Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group

From: Peter Zijlstra
Date: Mon Aug 30 2021 - 10:40:17 EST


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@xxxxxxxxxx>

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.