In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
cgroup: Avoid iteration of dying tasks with zero refcount<\/p>\n
The commit 260fbcb92bbea (“cgroup: Move dying_tasks cleanup from
\ncgroup_task_release() to cgroup_task_free()”) extended the lifetime of
\ntasks on the dying_tasks list.
\nThe iterators have provision to go through dying_tasks because of
\ndying threadgroup leaders or explicit CSS_TASK_ITER_WITH_DEAD, however,
\nit was expected that such tasks can obtain a new reference (that is
\npossible before cgroup_task_release()\/put_task_struct_rcu_user()).
\nThe tasks after cgroup_task_release() and before cgroup_task_free()
\nare subject to race when they may or may not have ->usage count > 0.<\/p>\n
The race window is between css_task_iter_next() invocations
\nwhen css_set_lock is released and we may arrive at a new ->task_pos.
\nThe iterator should not attempt to resurrect tasks whose ->usage count
\ndropped to zero. (When that happens, __put_task_struct_rcu_cb() is
\nalready imminent and the returned task_struct would could be used
\nafter free.)<\/p>\n
As for the fix, we cannot simply check the signal->live count of a task
\non the dying list because that won’t distinguish regular zombies waiting
\nto be reaped from RCU remnant tasks that are going to be free’d.
\nTherefore add an extra check to rule out ->usage==0 tasks from any
\niteration.<\/p>\n
The repeat: loop in css_task_iter_advance() doesn’t consider ->usage
\ncount, so add a new loop to css_task_iter_next() to skip de-used tasks
\non the dying_list.<\/p>\n
Rough illustration of the possible race<\/p>\n
R (reader of cgroup.procs) T (thread) L (group leader)
\n ——————————— ——————————– ——————————–
\n L exits, signal->live > 0
\n cgroup_task_dead(L)
\n css_set_skip_task_iters() \/\/ skips only cset->tasks
\n list_add_tail(&L->cg_list, &cset->dying_tasks)
\n css_task_iter_next()
\n take css_set_lock
\n css_task_iter_advance()
\n leader && signal->live != 0
\n => it->task_pos = &L->cg_list
\n release css_set_lock
\n T exits
\n –signal->live == 0
\n\t\t\t\t cgroup_task_dead(T) \/\/ css_set_lock
\n release_task(T)
\n cgroup_task_release(T)
\n release_task(L) \/\/ zap_leader
\n cgroup_task_release(L)
\n put_task_struct_rcu_user(L)
\n …RCU…
\n put_task_struct(L)
\n L->usage = 0
\n \/* L still on dying_tasks *\/
\n …RCU…
\n __put_task_struct(L)
\n css_task_iter_next() \/\/ another iteration
\n take css_set_lock
\n it->task_pos = &L->cg_list
\n get_task_struct(L)
\n => addition on 0
\n drop css_set_lock
\n cgroup_task_free(L)
\n css_set_skip_task_iters() \/\/ dying skip comes too late
\n free_task(L)
\n cgroup_procs_show()
\n task_pid_vnr(L)<\/p>\n","protected":false},"excerpt":{"rendered":"
In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
cgroup: Avoid iteration of dying tasks with zero refcount<\/p>\n
The commit 260fbcb92bbea (“cgroup: Move dying_tasks cleanup from
\ncgroup_<\/p>\n","protected":false},"template":"","meta":[],"categories":[],"tags":[],"class_list":["post-29676","vuln","type-vuln","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln\/29676","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln"}],"about":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/types\/vuln"}],"wp:attachment":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=29676"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=29676"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=29676"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}