Messages in this thread | | | Date | Mon, 30 Aug 2021 16:39:54 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] kernel/sched: Fix sched_fork() access an invalid sched_task_group |
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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@huawei.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.
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