Messages in this thread | | | From | Andy Chiu <> | Date | Tue, 27 Jun 2023 23:15:01 +0800 | Subject | Re: [PATCH v2] riscv: Discard vector state on syscalls |
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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.
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.
Thanks, Andy
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