[PATCH v5 1/5] riscv: save the SR_SUM status over switches
Alexandre Ghiti
alex at ghiti.fr
Mon Mar 31 08:20:22 PDT 2025
Hi Cyril,
On 31/03/2025 05:16, Cyril Bur wrote:
>
>
> On 21/3/2025 8:09 pm, Alexandre Ghiti wrote:
>> Hi Cyril/Ben,
>>
>> On 20/03/2025 23:44, Cyril Bur wrote:
>>> From: Ben Dooks <ben.dooks at codethink.co.uk>
>>>
>>> When threads/tasks are switched we need to ensure the old execution's
>>> SR_SUM state is saved and the new thread has the old SR_SUM state
>>> restored.
>>>
>>> The issue is seen under heavy load especially with the syz-stress tool
>>> running, with crashes as follows in schedule_tail:
>>>
>>> Unable to handle kernel access to user memory without uaccess routines
>>> at virtual address 000000002749f0d0
>>> Oops [#1]
>>> Modules linked in:
>>> CPU: 1 PID: 4875 Comm: syz-executor.0 Not tainted
>>> 5.12.0-rc2-syzkaller-00467-g0d7588ab9ef9 #0
>>> Hardware name: riscv-virtio,qemu (DT)
>>> epc : schedule_tail+0x72/0xb2 kernel/sched/core.c:4264
>>> ra : task_pid_vnr include/linux/sched.h:1421 [inline]
>>> ra : schedule_tail+0x70/0xb2 kernel/sched/core.c:4264
>>> epc : ffffffe00008c8b0 ra : ffffffe00008c8ae sp : ffffffe025d17ec0
>>> gp : ffffffe005d25378 tp : ffffffe00f0d0000 t0 : 0000000000000000
>>> t1 : 0000000000000001 t2 : 00000000000f4240 s0 : ffffffe025d17ee0
>>> s1 : 000000002749f0d0 a0 : 000000000000002a a1 : 0000000000000003
>>> a2 : 1ffffffc0cfac500 a3 : ffffffe0000c80cc a4 : 5ae9db91c19bbe00
>>> a5 : 0000000000000000 a6 : 0000000000f00000 a7 : ffffffe000082eba
>>> s2 : 0000000000040000 s3 : ffffffe00eef96c0 s4 : ffffffe022c77fe0
>>> s5 : 0000000000004000 s6 : ffffffe067d74e00 s7 : ffffffe067d74850
>>> s8 : ffffffe067d73e18 s9 : ffffffe067d74e00 s10: ffffffe00eef96e8
>>> s11: 000000ae6cdf8368 t3 : 5ae9db91c19bbe00 t4 : ffffffc4043cafb2
>>> t5 : ffffffc4043cafba t6 : 0000000000040000
>>> status: 0000000000000120 badaddr: 000000002749f0d0 cause:
>>> 000000000000000f
>>> Call Trace:
>>> [<ffffffe00008c8b0>] schedule_tail+0x72/0xb2 kernel/sched/core.c:4264
>>> [<ffffffe000005570>] ret_from_exception+0x0/0x14
>>> Dumping ftrace buffer:
>>> (ftrace buffer empty)
>>> ---[ end trace b5f8f9231dc87dda ]---
>>>
>>> The issue comes from the put_user() in schedule_tail
>>> (kernel/sched/core.c)
>>> doing the following:
>>>
>>> asmlinkage __visible void schedule_tail(struct task_struct *prev)
>>> {
>>> ...
>>> if (current->set_child_tid)
>>> put_user(task_pid_vnr(current),
>>> current->set_child_tid);
>>> ...
>>> }
>>>
>>> the put_user() macro causes the code sequence to come out as follows:
>>>
>>> 1: __enable_user_access()
>>> 2: reg = task_pid_vnr(current);
>>> 3: *current->set_child_tid = reg;
>>> 4: __disable_user_access()
>>>
>>> This means the task_pid_vnr() is being called with user-access enabled
>>> which itself is not a good idea, but that is a separate issue. Here we
>>> have a function that /might/ sleep being called with the SR_SUM and if
>>> it does, then it returns with the SR_SUM flag possibly cleared thus
>>> causing the above abort.
>>>
>>> To try and deal with this, and stop the SR_SUM leaking out into other
>>> threads (this has also been tested and see under stress. It can rarely
>>> happen but it /does/ under load) make sure the __switch_to() will save
>>> and restore the SR_SUM flag, and clear it possibly for the next thread
>>> if it does not need it.
>>>
>>> Note, test code to be supplied once other checks have been finished.
>>>
>>> There may be further issues with the mstatus flags with this, this
>>> can be discussed further once some initial testing has been done.
>>
>>
>> The whole changelog is outdated, it needs to be reworded:
>>
>> "To prevent the evaluation of preemptible functions in unsafe_get/
>> put_XXX() which could clear SUM bit set by get_user_access()... etc etc"
>>
>
> Hi Alex and Ben,
>
> Commit messages are not my forte and this one is complex. I've been
> wondering how to reword it - I feel that Bens text should live on, it
> isn't incorrect. Also I would very much appreciate if you could look
> over what I've written Ben.
>
> I'll start it off the same and hopefully I've messaged the end
> successfully:
>
>
> From: Ben Dooks <ben.dooks at codethink.co.uk>
>
> When threads/tasks are switched we need to ensure the old execution's
> SR_SUM state is saved and the new thread has the old SR_SUM state
> restored.
>
> The issue is seen under heavy load especially with the syz-stress tool
> running, with crashes as follows in schedule_tail:
"The issue was seen ... "
>
> Unable to handle kernel access to user memory without uaccess routines
> at virtual address 000000002749f0d0
> Oops [#1]
> Modules linked in:
> CPU: 1 PID: 4875 Comm: syz-executor.0 Not tainted
> 5.12.0-rc2-syzkaller-00467-g0d7588ab9ef9 #0
> Hardware name: riscv-virtio,qemu (DT)
> epc : schedule_tail+0x72/0xb2 kernel/sched/core.c:4264
> ra : task_pid_vnr include/linux/sched.h:1421 [inline]
> ra : schedule_tail+0x70/0xb2 kernel/sched/core.c:4264
> epc : ffffffe00008c8b0 ra : ffffffe00008c8ae sp : ffffffe025d17ec0
> gp : ffffffe005d25378 tp : ffffffe00f0d0000 t0 : 0000000000000000
> t1 : 0000000000000001 t2 : 00000000000f4240 s0 : ffffffe025d17ee0
> s1 : 000000002749f0d0 a0 : 000000000000002a a1 : 0000000000000003
> a2 : 1ffffffc0cfac500 a3 : ffffffe0000c80cc a4 : 5ae9db91c19bbe00
> a5 : 0000000000000000 a6 : 0000000000f00000 a7 : ffffffe000082eba
> s2 : 0000000000040000 s3 : ffffffe00eef96c0 s4 : ffffffe022c77fe0
> s5 : 0000000000004000 s6 : ffffffe067d74e00 s7 : ffffffe067d74850
> s8 : ffffffe067d73e18 s9 : ffffffe067d74e00 s10: ffffffe00eef96e8
> s11: 000000ae6cdf8368 t3 : 5ae9db91c19bbe00 t4 : ffffffc4043cafb2
> t5 : ffffffc4043cafba t6 : 0000000000040000
> status: 0000000000000120 badaddr: 000000002749f0d0 cause:
> 000000000000000f
> Call Trace:
> [<ffffffe00008c8b0>] schedule_tail+0x72/0xb2 kernel/sched/core.c:4264
> [<ffffffe000005570>] ret_from_exception+0x0/0x14
> Dumping ftrace buffer:
> (ftrace buffer empty)
> ---[ end trace b5f8f9231dc87dda ]---
>
> The issue comes from the put_user() in schedule_tail
> (kernel/sched/core.c) doing the following:
>
> asmlinkage __visible void schedule_tail(struct task_struct *prev)
> {
> ...
> if (current->set_child_tid)
> put_user(task_pid_vnr(current), current->set_child_tid);
> ...
> }
>
> the put_user() macro causes the code sequence to come out as follows:
>
> 1: __enable_user_access()
> 2: reg = task_pid_vnr(current);
> 3: *current->set_child_tid = reg;
> 4: __disable_user_access()
>
"The problem is that we may have a sleeping function as argument which
could clear SR_SUM causing the panic above. So this was fixed by
evaluating the argument of the put_user() macro outside the user-enabled
section in commit XXXX ("")"
> This means the task_pid_vnr() is being called with user-access enabled
> which itself is not a good idea, but that is a separate issue. Here we
> have a function that /might/ sleep being called with the SR_SUM and if
> it does, then it returns with the SR_SUM flag possibly cleared thus
> causing the above abort.
^^ I would remove that then.
>
> To try and deal with this, and stop the SR_SUM leaking out into other
> threads (this has also been tested and see under stress. It can rarely
> happen but it /does/ under load) make sure the __switch_to() will save
> and restore the SR_SUM flag, and clear it possibly for the next thread
> if it does not need it.
>
I'd move that after "we must ensure code flow..." and I'd remove "and
clear it possibly..." since we won't clear it in the next version.
> In order for riscv to take advantage of unsafe_get/put_XXX() macros
"and to avoid the same issue we had with put_user() and sleeping functions"
> we must ensure code flow can go through switch_to() from within a
> region of code with SR_SUM enabled and come back with SR_SUM still
> enabled. This patch addresses the problem allowing future work to
> enable full use of unsafe_get/put_XXX() macros without needing to take
> a CSR bit flip cost on every access.
>
>
>>
>>>
>>> Reported-by: syzbot+e74b94fe601ab9552d69 at syzkaller.appspotmail.com
>>> Signed-off-by: Ben Dooks <ben.dooks at codethink.co.uk>
>>> Signed-off-by: Cyril Bur <cyrilbur at tenstorrent.com>
>>> ---
>>> arch/riscv/include/asm/processor.h | 1 +
>>> arch/riscv/kernel/asm-offsets.c | 5 +++++
>>> arch/riscv/kernel/entry.S | 8 ++++++++
>>> 3 files changed, 14 insertions(+)
>>>
>>> diff --git a/arch/riscv/include/asm/processor.h
>>> b/arch/riscv/include/ asm/processor.h
>>> index 5f56eb9d114a..0de05d652e0f 100644
>>> --- a/arch/riscv/include/asm/processor.h
>>> +++ b/arch/riscv/include/asm/processor.h
>>> @@ -103,6 +103,7 @@ struct thread_struct {
>>> struct __riscv_d_ext_state fstate;
>>> unsigned long bad_cause;
>>> unsigned long envcfg;
>>> + unsigned long flags;
>>
>>
>> I would prefer the use of status since it stores the sstatus csr.
>
> I will change it in the next version of my series.
>
>>
>>
>>> u32 riscv_v_flags;
>>> u32 vstate_ctrl;
>>> struct __riscv_v_ext_state vstate;
>>> diff --git a/arch/riscv/kernel/asm-offsets.c
>>> b/arch/riscv/kernel/asm- offsets.c
>>> index e89455a6a0e5..556ebcbb7e22 100644
>>> --- a/arch/riscv/kernel/asm-offsets.c
>>> +++ b/arch/riscv/kernel/asm-offsets.c
>>> @@ -34,6 +34,7 @@ void asm_offsets(void)
>>> OFFSET(TASK_THREAD_S9, task_struct, thread.s[9]);
>>> OFFSET(TASK_THREAD_S10, task_struct, thread.s[10]);
>>> OFFSET(TASK_THREAD_S11, task_struct, thread.s[11]);
>>> + OFFSET(TASK_THREAD_FLAGS, task_struct, thread.flags);
>>> OFFSET(TASK_TI_CPU, task_struct, thread_info.cpu);
>>> OFFSET(TASK_TI_FLAGS, task_struct, thread_info.flags);
>>> @@ -347,6 +348,10 @@ void asm_offsets(void)
>>> offsetof(struct task_struct, thread.s[11])
>>> - offsetof(struct task_struct, thread.ra)
>>> );
>>> + DEFINE(TASK_THREAD_FLAGS_RA,
>>> + offsetof(struct task_struct, thread.flags)
>>> + - offsetof(struct task_struct, thread.ra)
>>> + );
>>> DEFINE(TASK_THREAD_F0_F0,
>>> offsetof(struct task_struct, thread.fstate.f[0])
>>> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
>>> index 33a5a9f2a0d4..c278b3ac37b9 100644
>>> --- a/arch/riscv/kernel/entry.S
>>> +++ b/arch/riscv/kernel/entry.S
>>> @@ -397,9 +397,17 @@ SYM_FUNC_START(__switch_to)
>>> REG_S s9, TASK_THREAD_S9_RA(a3)
>>> REG_S s10, TASK_THREAD_S10_RA(a3)
>>> REG_S s11, TASK_THREAD_S11_RA(a3)
>>> +
>>> + /* save (and disable the user space access flag) */
>>> + li s0, SR_SUM
>>> + csrrc s1, CSR_STATUS, s0
>>
>>
>> Here (again), I don't think we need to clear sstatus.
>
> I don't think so either, the bit definitely has no effect in userspace:
> > SUM has no effect when page-based virtual memory is not in effect,
> nor when executing in U-mode.
>
> However, I wonder if Ben had a plan? If not I'll make it a read next
> version of the series.
>
> Thanks,
>
> Cyril
>
>>
>>
>>> + REG_S s1, TASK_THREAD_FLAGS_RA(a3)
>>> +
>>> /* Save the kernel shadow call stack pointer */
>>> scs_save_current
>>> /* Restore context from next->thread */
>>> + REG_L s0, TASK_THREAD_FLAGS_RA(a4)
>>> + csrs CSR_STATUS, s0
>>> REG_L ra, TASK_THREAD_RA_RA(a4)
>>> REG_L sp, TASK_THREAD_SP_RA(a4)
>>> REG_L s0, TASK_THREAD_S0_RA(a4)
>>
>>
>> Thanks,
>>
>> Alex
>>
>
>
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Hopefully that's better!
Thanks,
Alex
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