[PATCH v5 1/5] riscv: save the SR_SUM status over switches
Cyril Bur
cyrilbur at tenstorrent.com
Sun Mar 30 20:16:57 PDT 2025
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:
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.
In order for riscv to take advantage of unsafe_get/put_XXX() macros 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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