[PATCH 6.6 316/396] tmpfs: fix race on handling dquot rbtree

Greg Kroah-Hartman
2024-04-01T16:52:02Z

From: Carlos Maiolino <cem@kernel.org>

commit 0a69b6b3a026543bc215ccc866d0aea5579e6ce2 upstream.

A syzkaller reproducer found a race while attempting to remove dquot
information from the rb tree.

Fetching the rb_tree root node must also be protected by the
dqopt->dqio_sem, otherwise, giving the right timing, shmem_release_dquot()
will trigger a warning because it couldn't find a node in the tree, when
the real reason was the root node changing before the search starts:

Thread 1 Thread 2
- shmem_release_dquot() - shmem_{acquire,release}_dquot()

- fetch ROOT - Fetch ROOT

- acquire dqio_sem
- wait dqio_sem

- do something, triger a tree rebalance
- release dqio_sem

- acquire dqio_sem
- start searching for the node, but
from the wrong location, missing
the node, and triggering a warning.

Link: https://lkml.kernel.org/r/20240320124011.398847-1-cem@kernel.org
Fixes: eafc474e2029 ("shmem: prepare shmem quota infrastructure")
Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reported-by: Ubisectech Sirius <bugreport@ubisectech.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/shmem_quota.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/shmem_quota.c b/mm/shmem_quota.c
index 062d1c1097ae..ce514e700d2f 100644
--- a/mm/shmem_quota.c
+++ b/mm/shmem_quota.c
@@ -116,7 +116,7 @@ static int shmem_free_file_info(struct super_block *sb, int type)
static int shmem_get_next_id(struct super_block *sb, struct kqid *qid)
{
struct mem_dqinfo *info = sb_dqinfo(sb, qid->type);
- struct rb_node *node = ((struct rb_root *)info->dqi_priv)->rb_node;
+ struct rb_node *node;
qid_t id = from_kqid(&init_user_ns, *qid);
struct quota_info *dqopt = sb_dqopt(sb);
struct quota_id *entry = NULL;
@@ -126,6 +126,7 @@ static int shmem_get_next_id(struct super_block *sb, struct kqid *qid)
return -ESRCH;

down_read(&dqopt->dqio_sem);
+ node = ((struct rb_root *)info->dqi_priv)->rb_node;
while (node) {
entry = rb_entry(node, struct quota_id, node);

@@ -165,7 +166,7 @@ static int shmem_get_next_id(struct super_block *sb, struct kqid *qid)
static int shmem_acquire_dquot(struct dquot *dquot)
{
struct mem_dqinfo *info = sb_dqinfo(dquot->dq_sb, dquot->dq_id.type);
- struct rb_node **n = &((struct rb_root *)info->dqi_priv)->rb_node;
+ struct rb_node **n;
struct shmem_sb_info *sbinfo = dquot->dq_sb->s_fs_info;
struct rb_node *parent = NULL, *new_node = NULL;
struct quota_id *new_entry, *entry;
@@ -176,6 +177,8 @@ static int shmem_acquire_dquot(struct dquot *dquot)
mutex_lock(&dquot->dq_lock);

down_write(&dqopt->dqio_sem);
+ n = &((struct rb_root *)info->dqi_priv)->rb_node;
+
while (*n) {
parent = *n;
entry = rb_entry(parent, struct quota_id, node);
@@ -264,7 +267,7 @@ static bool shmem_is_empty_dquot(struct dquot *dquot)
static int shmem_release_dquot(struct dquot *dquot)
{
struct mem_dqinfo *info = sb_dqinfo(dquot->dq_sb, dquot->dq_id.type);
- struct rb_node *node = ((struct rb_root *)info->dqi_priv)->rb_node;
+ struct rb_node *node;
qid_t id = from_kqid(&init_user_ns, dquot->dq_id);
struct quota_info *dqopt = sb_dqopt(dquot->dq_sb);
struct quota_id *entry = NULL;
@@ -275,6 +278,7 @@ static int shmem_release_dquot(struct dquot *dquot)
goto out_dqlock;

down_write(&dqopt->dqio_sem);
+ node = ((struct rb_root *)info->dqi_priv)->rb_node;
while (node) {
entry = rb_entry(node, struct quota_id, node);

--
2.44.0