linux-kernel - Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task

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Date:   Mon, 30 Aug 2021 16:39:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Zhang Qiao <zhangqiao22@...wei.com>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        tj@...nel.org
Subject: Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid
 sched_task_group

On Thu, Aug 26, 2021 at 07:26:35PM +0800, Zhang Qiao wrote:
> 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@...wei.com>

Hmm, I think you're right. Did something recently chagne in cgroup land
to make this more visible? This code hasn't changed in like 9 years.