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

From: Andy Chiu
Date: Wed Jun 28 2023 - 13:03:39 EST

On Wed, Jun 28, 2023 at 6:35 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
>
> Andy Chiu <andy.chiu@xxxxxxxxxx> writes:
>
> > On Tue, Jun 27, 2023 at 12:57 AM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
> >>
> >> From: Björn Töpel <bjorn@xxxxxxxxxxxx>
> >>
> >> 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@xxxxxxxxxxxxxxxxx/ # [1]
> >> Suggested-by: Darius Rad <darius@xxxxxxxxxxxx>
> >> Suggested-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> >> Suggested-by: Rémi Denis-Courmont <remi@xxxxxxxxxx>
> >> Signed-off-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
> >> ---
> >> 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?

Yep, consider that our tracer process attaches to a tracee with
PTRACE_SYSCALL. Then, the tracee will let the tracer to inspect it
whenever it makes a syscall. The tracer wants to inspect V-registers
at these PTRACE_SYSCALL stops. Assume the tracee context switches out
before being inspected (Sadly I didn't find this part in the code, so
maybe I was wrong). Now, we set all V-regs to -1 and VS to 'On'
entering a syscall. However, -1 will not be saved into datap, which
the tracer copies from, because riscv_v_vstate_save() only saves
whenever VS is 'Dirty'. We intentionally want this because it saves
unnecessary context saves. As a result, what we will get with REGSET_V
will not reflect the latest one, and what we set will get lost since
VS='ON' restores V to -1.

Since we are planning to discard V registers on syscall, does it make
sense to also make ptrace aware of this? Or, just leave it as-it
because reading/writing V register at syscall is not meaningful
already.

Thanks,
Andy