The dquot_mark_dquot_dirty() using dquot references from the inode
should be protected by dquot_srcu. quota_off code takes care to call
synchronize_srcu(&dquot_srcu) to not drop dquot references while they
are used by other users. But dquot_transfer() breaks this assumption.
We call dquot_transfer() to drop the last reference of dquot and add
it to free_dquots, but there may still be other users using the dquot
at this time, as shown in the function graph below:
cpu1 cpu2
_________________|_________________
wb_do_writeback CHOWN(1)
...
ext4_da_update_reserve_space
dquot_claim_block
...
dquot_mark_dquot_dirty // try to dirty old quota
test_bit(DQ_ACTIVE_B, &dquot->dq_flags) // still ACTIVE
if (test_bit(DQ_MOD_B, &dquot->dq_flags))
// test no dirty, wait dq_list_lock
...
dquot_transfer
__dquot_transfer
dqput_all(transfer_from) // rls old dquot
dqput // last dqput
dquot_release
clear_bit(DQ_ACTIVE_B, &dquot->dq_flags)
atomic_dec(&dquot->dq_count)
put_dquot_last(dquot)
list_add_tail(&dquot->dq_free, &free_dquots)
// add the dquot to free_dquots
if (!test_and_set_bit(DQ_MOD_B, &dquot->dq_flags))
add dqi_dirty_list // add released dquot to dirty_list
This can cause various issues, such as dquot being destroyed by
dqcache_shrink_scan() after being added to free_dquots, which can trigger
a UAF in dquot_mark_dquot_dirty(); or after dquot is added to free_dquots
and then to dirty_list, it is added to free_dquots again after
dquot_writeback_dquots() is executed, which causes the free_dquots list to
be corrupted and triggers a UAF when dqcache_shrink_scan() is called for
freeing dquot twice.
As Honza said, we need to fix dquot_transfer() to follow the guarantees
dquot_srcu should provide. But calling synchronize_srcu() directly from
dquot_transfer() is too expensive (and mostly unnecessary). So we add
dquot whose last reference should be dropped to the new global dquot
list releasing_dquots, and then queue work item which would call
synchronize_srcu() and after that perform the final cleanup of all the
dquots on releasing_dquots.
Fixes: 4580b30ea887 ("quota: Do not dirty bad dquots")
Suggested-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
fs/quota/dquot.c | 85 ++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 71 insertions(+), 14 deletions(-)
diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
index 09787e4f6a00..e8e702ac64e5 100644
--- a/fs/quota/dquot.c
+++ b/fs/quota/dquot.c
@@ -270,6 +270,9 @@ static qsize_t inode_get_rsv_space(struct inode *inode);
static qsize_t __inode_get_rsv_space(struct inode *inode);
static int __dquot_initialize(struct inode *inode, int type);
+static void quota_release_workfn(struct work_struct *work);
+static DECLARE_DELAYED_WORK(quota_release_work, quota_release_workfn);
+
static inline unsigned int
hashfn(const struct super_block *sb, struct kqid qid)
{
@@ -569,6 +572,8 @@ static void invalidate_dquots(struct super_block *sb, int type)
struct dquot *dquot, *tmp;
restart:
+ flush_delayed_work("a_release_work);
+
spin_lock(&dq_list_lock);
list_for_each_entry_safe(dquot, tmp, &inuse_list, dq_inuse) {
if (dquot->dq_sb != sb)
@@ -577,6 +582,12 @@ static void invalidate_dquots(struct super_block *sb, int type)
continue;
/* Wait for dquot users */
if (atomic_read(&dquot->dq_count)) {
+ /* dquot in releasing_dquots, flush and retry */
+ if (!list_empty(&dquot->dq_free)) {
+ spin_unlock(&dq_list_lock);
+ goto restart;
+ }
+
atomic_inc(&dquot->dq_count);
spin_unlock(&dq_list_lock);
/*
@@ -760,6 +771,8 @@ dqcache_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
struct dquot *dquot;
unsigned long freed = 0;
+ flush_delayed_work("a_release_work);
+
spin_lock(&dq_list_lock);
while (!list_empty(&free_dquots) && sc->nr_to_scan) {
dquot = list_first_entry(&free_dquots, struct dquot, dq_free);
@@ -787,6 +800,60 @@ static struct shrinker dqcache_shrinker = {
.seeks = DEFAULT_SEEKS,
};
+/*
+ * Safely release dquot and put reference to dquot.
+ */
+static void quota_release_workfn(struct work_struct *work)
+{
+ struct dquot *dquot;
+ struct list_head rls_head;
+
+ spin_lock(&dq_list_lock);
+ /* Exchange the list head to avoid livelock. */
+ list_replace_init(&releasing_dquots, &rls_head);
+ spin_unlock(&dq_list_lock);
+
+restart:
+ synchronize_srcu(&dquot_srcu);
+ spin_lock(&dq_list_lock);
+ while (!list_empty(&rls_head)) {
+ dquot = list_first_entry(&rls_head, struct dquot, dq_free);
+ if (dquot_dirty(dquot)) {
+ spin_unlock(&dq_list_lock);
+ /* Commit dquot before releasing */
+ dquot_write_dquot(dquot);
+ goto restart;
+ }
+ /* Always clear DQ_ACTIVE_B, unless racing with dqget() */
+ if (dquot_active(dquot)) {
+ spin_unlock(&dq_list_lock);
+ dquot->dq_sb->dq_op->release_dquot(dquot);
+ spin_lock(&dq_list_lock);
+ }
+ /*
+ * During the execution of dquot_release() outside the
+ * dq_list_lock, another process may have completed
+ * dqget()/dqput()/mark_dirty().
+ */
+ if (atomic_read(&dquot->dq_count) == 1 &&
+ (dquot_active(dquot) || dquot_dirty(dquot))) {
+ spin_unlock(&dq_list_lock);
+ goto restart;
+ }
+ /*
+ * Now it is safe to remove this dquot from releasing_dquots
+ * and reduce its reference count.
+ */
+ remove_free_dquot(dquot);
+ atomic_dec(&dquot->dq_count);
+
+ /* We may be racing with some other dqget(). */
+ if (!atomic_read(&dquot->dq_count))
+ put_dquot_last(dquot);
+ }
+ spin_unlock(&dq_list_lock);
+}
+
/*
* Put reference to dquot
*/
@@ -803,7 +870,7 @@ void dqput(struct dquot *dquot)
}
#endif
dqstats_inc(DQST_DROPS);
-we_slept:
+
spin_lock(&dq_list_lock);
if (atomic_read(&dquot->dq_count) > 1) {
/* We have more than one user... nothing to do */
@@ -815,25 +882,15 @@ void dqput(struct dquot *dquot)
spin_unlock(&dq_list_lock);
return;
}
+
/* Need to release dquot? */
- if (dquot_dirty(dquot)) {
- spin_unlock(&dq_list_lock);
- /* Commit dquot before releasing */
- dquot_write_dquot(dquot);
- goto we_slept;
- }
- if (dquot_active(dquot)) {
- spin_unlock(&dq_list_lock);
- dquot->dq_sb->dq_op->release_dquot(dquot);
- goto we_slept;
- }
- atomic_dec(&dquot->dq_count);
#ifdef CONFIG_QUOTA_DEBUG
/* sanity check */
BUG_ON(!list_empty(&dquot->dq_free));
#endif
- put_dquot_last(dquot);
+ put_releasing_dquots(dquot);
spin_unlock(&dq_list_lock);
+ queue_delayed_work(system_unbound_wq, "a_release_work, 1);
}
EXPORT_SYMBOL(dqput);
--
2.31.1
