On 25/01/2022 20:34, Tadeusz Struk wrote:
Syzbot found a GPF in reweight_entity. This has been bisected to commit 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
There is a race between sched_post_fork() and setpriority(PRIO_PGRP) within a thread group that causes a null-ptr-deref in reweight_entity() in CFS. The scenario is that the main process spawns number of new threads, which then call setpriority(PRIO_PGRP, 0, -20), wait, and exit. For each of the new threads the copy_process() gets invoked, which adds the new task_struct and calls sched_post_fork() for it.
In the above scenario there is a possibility that setpriority(PRIO_PGRP) and 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(), as it will try to access the run queue pointer, which hasn't been set. This results it a crash as shown below:
KASAN: null-ptr-deref in range [0x00000000000000a0-0x00000000000000a7] CPU: 0 PID: 2392 Comm: reduced_repro Not tainted 5.16.0-11201-gb42c5a161ea3 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 cfs_rq pointer for the task has been set in sched_fork(), which is called much earlier in copy_process(), before the new task is added to the thread_group. Now it is done in the sched_post_fork(), which is called after that. To fix the issue the update_load condition passed to set_load_weight() in set_user_nice() and __sched_setscheduler() has been changed from always true to true if the task->state != TASK_NEW.
Cc: Ingo Molnar mingo@redhat.com Cc: Peter Zijlstra peterz@infradead.org Cc: Juri Lelli juri.lelli@redhat.com Cc: Vincent Guittot vincent.guittot@linaro.org Cc: Dietmar Eggemann dietmar.eggemann@arm.com Cc: Steven Rostedt rostedt@goodmis.org Cc: Ben Segall bsegall@google.com Cc: Mel Gorman mgorman@suse.de Cc: Daniel Bristot de Oliveira bristot@redhat.com Cc: Zhang Qiao zhangqiao22@huawei.com Cc: Daniel Jordan daniel.m.jordan@oracle.com Cc: stable@vger.kernel.org Cc: linux-kernel@vger.kernel.org
Link: https://syzkaller.appspot.com/bug?id=9d9c27adc674e3a7932b22b61c79a02da82cbdc... Fixes: 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group") Reported-by: syzbot+af7a719bc92395ee41b3@syzkaller.appspotmail.com Signed-off-by: Tadeusz Struk tadeusz.struk@linaro.org
Changes in v3:
- Removed the new check and changed the update_load condition from always true to true if p->state != TASK_NEW
Changes in v2:
- Added a check in set_user_nice(), and return from there if the task is not fully setup instead of returning from reweight_entity()
kernel/sched/core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 848eaa0efe0e..3d7ede06b971 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -6921,7 +6921,7 @@ void set_user_nice(struct task_struct *p, long nice) put_prev_task(rq, p); p->static_prio = NICE_TO_PRIO(nice);
- set_load_weight(p, true);
- set_load_weight(p, !(READ_ONCE(p->__state) & TASK_NEW)); old_prio = p->prio; p->prio = effective_prio(p);
@@ -7212,7 +7212,7 @@ static void __setscheduler_params(struct task_struct *p, */ p->rt_priority = attr->sched_priority; p->normal_prio = normal_prio(p);
- set_load_weight(p, true);
- set_load_weight(p, !(READ_ONCE(p->__state) & TASK_NEW));
}
Can we then not get rid of `bool update_load` parameter of set_load_weight() completely?
@@ -1214,8 +1214,9 @@ int tg_nop(struct task_group *tg, void *data) } #endif
-static void set_load_weight(struct task_struct *p, bool update_load) +static void set_load_weight(struct task_struct *p) { + int task_new = READ_ONCE(p->__state) & TASK_NEW; int prio = p->static_prio - MAX_RT_PRIO; struct load_weight *load = &p->se.load;
@@ -1232,7 +1233,7 @@ static void set_load_weight(struct task_struct *p, bool update_load) * SCHED_OTHER tasks have to update their load when changing their * weight */ - if (update_load && p->sched_class == &fair_sched_class) { + if (!task_new && p->sched_class == &fair_sched_class) { reweight_task(p, prio); } else { load->weight = scale_load(sched_prio_to_weight[prio]);
p in sched_fork() would have `p->__state = TASK_NEW` and in sched_init() `init_task->sched_class is NULL so != &fair_sched_class`.