blk-cgroup: cleanup and bugfixs in blk-cgroup #459
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January 13, 2026 15:29
When switching an IO scheduler on a block device, blkcg_activate_policy() allocates blkg_policy_data (pd) for all blkgs attached to the queue. However, blkcg_activate_policy() may race with concurrent blkcg deletion, leading to use-after-free and memory leak issues. The use-after-free occurs in the following race: T1 (blkcg_activate_policy): - Successfully allocates pd for blkg1 (loop0->queue, blkcgA) - Fails to allocate pd for blkg2 (loop0->queue, blkcgB) - Enters the enomem rollback path to release blkg1 resources T2 (blkcg deletion): - blkcgA is deleted concurrently - blkg1 is freed via blkg_free_workfn() - blkg1->pd is freed T1 (continued): - Rollback path accesses blkg1->pd->online after pd is freed - Triggers use-after-free In addition, blkg_free_workfn() frees pd before removing the blkg from q->blkg_list. This allows blkcg_activate_policy() to allocate a new pd for a blkg that is being destroyed, leaving the newly allocated pd unreachable when the blkg is finally freed. Fix these races by extending blkcg_mutex coverage to serialize blkcg_activate_policy() rollback and blkg destruction, ensuring pd lifecycle is synchronized with blkg list visibility. Link: https://lore.kernel.org/all/[email protected]/ Fixes: f1c006f ("blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()") Signed-off-by: Zheng Qixing <[email protected]>
When switching IO schedulers on a block device, blkcg_activate_policy()
can race with concurrent blkcg deletion, leading to a use-after-free in
rcu_accelerate_cbs.
T1: T2:
blkg_destroy
kill(&blkg->refcnt) // blkg->refcnt=1->0
blkg_release // call_rcu(__blkg_release)
...
blkg_free_workfn
->pd_free_fn(pd)
elv_iosched_store
elevator_switch
...
iterate blkg list
blkg_get(blkg) // blkg->refcnt=0->1
list_del_init(&blkg->q_node)
blkg_put(pinned_blkg) // blkg->refcnt=1->0
blkg_release // call_rcu again
rcu_accelerate_cbs // uaf
Fix this by replacing blkg_get() with blkg_tryget(), which fails if
the blkg's refcount has already reached zero. If blkg_tryget() fails,
skip processing this blkg since it's already being destroyed.
Link: https://lore.kernel.org/all/[email protected]/
Fixes: f1c006f ("blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()")
Signed-off-by: Zheng Qixing <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Move the teardown sequence which offlines and frees per-policy blkg_policy_data (pd) into a helper for readability. No functional change intended. Signed-off-by: Zheng Qixing <[email protected]> Reviewed-by: Christoph Hellwig <[email protected]> Reviewed-by: Yu Kuai <[email protected]>
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Pull request for series with
subject: blk-cgroup: cleanup and bugfixs in blk-cgroup
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1039644