[PATCH] riscv: Discard vector state on syscalls

[Björn Töpel]

From: Björn Töpel <bjorn at 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 cleared, 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 clearing 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: Palmer Dabbelt <palmer at rivosinc.com>
Suggested-by: Rémi Denis-Courmont <remi at remlab.net>
Signed-off-by: Björn Töpel <bjorn at rivosinc.com>
---

I figured I'd sent out a proper patch. I like Andy's optimization
patch, but TBH I think we should do that as a follow up.

As Rémi pointed out, the clearing might be opted out, but I left it in
here.


Björn

---
 arch/riscv/include/asm/vector.h | 25 +++++++++++++++++++++++++
 arch/riscv/kernel/traps.c       |  2 ++
 2 files changed, 27 insertions(+)

diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
index 04c0b07bf6cd..692ce55e4a69 100644
--- a/arch/riscv/include/asm/vector.h
+++ b/arch/riscv/include/asm/vector.h
@@ -163,6 +163,30 @@ static inline void __switch_to_vector(struct task_struct *prev,
 void riscv_v_vstate_ctrl_init(struct task_struct *tsk);
 bool riscv_v_vstate_ctrl_user_allowed(void);
 
+static inline void riscv_v_vstate_discard(struct pt_regs *regs)
+{
+	unsigned long vl, vtype_inval = 1UL << (BITS_PER_LONG - 1);
+
+	if (!riscv_v_vstate_query(regs))
+		return;
+
+	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, 0\n\t"
+		"vmv.v.i	v8, 0\n\t"
+		"vmv.v.i	v16, 0\n\t"
+		"vmv.v.i	v24, 0\n\t"
+		"vsetvl		%0, x0, %1\n\t"
+		".option pop\n\t"
+		: "=&r" (vl) : "r" (vtype_inval) : "memory");
+	riscv_v_disable();
+
+	riscv_v_vstate_on(regs);
+}
+
 #else /* ! CONFIG_RISCV_ISA_V  */
 
 struct pt_regs;
@@ -178,6 +202,7 @@ 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 05ffdcd1424e..00c68b57ff88 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -295,6 +295,8 @@ 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: 4681dacadeefa5ca6017e00736adc1d7dc963c6a
-- 
2.39.2