Re: [PATCH 5.10 000/381] 5.10.180-rc1 review

[Greg Thelen <gthelen@google.com>]

Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:

> This is the start of the stable review cycle for the 5.10.180 release.
> There are 381 patches in this series, all will be posted as a response
> to this one.  If anyone has any issues with these being applied, please
> let me know.
>
> Responses should be made by Wed, 17 May 2023 16:16:37 +0000.
> Anything received after that time might be too late.
>
> The whole patch series can be found in one patch at:
> 	https://www.kernel.org/pub/linux/kernel/v5.x/stable-review/patch-5.10.180-rc1.gz
> or in the git tree and branch at:
> 	git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git linux-5.10.y
> and the diffstat can be found below.
>
> thanks,
>
> greg k-h
>
> -------------
> Pseudo-Shortlog of commits:

[...]

> Baokun Li <libaokun1@huawei.com>
>     writeback, cgroup: fix null-ptr-deref write in bdi_split_work_to_wbs

Sorry for not noticing this sooner, but I think there's a benign issue
in this backport.

v5.10.180 commit 2b00b2a0e642 ("writeback, cgroup: fix null-ptr-deref
write in bdi_split_work_to_wbs") contains:

+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);
+}

[...]

@@ -397,7 +406,7 @@ static void cgwb_release_workfn(struct work_struct *work)
        fprop_local_destroy_percpu(&wb->memcg_completions);
        percpu_ref_exit(&wb->refcnt);
        wb_exit(wb);
-       kfree_rcu(wb, rcu);
+       call_rcu(&wb->rcu, cgwb_free_rcu);
 }

Notice there are now 2 percpu_ref_exit() calls. The upstream, and 5.15.y
patches remove the cgwb_release_workfn() calls to percpu_ref_exit(). The
5.10.y fixup is below. It's not essential but might be worth applying to
track upstream.

From 416e0e8ab5ff41676d04dc819bd667c6ad3f7555 Mon Sep 17 00:00:00 2001
From: Greg Thelen <gthelen@google.com>
Date: Sat, 20 May 2023 12:46:24 -0700
Subject: [PATCH] writeback, cgroup: remove extra percpu_ref_exit()

5.10 stable commit 2b00b2a0e642 ("writeback, cgroup: fix null-ptr-deref
write in bdi_split_work_to_wbs") is a backport of upstream 6.3 commit
1ba1199ec574.

In the 5.10 stable commit backport percpu_ref_exit() is called twice:
first in cgwb_release_workfn() and then in cgwb_free_rcu(). The 2nd call
is benign as percpu_ref_exit() internally detects there's nothing to do.

This fixes an non-upstream issue that only applies to 5.10.y.

Fixes: 2b00b2a0e642 ("writeback, cgroup: fix null-ptr-deref write in bdi_split_work_to_wbs")
Signed-off-by: Greg Thelen <gthelen@google.com>
---
 mm/backing-dev.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index b28f629c3527..dd08ab928e07 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -404,7 +404,6 @@ static void cgwb_release_workfn(struct work_struct *work)
 	blkcg_unpin_online(blkcg);
 
 	fprop_local_destroy_percpu(&wb->memcg_completions);
-	percpu_ref_exit(&wb->refcnt);
 	wb_exit(wb);
 	call_rcu(&wb->rcu, cgwb_free_rcu);
 }
-- 
2.40.1.698.g37aff9b760-goog