Re: [PATCH v2] riscv: Discard vector state on syscalls

["Björn Töpel" <bjorn@kernel.org>]

Andy Chiu <andy.chiu@sifive.com> writes:

> On Tue, Jun 27, 2023 at 12:57 AM Björn Töpel <bjorn@kernel.org> wrote:
>>
>> From: Björn Töpel <bjorn@rivosinc.com>
>>
>> The RISC-V vector specification states:
>>   Executing a system call causes all caller-saved vector registers
>>   (v0-v31, vl, vtype) and vstart to become unspecified.
>>
>> The vector registers are set to all 1s, vill is set (invalid), and the
>> vector status is set to Initial.
>>
>> That way we can prevent userspace from accidentally relying on the
>> stated save.
>>
>> Rémi pointed out [1] that writing to the registers might be
>> superfluous, and setting vill is sufficient.
>>
>> Link: https://lore.kernel.org/linux-riscv/12784326.9UPPK3MAeB@basile.remlab.net/ # [1]
>> Suggested-by: Darius Rad <darius@bluespec.com>
>> Suggested-by: Palmer Dabbelt <palmer@rivosinc.com>
>> Suggested-by: Rémi Denis-Courmont <remi@remlab.net>
>> Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
>> ---
>> v1->v2:
>>   Proper register restore for initial state (Andy)
>>   Set registers to 1s, and not 0s (Darius)
>> ---
>>  arch/riscv/include/asm/vector.h | 42 ++++++++++++++++++++++++++++++---
>>  arch/riscv/kernel/traps.c       |  2 ++
>>  2 files changed, 41 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
>> index 04c0b07bf6cd..93d702d9988c 100644
>> --- a/arch/riscv/include/asm/vector.h
>> +++ b/arch/riscv/include/asm/vector.h
>> @@ -139,14 +139,49 @@ static inline void riscv_v_vstate_save(struct task_struct *task,
>>         }
>>  }
>>
>> +static inline void __riscv_v_vstate_discard(void)
>> +{
>> +       unsigned long vl, vtype_inval = 1UL << (BITS_PER_LONG - 1);
>> +
>> +       riscv_v_enable();
>> +       asm volatile (
>> +               ".option push\n\t"
>> +               ".option arch, +v\n\t"
>> +               "vsetvli        %0, x0, e8, m8, ta, ma\n\t"
>> +               "vmv.v.i        v0, -1\n\t"
>> +               "vmv.v.i        v8, -1\n\t"
>> +               "vmv.v.i        v16, -1\n\t"
>> +               "vmv.v.i        v24, -1\n\t"
>> +               "vsetvl         %0, x0, %1\n\t"
>> +               ".option pop\n\t"
>> +               : "=&r" (vl) : "r" (vtype_inval) : "memory");
>> +       riscv_v_disable();
>> +}
>> +
>> +static inline void riscv_v_vstate_discard(struct pt_regs *regs)
>> +{
>> +       if (!riscv_v_vstate_query(regs))
>> +               return;
>> +
>> +       __riscv_v_vstate_discard();
>> +       riscv_v_vstate_on(regs);
>> +}
>> +
>>  static inline void riscv_v_vstate_restore(struct task_struct *task,
>>                                           struct pt_regs *regs)
>>  {
>> -       if ((regs->status & SR_VS) != SR_VS_OFF) {
>> -               struct __riscv_v_ext_state *vstate = &task->thread.vstate;
>> -
>> +       struct __riscv_v_ext_state *vstate = &task->thread.vstate;
>> +       unsigned long status = regs->status & SR_VS;
>> +
>> +       switch (status) {
>> +       case SR_VS_INITIAL:
>> +               __riscv_v_vstate_discard();
>> +               break;
>> +       case SR_VS_CLEAN:
>> +       case SR_VS_DIRTY:
>>                 __riscv_v_vstate_restore(vstate, vstate->datap);
>>                 __riscv_v_vstate_clean(regs);
>> +               break;
>>         }
>>  }
>>
>> @@ -178,6 +213,7 @@ static inline bool riscv_v_vstate_ctrl_user_allowed(void) { return false; }
>>  #define __switch_to_vector(__prev, __next)     do {} while (0)
>>  #define riscv_v_vstate_off(regs)               do {} while (0)
>>  #define riscv_v_vstate_on(regs)                        do {} while (0)
>> +#define riscv_v_vstate_discard(regs)           do {} while (0)
>>
>>  #endif /* CONFIG_RISCV_ISA_V */
>>
>> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
>> index 5158961ea977..5ff63a784a6d 100644
>> --- a/arch/riscv/kernel/traps.c
>> +++ b/arch/riscv/kernel/traps.c
>> @@ -296,6 +296,8 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>>                 regs->epc += 4;
>>                 regs->orig_a0 = regs->a0;
>>
>> +               riscv_v_vstate_discard(regs);
>> +
>>                 syscall = syscall_enter_from_user_mode(regs, syscall);
>>
>>                 if (syscall < NR_syscalls)
>>
>> base-commit: 488833ccdcac118da16701f4ee0673b20ba47fe3
>> --
>> 2.39.2
>>
>
> Hi, the above part looks good to me. In the context of kernel-mode
> vector, it would also be good to just discard V-context at the syscall
> entry. So the kernel can freely use Vector if needed. I will rebase my
> work on top of yours.

Ok!

> Another part that just came into my mind is the one for ptrace. Do we
> need to disallow, or immediately return all -1 if the tracee process
> is in the syscall path? It seems that we are likely to get stale
> values on datap if a tracee is being traced during a syscall.

Hmm, could you elaborate a bit on when the tracer would get stale regs?

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