CVE-2021-47209: In the Linux kernel, the following vulnerability has been resolved: sched/fair: Prevent dead task groups from regaining cfs_rq's Kevin is reporting crashes…

PriorityP421medium5.5CVSS 3.1

AVLACLPRLUINSUCNINAH

EPSS

0.21%

10.8th percentile

In the Linux kernel, the following vulnerability has been resolved: sched/fair: Prevent dead task groups from regaining cfs_rq's Kevin is reporting crashes which point to a use-after-free of a cfs_rq in update_blocked_averages(). Initial debugging revealed that we've live cfs_rq's (on_list=1) in an about to be kfree()'d task group in free_fair_sched_group(). However, it was unclear how that can happen. His kernel config happened to lead to a layout of struct sched_entity that put the 'my_q' member directly into the middle of the object which makes it incidentally overlap with SLUB's freelist pointer. That, in combination with SLAB_FREELIST_HARDENED's freelist pointer mangling, leads to a reliable access violation in form of a #GP which made the UAF fail fast. Michal seems to have run into the same issue[1]. He already correctly diagnosed that commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle") is causing the preconditions for the UAF to happen by re-adding cfs_rq's also to task groups that have no more running tasks, i.e. also to dead ones. His analysis, however, misses the real root cause and it cannot be seen from the crash backtrace only, as the real offender is tg_unthrottle_up() getting called via sched_cfs_period_timer() via the timer interrupt at an inconvenient time. When unregister_fair_sched_group() unlinks all cfs_rq's from the dying task group, it doesn't protect itself from getting interrupted. If the timer interrupt triggers while we iterate over all CPUs or after unregister_fair_sched_group() has finished but prior to unlinking the task group, sched_cfs_period_timer() will execute and walk the list of task groups, trying to unthrottle cfs_rq's, i.e. re-add them to the dying task group. These will later -- in free_fair_sched_group() -- be kfree()'ed while still being linked, leading to the fireworks Kevin and Michal are seeing. To fix this race, ensure the dying task group gets unlinked first. However, simply switc

Affected

8 ranges

Vendor	Product	Version range	Fixed in
debian	linux	< linux 5.15.5-1 (bookworm)	linux 5.15.5-1 (bookworm)
linux	linux	—	—
linux	linux	>= a7b359fc6a37faaf472125867c8dc5a068c90982 < 512e21c150c1c3ee298852660f3a796e267e62ec	512e21c150c1c3ee298852660f3a796e267e62ec
linux	linux	>= a7b359fc6a37faaf472125867c8dc5a068c90982 < b027789e5e50494c2325cc70c8642e7fd6059479	b027789e5e50494c2325cc70c8642e7fd6059479
linux	linux_kernel	>= 0 < 5.15.5-1	5.15.5-1
linux	linux_kernel	>= 0 < 5.15.5-1	5.15.5-1
linux	linux_kernel	>= 0 < 5.15.5-1	5.15.5-1
linux	linux_kernel	>= 5.13 < 5.15.5	5.15.5

CVSS provenance

nvdv3.15.5MEDIUMCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

osv5.5MEDIUM

vendor_debian5.5MEDIUM

vendor_redhat5.5MEDIUM

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