| Message ID | 20221211222058.2946830-2-longman@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | blk-cgroup: Fix potential UAF & miscellaneous cleanup | expand |
On Sun, Dec 11, 2022 at 05:20:56PM -0500, Waiman Long wrote: > static void blkcg_destroy_blkgs(struct blkcg *blkcg) > { > + /* > + * blkcg_destroy_blkgs() shouldn't be called with all the blkcg > + * references gone. > + */ > + if (WARN_ON_ONCE(!css_tryget(&blkcg->css))) > + return; Wouldn't it make more sense to use percpu_ref_is_zero()? It's not like the obtained extra reference does anything, right? Thanks.
On 12/12/22 17:13, Tejun Heo wrote: > On Sun, Dec 11, 2022 at 05:20:56PM -0500, Waiman Long wrote: >> static void blkcg_destroy_blkgs(struct blkcg *blkcg) >> { >> + /* >> + * blkcg_destroy_blkgs() shouldn't be called with all the blkcg >> + * references gone. >> + */ >> + if (WARN_ON_ONCE(!css_tryget(&blkcg->css))) >> + return; > Wouldn't it make more sense to use percpu_ref_is_zero()? It's not like the > obtained extra reference does anything, right? Yes, that makes sense. Will incorporate the change in the next version. Thanks, Longman
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c index 1bb939d3b793..21cc88349f21 100644 --- a/block/blk-cgroup.c +++ b/block/blk-cgroup.c @@ -1084,6 +1084,13 @@ struct list_head *blkcg_get_cgwb_list(struct cgroup_subsys_state *css) */ static void blkcg_destroy_blkgs(struct blkcg *blkcg) { + /* + * blkcg_destroy_blkgs() shouldn't be called with all the blkcg + * references gone. + */ + if (WARN_ON_ONCE(!css_tryget(&blkcg->css))) + return; + might_sleep(); spin_lock_irq(&blkcg->lock); @@ -1110,6 +1117,7 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg) } spin_unlock_irq(&blkcg->lock); + css_put(&blkcg->css); } /** diff --git a/mm/backing-dev.c b/mm/backing-dev.c index c30419a5e119..36f75b072325 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -390,11 +390,15 @@ static void cgwb_release_workfn(struct work_struct *work) wb_shutdown(wb); css_put(wb->memcg_css); - css_put(wb->blkcg_css); mutex_unlock(&wb->bdi->cgwb_release_mutex); - /* triggers blkg destruction if no online users left */ + /* + * Triggers blkg destruction if no online users left + * The final blkcg css_put() has to be done after blkcg_unpin_online() + * to avoid use-after-free. + */ blkcg_unpin_online(wb->blkcg_css); + css_put(wb->blkcg_css); fprop_local_destroy_percpu(&wb->memcg_completions);
Commit 59b57717fff8 ("blkcg: delay blkg destruction until after writeback has finished") delayed call to blkcg_destroy_blkgs() to cgwb_release_workfn(). However, it is done after a css_put() of blkcg which may be the final put that causes the blkcg to be freed as RCU read lock isn't held. Another place where blkcg_destroy_blkgs() can be called indirectly via blkcg_unpin_online() is from the offline_css() function called from css_killed_work_fn(). Over there, the potentially final css_put() call is issued after offline_css(). By adding a css_tryget() into blkcg_destroy_blkgs() and warning its failure, the following stack trace was produced in a test system on bootup. [ 34.254240] RIP: 0010:blkcg_destroy_blkgs+0x16a/0x1a0 : [ 34.339943] Call Trace: [ 34.344510] blkcg_unpin_online+0x38/0x60 [ 34.348523] cgwb_release_workfn+0x6a/0x200 [ 34.352708] process_one_work+0x1e5/0x3b0 [ 34.360758] worker_thread+0x50/0x3a0 [ 34.368447] kthread+0xd9/0x100 [ 34.376386] ret_from_fork+0x22/0x30 This confirms that a potential UAF situation can really happen in cgwb_release_workfn(). Fix that by delaying the css_put() until after the blkcg_unpin_online() call. Also use css_tryget() in blkcg_destroy_blkgs() and issue a warning if css_tryget() fails. The reproducing system can no longer produce a warning with this patch. All the runnable block/0* tests including block/027 were run successfully without failure. Fixes: 59b57717fff8 ("blkcg: delay blkg destruction until after writeback has finished") Suggested-by: Michal Koutný <mkoutny@suse.com> Reported-by: Yi Zhang <yi.zhang@redhat.com> Signed-off-by: Waiman Long <longman@redhat.com> --- block/blk-cgroup.c | 8 ++++++++ mm/backing-dev.c | 8 ++++++-- 2 files changed, 14 insertions(+), 2 deletions(-)