The patch titled Subject: writeback, cgroup: fix null-ptr-deref write in bdi_split_work_to_wbs has been added to the -mm mm-hotfixes-unstable branch. Its filename is writeback-cgroup-fix-null-ptr-deref-write-in-bdi_split_work_to_wbs.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days

------------------------------------------------------ From: Baokun Li libaokun1@huawei.com Subject: writeback, cgroup: fix null-ptr-deref write in bdi_split_work_to_wbs Date: Mon, 10 Apr 2023 21:08:26 +0800

KASAN report null-ptr-deref: ================================================================== BUG: KASAN: null-ptr-deref in bdi_split_work_to_wbs+0x5c5/0x7b0 Write of size 8 at addr 0000000000000000 by task sync/943 CPU: 5 PID: 943 Comm: sync Tainted: 6.3.0-rc5-next-20230406-dirty #461 Call Trace: <TASK> dump_stack_lvl+0x7f/0xc0 print_report+0x2ba/0x340 kasan_report+0xc4/0x120 kasan_check_range+0x1b7/0x2e0 __kasan_check_write+0x24/0x40 bdi_split_work_to_wbs+0x5c5/0x7b0 sync_inodes_sb+0x195/0x630 sync_inodes_one_sb+0x3a/0x50 iterate_supers+0x106/0x1b0 ksys_sync+0x98/0x160 [...] ==================================================================

The race that causes the above issue is as follows:

cpu1 cpu2 -------------------------|------------------------- inode_switch_wbs INIT_WORK(&isw->work, inode_switch_wbs_work_fn) queue_rcu_work(isw_wq, &isw->work) // queue_work async inode_switch_wbs_work_fn wb_put_many(old_wb, nr_switched) percpu_ref_put_many ref->data->release(ref) cgwb_release queue_work(cgwb_release_wq, &wb->release_work) // queue_work async &wb->release_work cgwb_release_workfn ksys_sync iterate_supers sync_inodes_one_sb sync_inodes_sb bdi_split_work_to_wbs kmalloc(sizeof(*work), GFP_ATOMIC) // alloc memory failed percpu_ref_exit ref->data = NULL kfree(data) wb_get(wb) percpu_ref_get(&wb->refcnt) percpu_ref_get_many(ref, 1) atomic_long_add(nr, &ref->data->count) atomic64_add(i, v) // trigger null-ptr-deref

bdi_split_work_to_wbs() traverses &bdi->wb_list to split work into all wbs. If the allocation of new work fails, the on-stack fallback will be used and the reference count of the current wb is increased afterwards. If cgroup writeback membership switches occur before getting the reference count and the current wb is released as old_wd, then calling wb_get() or wb_put() will trigger the null pointer dereference above.

This issue was introduced in v4.3-rc7 (see fix tag1). Both sync_inodes_sb() and __writeback_inodes_sb_nr() calls to bdi_split_work_to_wbs() can trigger this issue. For scenarios called via sync_inodes_sb(), originally commit 7fc5854f8c6e ("writeback: synchronize sync(2) against cgroup writeback membership switches") reduced the possibility of the issue by adding wb_switch_rwsem, but in v5.14-rc1 (see fix tag2) removed the "inode_io_list_del_locked(inode, old_wb)" from inode_switch_wbs_work_fn() so that wb->state contains WB_has_dirty_io, thus old_wb is not skipped when traversing wbs in bdi_split_work_to_wbs(), and the issue becomes easily reproducible again.

To solve this problem, percpu_ref_exit() is called under RCU protection to avoid race between cgwb_release_workfn() and bdi_split_work_to_wbs(). Moreover, replace wb_get() with wb_tryget() in bdi_split_work_to_wbs(), and skip the current wb if wb_tryget() fails because the wb has already been shutdown.

Link: https://lkml.kernel.org/r/20230410130826.1492525-1-libaokun1@huawei.com Fixes: b817525a4a80 ("writeback: bdi_writeback iteration must not skip dying ones") Signed-off-by: Baokun Li libaokun1@huawei.com Reviewed-by: Jan Kara jack@suse.cz Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: Andreas Dilger adilger.kernel@dilger.ca Cc: Christian Brauner brauner@kernel.org Cc: Dennis Zhou dennis@kernel.org Cc: Hou Tao houtao1@huawei.com Cc: Tejun Heo tj@kernel.org Cc: yangerkun yangerkun@huawei.com Cc: Zhang Yi yi.zhang@huawei.com Cc: Jens Axboe axboe@kernel.dk Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/fs-writeback.c | 17 ++++++++++------- mm/backing-dev.c | 12 ++++++++++-- 2 files changed, 20 insertions(+), 9 deletions(-)

--- a/fs/fs-writeback.c~writeback-cgroup-fix-null-ptr-deref-write-in-bdi_split_work_to_wbs +++ a/fs/fs-writeback.c @@ -978,6 +978,16 @@ restart: continue; }

+ /* + * If wb_tryget fails, the wb has been shutdown, skip it. + * + * Pin @wb so that it stays on @bdi->wb_list. This allows + * continuing iteration from @wb after dropping and + * regrabbing rcu read lock. + */ + if (!wb_tryget(wb)) + continue; + /* alloc failed, execute synchronously using on-stack fallback */ work = &fallback_work; *work = *base_work; @@ -986,13 +996,6 @@ restart: work->done = &fallback_work_done;

wb_queue_work(wb, work); - - /* - * Pin @wb so that it stays on @bdi->wb_list. This allows - * continuing iteration from @wb after dropping and - * regrabbing rcu read lock. - */ - wb_get(wb); last_wb = wb;

rcu_read_unlock(); --- a/mm/backing-dev.c~writeback-cgroup-fix-null-ptr-deref-write-in-bdi_split_work_to_wbs +++ a/mm/backing-dev.c @@ -507,6 +507,15 @@ static LIST_HEAD(offline_cgwbs); static void cleanup_offline_cgwbs_workfn(struct work_struct *work); static DECLARE_WORK(cleanup_offline_cgwbs_work, cleanup_offline_cgwbs_workfn);

+static void cgwb_free_rcu(struct rcu_head *rcu_head) +{ + struct bdi_writeback *wb = container_of(rcu_head, + struct bdi_writeback, rcu); + + percpu_ref_exit(&wb->refcnt); + kfree(wb); +} + static void cgwb_release_workfn(struct work_struct *work) { struct bdi_writeback *wb = container_of(work, struct bdi_writeback, @@ -529,11 +538,10 @@ static void cgwb_release_workfn(struct w list_del(&wb->offline_node); spin_unlock_irq(&cgwb_lock);

- percpu_ref_exit(&wb->refcnt); wb_exit(wb); bdi_put(bdi); WARN_ON_ONCE(!list_empty(&wb->b_attached)); - kfree_rcu(wb, rcu); + call_rcu(&wb->rcu, cgwb_free_rcu); }

static void cgwb_release(struct percpu_ref *refcnt) _

Patches currently in -mm which might be from libaokun1@huawei.com are

writeback-cgroup-fix-null-ptr-deref-write-in-bdi_split_work_to_wbs.patch