Re: [PATCH v6 1/5] riscv: save the SR_SUM status over switches

[Ben Dooks <ben.dooks@codethink.co.uk>]

On 22/04/2025 11:22, Alexandre Ghiti wrote:
> Hi Cyril,
> 
> On 10/04/2025 09:05, Cyril Bur wrote:
>> From: Ben Dooks <ben.dooks@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 was 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()
>>
>> The problem is that we may have a sleeping function as argument which
>> could clear SR_SUM causing the panic above. This was fixed by
>> evaluating the argument of the put_user() macro outside the user-enabled
>> section in commit 285a76bb2cf5 ("riscv: evaluate put_user() arg before
>> enabling user access")"
>>
>> 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. Make switch_to() save and restore SR_SUM.
>>
>> Reported-by: syzbot+e74b94fe601ab9552d69@syzkaller.appspotmail.com
>> Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
>> Signed-off-by: Cyril Bur <cyrilbur@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..58fd11c89fe9 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 status;
>>       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 16490755304e..969c65b1fe41 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_STATUS, task_struct, thread.status);
>>       OFFSET(TASK_TI_CPU, task_struct, thread_info.cpu);
>>       OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, 
>> thread_info.preempt_count);
>> @@ -346,6 +347,10 @@ void asm_offsets(void)
>>             offsetof(struct task_struct, thread.s[11])
>>           - offsetof(struct task_struct, thread.ra)
>>       );
>> +    DEFINE(TASK_THREAD_STATUS_RA,
>> +          offsetof(struct task_struct, thread.status)
>> +        - 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..00bd0de9faa2 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 the user space access flag */
>> +    li    s0, SR_SUM
> 
> 
> This is not needed anymore ^ but I'll remove it when merging your patchset.
> 

Could you be more specific about what "this" is?

If we don't save/restore the SR_SUM bit I think our old friend
the sched_tail bug will just return.


>> +    csrr  s1, CSR_STATUS
>> +    REG_S s1, TASK_THREAD_STATUS_RA(a3)
>> +
>>       /* Save the kernel shadow call stack pointer */
>>       scs_save_current
>>       /* Restore context from next->thread */
>> +    REG_L s0,  TASK_THREAD_STATUS_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)
> 
> Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> 
> Thanks for the multiple revisions!
> 
> Alex
> 
> 
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> 


-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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