Messages in this thread Patch in this message | | | From | Zhang Qiao <> | Subject | [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group | Date | Thu, 26 Aug 2021 19:26:35 +0800 |
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There is a small race between copy_process() and sched_fork() where child->sched_task_group point to an already freed pointer.
parent doing fork() | someone moving the parent to another cgroup -------------------------------+------------------------------- copy_process() + dup_task_struct()<1> parent move to another cgroup, and free the old cgroup. <2> + sched_fork() + __set_task_cpu()<3> + task_fork_fair() + sched_slice()<4>
In the worst case, this bug can lead to "use-after-free" and cause panic as shown above, (1)parent copy its sched_task_group to child at <1>; (2)someone move the parent to another cgroup and free the old cgroup at <2>; (3)the sched_task_group and cfs_rq that belong to the old cgroup will be accessed at <3> and <4>, which cause a panic:
[89249.732198] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 [89249.732701] PGD 8000001fa0a86067 P4D 8000001fa0a86067 PUD 2029955067 PMD 0 [89249.733005] Oops: 0000 [#1] SMP PTI [89249.733288] CPU: 7 PID: 648398 Comm: ebizzy Kdump: loaded Tainted: G OE --------- - - 4.18.0.x86_64+ #1 [89249.734318] RIP: 0010:sched_slice+0x84/0xc0 .... [89249.737910] Call Trace: [89249.738181] task_fork_fair+0x81/0x120 [89249.738457] sched_fork+0x132/0x240 [89249.738732] copy_process.part.5+0x675/0x20e0 [89249.739010] ? __handle_mm_fault+0x63f/0x690 [89249.739286] _do_fork+0xcd/0x3b0 [89249.739558] do_syscall_64+0x5d/0x1d0 [89249.739830] entry_SYSCALL_64_after_hwframe+0x65/0xca [89249.740107] RIP: 0033:0x7f04418cd7e1
When a new process is forked, cgroup_post_fork() associates it with the cgroup of its parent. Therefore this commit move the __set_task_cpu() and task_fork() that access some cgroup-related fields(sched_task_group and cfs_rq) to sched_post_fork() and call sched_post_fork() after cgroup_post_fork().
Fixes: 8323f26ce342 ("sched: Fix race in task_group") Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com> --- kernel/fork.c | 2 +- kernel/sched/core.c | 33 +++++++++++++-------------------- 2 files changed, 14 insertions(+), 21 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c index bc94b2cc5995..a4acd87f00f9 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2330,8 +2330,8 @@ static __latent_entropy struct task_struct *copy_process( write_unlock_irq(&tasklist_lock); proc_fork_connector(p); - sched_post_fork(p); cgroup_post_fork(p, args); + sched_post_fork(p); perf_event_fork(p); trace_task_newtask(p, clone_flags); diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 20ffcc044134..041f21e8b266 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4069,8 +4069,6 @@ int sysctl_schedstats(struct ctl_table *table, int write, void *buffer, */ int sched_fork(unsigned long clone_flags, struct task_struct *p) { - unsigned long flags; - __sched_fork(clone_flags, p); /* * We mark the process as NEW here. This guarantees that @@ -4116,24 +4114,6 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p) init_entity_runnable_average(&p->se); - /* - * The child is not yet in the pid-hash so no cgroup attach races, - * and the cgroup is pinned to this child due to cgroup_fork() - * is ran before sched_fork(). - * - * Silence PROVE_RCU. - */ - raw_spin_lock_irqsave(&p->pi_lock, flags); - rseq_migrate(p); - /* - * We're setting the CPU for the first time, we don't migrate, - * so use __set_task_cpu(). - */ - __set_task_cpu(p, smp_processor_id()); - if (p->sched_class->task_fork) - p->sched_class->task_fork(p); - raw_spin_unlock_irqrestore(&p->pi_lock, flags); - #ifdef CONFIG_SCHED_INFO if (likely(sched_info_on())) memset(&p->sched_info, 0, sizeof(p->sched_info)); @@ -4151,6 +4131,19 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p) void sched_post_fork(struct task_struct *p) { + unsigned long flags; + + raw_spin_lock_irqsave(&p->pi_lock, flags); + rseq_migrate(p); + /* + * We're setting the CPU for the first time, we don't migrate, + * so use __set_task_cpu(). + */ + __set_task_cpu(p, smp_processor_id()); + if (p->sched_class->task_fork) + p->sched_class->task_fork(p); + raw_spin_unlock_irqrestore(&p->pi_lock, flags); + uclamp_post_fork(p); } -- 2.17.1
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