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 <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Vincent Guittot <[email protected]>
Cc: Dietmar Eggemann <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Ben Segall <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Daniel Bristot de Oliveira <[email protected]>
Cc: Zhang Qiao <[email protected]>
Cc: Daniel Jordan <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://syzkaller.appspot.com/bug?id=9d9c27adc674e3a7932b22b61c79a02da82cbdc1
Fixes: 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
Reported-by: [email protected]
Signed-off-by: Tadeusz Struk <[email protected]>
---
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));
}
/*
--
2.34.1
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 <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Juri Lelli <[email protected]>
> Cc: Vincent Guittot <[email protected]>
> Cc: Dietmar Eggemann <[email protected]>
> Cc: Steven Rostedt <[email protected]>
> Cc: Ben Segall <[email protected]>
> Cc: Mel Gorman <[email protected]>
> Cc: Daniel Bristot de Oliveira <[email protected]>
> Cc: Zhang Qiao <[email protected]>
> Cc: Daniel Jordan <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
>
> Link: https://syzkaller.appspot.com/bug?id=9d9c27adc674e3a7932b22b61c79a02da82cbdc1
> Fixes: 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
> Reported-by: [email protected]
> Signed-off-by: Tadeusz Struk <[email protected]>
> ---
> 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`.
On 1/26/22 05:53, Dietmar Eggemann wrote:
> 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]);
That will also work. Let's agree on the best way to fix it.
Peter, what's your preference?
--
Thanks,
Tadeusz
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 remove the update_load param from the
update_load param() function and call reweight_task() only if the task
flag doesn't have the TASK_NEW flag set.
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Vincent Guittot <[email protected]>
Cc: Dietmar Eggemann <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Ben Segall <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Daniel Bristot de Oliveira <[email protected]>
Cc: Zhang Qiao <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://syzkaller.appspot.com/bug?id=9d9c27adc674e3a7932b22b61c79a02da82cbdc1
Fixes: 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
Reported-by: [email protected]
Signed-off-by: Tadeusz Struk <[email protected]>
---
Changes in v4:
- Removed the update_load param from set_load_weight() and call
reweight_task() based on the TASK_NEW flag
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 | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 848eaa0efe0e..a0ef4670e695 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1214,10 +1214,12 @@ 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 prio = p->static_prio - MAX_RT_PRIO;
struct load_weight *load = &p->se.load;
+ bool update_load = !(READ_ONCE(p->__state) & TASK_NEW);
+
/*
* SCHED_IDLE tasks get minimal weight:
@@ -4406,7 +4408,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
p->static_prio = NICE_TO_PRIO(0);
p->prio = p->normal_prio = p->static_prio;
- set_load_weight(p, false);
+ set_load_weight(p);
/*
* We don't need the reset flag anymore after the fork. It has
@@ -6921,7 +6923,7 @@ 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);
old_prio = p->prio;
p->prio = effective_prio(p);
@@ -7212,7 +7214,7 @@ 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);
}
/*
@@ -9445,7 +9447,7 @@ void __init sched_init(void)
#endif
}
- set_load_weight(&init_task, false);
+ set_load_weight(&init_task);
/*
* The boot idle thread does lazy MMU switching as well:
--
2.34.1
On 2/3/22 01:40, Dietmar Eggemann wrote:
>> -static void set_load_weight(struct task_struct *p, bool update_load)
>> +static void set_load_weight(struct task_struct *p)
>> {
>> int prio = p->static_prio - MAX_RT_PRIO;
>> struct load_weight *load = &p->se.load;
>> + bool update_load = !(READ_ONCE(p->__state) & TASK_NEW);
> nit-pick: reverse fir tree order
>
> cat Documentation/process/maintainer-tip.rst | grep -A 10 "Variable
> declarations"
>
> I was able to recreate the initial issue with the reproducer on arm64
> Juno-r0 with CONFIG_CGROUP_SCHED=y .
>
> Reviewed-by: Dietmar Eggemann<[email protected]>
Thanks for reviewing. I will send a v5 soon.
--
Thanks,
Tadeusz
On 27/01/2022 21:56, 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 remove the update_load param from the
> update_load param() function and call reweight_task() only if the task
> flag doesn't have the TASK_NEW flag set.
>
> Cc: Ingo Molnar <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Juri Lelli <[email protected]>
> Cc: Vincent Guittot <[email protected]>
> Cc: Dietmar Eggemann <[email protected]>
> Cc: Steven Rostedt <[email protected]>
> Cc: Ben Segall <[email protected]>
> Cc: Mel Gorman <[email protected]>
> Cc: Daniel Bristot de Oliveira <[email protected]>
> Cc: Zhang Qiao <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
>
> Link: https://syzkaller.appspot.com/bug?id=9d9c27adc674e3a7932b22b61c79a02da82cbdc1
> Fixes: 4ef0c5c6b5ba ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
> Reported-by: [email protected]
> Signed-off-by: Tadeusz Struk <[email protected]>
> ---
> Changes in v4:
> - Removed the update_load param from set_load_weight() and call
> reweight_task() based on the TASK_NEW flag
>
> 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 | 12 +++++++-----
> 1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 848eaa0efe0e..a0ef4670e695 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1214,10 +1214,12 @@ 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 prio = p->static_prio - MAX_RT_PRIO;
> struct load_weight *load = &p->se.load;
> + bool update_load = !(READ_ONCE(p->__state) & TASK_NEW);
nit-pick: reverse fir tree order
cat Documentation/process/maintainer-tip.rst | grep -A 10 "Variable
declarations"
I was able to recreate the initial issue with the reproducer on arm64
Juno-r0 with CONFIG_CGROUP_SCHED=y .
Reviewed-by: Dietmar Eggemann <[email protected]>