Re: [PATCH] sched/fair: Fix fault in reweight_entity

[Tadeusz Struk <tadeusz.struk@linaro.org>]

On 1/19/22 07:43, Tadeusz Struk wrote:
>>> Looks like after this change there is a time window, when
>>> task_struct->se.cfs_rq can be NULL. This can be exploited to trigger
>>> null-ptr-deref by calling setpriority on that task.
>> Looks like isn't good enough, either there is, in which case you explain
>> the window, or there isn't in which case what are we doing here?
> 
> There surely is something wrong, otherwise it wouldn't crash.
> I will try to narrow down the reproducer to better understand what causes
> the fault.

The race is between sched_post_fork() and setpriority(PRIO_PGRP)
The scenario is that the main process spawns 3 new threads,
which then call setpriority(PRIO_PGRP, 0, -20), wait, and exit.
For each of the new thread the copy_process() gets invoked,
which then calls sched_fork() and finally sched_post_fork().

There is a possibility that setpriority(PRIO_PGRP)->set_one_prio() will be
called for a thread in the group that is just being created by copy_process(),
and for which the sched_post_fork() has not been executed yet.
This will trigger a null pointer dereference in reweight_entity()
because it will try to access the CFS run queue pointer, which hasn't been set, 
resulting it a crash as below:

KASAN: null-ptr-deref in range [0x00000000000000a0-0x00000000000000a7]
CPU: 0 PID: 2392 Comm: reduced_repro Not tainted 
5.16.0-11201-gb42c5a161ea3-dirty #13
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1.fc35 04/01/2014
RIP: 0010:reweight_entity+0x15d/0x440
RSP: 0018:ffffc900035dfcf8 EFLAGS: 00010006
Call Trace:
<TASK>
reweight_task+0xde/0x1c0
set_load_weight+0x21c/0x2b0
set_user_nice.part.0+0x2d1/0x519
set_user_nice.cold+0x8/0xd
set_one_prio+0x24f/0x263
__do_sys_setpriority+0x2d3/0x640
__x64_sys_setpriority+0x84/0x8b
do_syscall_64+0x35/0xb0
entry_SYSCALL_64_after_hwframe+0x44/0xae
</TASK>
---[ end trace 9dc80a9d378ed00a ]---

Before the mentioned change the rq pointer has been set in sched_fork(),
which is called much earlier in copy_process() as opposed to sched_post_fork(),
before the new task is added to the thread_group.

A stripped down version of the sysbot reproducer can be found here:
https://termbin.com/axkq

I can consistently reproduce the issue with it in 2-3 runs.

The solution is either we set the pointer p->se.cfs_rq to a dummy rq in 
sched_fork(), or return from the set_one_prio() without doing anything
if the rq is NULL, as it is done in the patch.
I will update the description and resend it tomorrow.

-- 
Thanks,
Tadeusz