]> git.ipfire.org Git - thirdparty/kernel/stable.git/commit
writeback: fix obtain a reference to a freeing memcg css
authorMuchun Song <songmuchun@bytedance.com>
Fri, 2 Apr 2021 09:11:45 +0000 (17:11 +0800)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Wed, 14 Jul 2021 15:00:06 +0000 (17:00 +0200)
commit3ee7e6cf9fe8ceb6f95de855ab04c52c21f71810
tree916406a42011fd1ec2244c52d7f85f75c051d995
parent91d53eb400c26d04fec9e070a73c02661ed1d493
writeback: fix obtain a reference to a freeing memcg css

[ Upstream commit 8b0ed8443ae6458786580d36b7d5f8125535c5d4 ]

The caller of wb_get_create() should pin the memcg, because
wb_get_create() relies on this guarantee. The rcu read lock
only can guarantee that the memcg css returned by css_from_id()
cannot be released, but the reference of the memcg can be zero.

  rcu_read_lock()
  memcg_css = css_from_id()
  wb_get_create(memcg_css)
      cgwb_create(memcg_css)
          // css_get can change the ref counter from 0 back to 1
          css_get(memcg_css)
  rcu_read_unlock()

Fix it by holding a reference to the css before calling
wb_get_create(). This is not a problem I encountered in the
real world. Just the result of a code review.

Fixes: 682aa8e1a6a1 ("writeback: implement unlocked_inode_to_wb transaction and use it for stat updates")
Link: https://lore.kernel.org/r/20210402091145.80635-1-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>
fs/fs-writeback.c