlock(&sbi->s_fc_lock);
>
> By the way, the WARN_ON added in patch 5 can detect this issue without
> enabling KASAN, but patch 5 also introduced softlocks and other UAFs.

Thanks for mentioning this. V8 of the patchset handles this race by
not releasing s_fc_lock in ext4_fc_submit_inode_data_all().

- Harshad
>
>
> Regards,
> Baokun
>