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@xxxxxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Juri Lelli <juri.lelli@xxxxxxxxxx> > Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx> > Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> > Cc: Ben Segall <bsegall@xxxxxxxxxx> > Cc: Mel Gorman <mgorman@xxxxxxx> > Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx> > Cc: Zhang Qiao <zhangqiao22@xxxxxxxxxx> > Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> > Cc: stable@xxxxxxxxxxxxxxx > Cc: linux-kernel@xxxxxxxxxxxxxxx > > Link: https://syzkaller.appspot.com/bug?id=9d9c27adc674e3a7932b22b61c79a02da82cbdc1 > Fixes: 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group") > Reported-by: syzbot+af7a719bc92395ee41b3@xxxxxxxxxxxxxxxxxxxxxxxxx > Signed-off-by: Tadeusz Struk <tadeusz.struk@xxxxxxxxxx> > --- > 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`.