Re: [PATCH v5 10/20] RISC-V: KVM: Handle MMIO exits for VCPU

Anup Patel
2019-08-22T12:33:43Z

On Thu, Aug 22, 2019 at 5:44 PM Alexander Graf <graf@amazon.com> wrote:
>
> On 22.08.19 10:44, Anup Patel wrote:
> > We will get stage2 page faults whenever Guest/VM access SW emulated
> > MMIO device or unmapped Guest RAM.
> >
> > This patch implements MMIO read/write emulation by extracting MMIO
> > details from the trapped load/store instruction and forwarding the
> > MMIO read/write to user-space. The actual MMIO emulation will happen
> > in user-space and KVM kernel module will only take care of register
> > updates before resuming the trapped VCPU.
> >
> > The handling for stage2 page faults for unmapped Guest RAM will be
> > implemeted by a separate patch later.
> >
> > Signed-off-by: Anup Patel <anup.patel@wdc.com>
> > Acked-by: Paolo Bonzini <pbonzini@redhat.com>
> > Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> > arch/riscv/include/asm/kvm_host.h | 11 +
> > arch/riscv/kvm/mmu.c | 7 +
> > arch/riscv/kvm/vcpu_exit.c | 436 +++++++++++++++++++++++++++++-
> > 3 files changed, 451 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/kvm_host.h b/arch/riscv/include/asm/kvm_host.h
> > index 18f1097f1d8d..4388bace6d70 100644
> > --- a/arch/riscv/include/asm/kvm_host.h
> > +++ b/arch/riscv/include/asm/kvm_host.h
> > @@ -53,6 +53,12 @@ struct kvm_arch {
> > phys_addr_t pgd_phys;
> > };
> >
> > +struct kvm_mmio_decode {
> > + unsigned long insn;
> > + int len;
> > + int shift;
> > +};
> > +
> > struct kvm_cpu_context {
> > unsigned long zero;
> > unsigned long ra;
> > @@ -141,6 +147,9 @@ struct kvm_vcpu_arch {
> > unsigned long irqs_pending;
> > unsigned long irqs_pending_mask;
> >
> > + /* MMIO instruction details */
> > + struct kvm_mmio_decode mmio_decode;
> > +
> > /* VCPU power-off state */
> > bool power_off;
> >
> > @@ -160,6 +169,8 @@ static inline void kvm_arch_vcpu_block_finish(struct kvm_vcpu *vcpu) {}
> > int kvm_riscv_setup_vsip(void);
> > void kvm_riscv_cleanup_vsip(void);
> >
> > +int kvm_riscv_stage2_map(struct kvm_vcpu *vcpu, gpa_t gpa, unsigned long hva,
> > + bool is_write);
> > void kvm_riscv_stage2_flush_cache(struct kvm_vcpu *vcpu);
> > int kvm_riscv_stage2_alloc_pgd(struct kvm *kvm);
> > void kvm_riscv_stage2_free_pgd(struct kvm *kvm);
> > diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
> > index 04dd089b86ff..2b965f9aac07 100644
> > --- a/arch/riscv/kvm/mmu.c
> > +++ b/arch/riscv/kvm/mmu.c
> > @@ -61,6 +61,13 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
> > return 0;
> > }
> >
> > +int kvm_riscv_stage2_map(struct kvm_vcpu *vcpu, gpa_t gpa, unsigned long hva,
> > + bool is_write)
> > +{
> > + /* TODO: */
> > + return 0;
> > +}
> > +
> > void kvm_riscv_stage2_flush_cache(struct kvm_vcpu *vcpu)
> > {
> > /* TODO: */
> > diff --git a/arch/riscv/kvm/vcpu_exit.c b/arch/riscv/kvm/vcpu_exit.c
> > index e4d7c8f0807a..efc06198c259 100644
> > --- a/arch/riscv/kvm/vcpu_exit.c
> > +++ b/arch/riscv/kvm/vcpu_exit.c
> > @@ -6,9 +6,371 @@
> > * Anup Patel <anup.patel@wdc.com>
> > */
> >
> > +#include <linux/bitops.h>
> > #include <linux/errno.h>
> > #include <linux/err.h>
> > #include <linux/kvm_host.h>
> > +#include <asm/csr.h>
> > +
> > +#define INSN_MATCH_LB 0x3
> > +#define INSN_MASK_LB 0x707f
> > +#define INSN_MATCH_LH 0x1003
> > +#define INSN_MASK_LH 0x707f
> > +#define INSN_MATCH_LW 0x2003
> > +#define INSN_MASK_LW 0x707f
> > +#define INSN_MATCH_LD 0x3003
> > +#define INSN_MASK_LD 0x707f
> > +#define INSN_MATCH_LBU 0x4003
> > +#define INSN_MASK_LBU 0x707f
> > +#define INSN_MATCH_LHU 0x5003
> > +#define INSN_MASK_LHU 0x707f
> > +#define INSN_MATCH_LWU 0x6003
> > +#define INSN_MASK_LWU 0x707f
> > +#define INSN_MATCH_SB 0x23
> > +#define INSN_MASK_SB 0x707f
> > +#define INSN_MATCH_SH 0x1023
> > +#define INSN_MASK_SH 0x707f
> > +#define INSN_MATCH_SW 0x2023
> > +#define INSN_MASK_SW 0x707f
> > +#define INSN_MATCH_SD 0x3023
> > +#define INSN_MASK_SD 0x707f
> > +
> > +#define INSN_MATCH_C_LD 0x6000
> > +#define INSN_MASK_C_LD 0xe003
> > +#define INSN_MATCH_C_SD 0xe000
> > +#define INSN_MASK_C_SD 0xe003
> > +#define INSN_MATCH_C_LW 0x4000
> > +#define INSN_MASK_C_LW 0xe003
> > +#define INSN_MATCH_C_SW 0xc000
> > +#define INSN_MASK_C_SW 0xe003
> > +#define INSN_MATCH_C_LDSP 0x6002
> > +#define INSN_MASK_C_LDSP 0xe003
> > +#define INSN_MATCH_C_SDSP 0xe002
> > +#define INSN_MASK_C_SDSP 0xe003
> > +#define INSN_MATCH_C_LWSP 0x4002
> > +#define INSN_MASK_C_LWSP 0xe003
> > +#define INSN_MATCH_C_SWSP 0xc002
> > +#define INSN_MASK_C_SWSP 0xe003
> > +
> > +#define INSN_LEN(insn) ((((insn) & 0x3) < 0x3) ? 2 : 4)
> > +
> > +#ifdef CONFIG_64BIT
> > +#define LOG_REGBYTES 3
> > +#else
> > +#define LOG_REGBYTES 2
> > +#endif
> > +#define REGBYTES (1 << LOG_REGBYTES)
> > +
> > +#define SH_RD 7
> > +#define SH_RS1 15
> > +#define SH_RS2 20
> > +#define SH_RS2C 2
> > +
> > +#define RV_X(x, s, n) (((x) >> (s)) & ((1 << (n)) - 1))
> > +#define RVC_LW_IMM(x) ((RV_X(x, 6, 1) << 2) | \
> > + (RV_X(x, 10, 3) << 3) | \
> > + (RV_X(x, 5, 1) << 6))
> > +#define RVC_LD_IMM(x) ((RV_X(x, 10, 3) << 3) | \
> > + (RV_X(x, 5, 2) << 6))
> > +#define RVC_LWSP_IMM(x) ((RV_X(x, 4, 3) << 2) | \
> > + (RV_X(x, 12, 1) << 5) | \
> > + (RV_X(x, 2, 2) << 6))
> > +#define RVC_LDSP_IMM(x) ((RV_X(x, 5, 2) << 3) | \
> > + (RV_X(x, 12, 1) << 5) | \
> > + (RV_X(x, 2, 3) << 6))
> > +#define RVC_SWSP_IMM(x) ((RV_X(x, 9, 4) << 2) | \
> > + (RV_X(x, 7, 2) << 6))
> > +#define RVC_SDSP_IMM(x) ((RV_X(x, 10, 3) << 3) | \
> > + (RV_X(x, 7, 3) << 6))
> > +#define RVC_RS1S(insn) (8 + RV_X(insn, SH_RD, 3))
> > +#define RVC_RS2S(insn) (8 + RV_X(insn, SH_RS2C, 3))
> > +#define RVC_RS2(insn) RV_X(insn, SH_RS2C, 5)
> > +
> > +#define SHIFT_RIGHT(x, y) \
> > + ((y) < 0 ? ((x) << -(y)) : ((x) >> (y)))
> > +
> > +#define REG_MASK \
> > + ((1 << (5 + LOG_REGBYTES)) - (1 << LOG_REGBYTES))
> > +
> > +#define REG_OFFSET(insn, pos) \
> > + (SHIFT_RIGHT((insn), (pos) - LOG_REGBYTES) & REG_MASK)
> > +
> > +#define REG_PTR(insn, pos, regs) \
> > + (ulong *)((ulong)(regs) + REG_OFFSET(insn, pos))
> > +
> > +#define GET_RM(insn) (((insn) >> 12) & 7)
> > +
> > +#define GET_RS1(insn, regs) (*REG_PTR(insn, SH_RS1, regs))
> > +#define GET_RS2(insn, regs) (*REG_PTR(insn, SH_RS2, regs))
> > +#define GET_RS1S(insn, regs) (*REG_PTR(RVC_RS1S(insn), 0, regs))
> > +#define GET_RS2S(insn, regs) (*REG_PTR(RVC_RS2S(insn), 0, regs))
> > +#define GET_RS2C(insn, regs) (*REG_PTR(insn, SH_RS2C, regs))
> > +#define GET_SP(regs) (*REG_PTR(2, 0, regs))
> > +#define SET_RD(insn, regs, val) (*REG_PTR(insn, SH_RD, regs) = (val))
> > +#define IMM_I(insn) ((s32)(insn) >> 20)
> > +#define IMM_S(insn) (((s32)(insn) >> 25 << 5) | \
> > + (s32)(((insn) >> 7) & 0x1f))
> > +#define MASK_FUNCT3 0x7000
> > +
> > +#define STR(x) XSTR(x)
> > +#define XSTR(x) #x
> > +
> > +/* TODO: Handle traps due to unpriv load and redirect it back to VS-mode */
> > +static ulong get_insn(struct kvm_vcpu *vcpu)
> > +{
> > + ulong __sepc = vcpu->arch.guest_context.sepc;
> > + ulong __hstatus, __sstatus, __vsstatus;
> > +#ifdef CONFIG_RISCV_ISA_C
> > + ulong rvc_mask = 3, tmp;
> > +#endif
> > + ulong flags, val;
> > +
> > + local_irq_save(flags);
> > +
> > + __vsstatus = csr_read(CSR_VSSTATUS);
> > + __sstatus = csr_read(CSR_SSTATUS);
> > + __hstatus = csr_read(CSR_HSTATUS);
> > +
> > + csr_write(CSR_VSSTATUS, __vsstatus | SR_MXR);
> > + csr_write(CSR_SSTATUS, vcpu->arch.guest_context.sstatus | SR_MXR);
> > + csr_write(CSR_HSTATUS, vcpu->arch.guest_context.hstatus | HSTATUS_SPRV);
>
> What happens when the insn load triggers a page fault, maybe because the
> guest was malicious and did
>
> 1) Run on page 0x1000
> 2) Remove map for 0x1000, do *not* flush TLB
> 3) Trigger MMIO
>
> That would DOS the host here, as the host kernel would continue running
> in guest address space, right?

Yes, we can certainly fault while accessing Guest instruction. We will
be fixing this issue in a followup series. We have mentioned this in cover
letter as well.

BTW, RISC-V spec is going to further improve to provide easy
access of faulting instruction to Hypervisor.
(Refer, https://github.com/riscv/riscv-isa-manual/issues/431)

Regards,
Anup

>
>
> Alex
>
> > +
> > +#ifndef CONFIG_RISCV_ISA_C
> > + asm ("\n"
> > +#ifdef CONFIG_64BIT
> > + STR(LWU) " %[insn], (%[addr])\n"
> > +#else
> > + STR(LW) " %[insn], (%[addr])\n"
> > +#endif
> > + : [insn] "=&r" (val) : [addr] "r" (__sepc));
> > +#else
> > + asm ("and %[tmp], %[addr], 2\n"
> > + "bnez %[tmp], 1f\n"
> > +#ifdef CONFIG_64BIT
> > + STR(LWU) " %[insn], (%[addr])\n"
> > +#else
> > + STR(LW) " %[insn], (%[addr])\n"
> > +#endif
> > + "and %[tmp], %[insn], %[rvc_mask]\n"
> > + "beq %[tmp], %[rvc_mask], 2f\n"
> > + "sll %[insn], %[insn], %[xlen_minus_16]\n"
> > + "srl %[insn], %[insn], %[xlen_minus_16]\n"
> > + "j 2f\n"
> > + "1:\n"
> > + "lhu %[insn], (%[addr])\n"
> > + "and %[tmp], %[insn], %[rvc_mask]\n"
> > + "bne %[tmp], %[rvc_mask], 2f\n"
> > + "lhu %[tmp], 2(%[addr])\n"
> > + "sll %[tmp], %[tmp], 16\n"
> > + "add %[insn], %[insn], %[tmp]\n"
> > + "2:"
> > + : [vsstatus] "+&r" (__vsstatus), [insn] "=&r" (val),
> > + [tmp] "=&r" (tmp)
> > + : [addr] "r" (__sepc), [rvc_mask] "r" (rvc_mask),
> > + [xlen_minus_16] "i" (__riscv_xlen - 16));
> > +#endif
> > +
> > + csr_write(CSR_HSTATUS, __hstatus);
> > + csr_write(CSR_SSTATUS, __sstatus);
> > + csr_write(CSR_VSSTATUS, __vsstatus);
> > +
> > + local_irq_restore(flags);
> > +
> > + return val;
> > +}
>