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

Alexander Graf
2019-08-22T13:25:59Z

On 22.08.19 14:33, Anup Patel wrote:
> 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.

I don't think the cover letter is the right place for such a comment.
Please definitely put it into the code as well, pointing out that this
is a known bug. Or even better yet: Fix it up properly :).

In fact, with a bug that dramatic, I'm not even sure we can safely
include the code. We're consciously allowing user space to DOS the kernel.

>
> 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)

Yes, we have similar extensions on other archs. Is this going to be an
optional addition or a mandatory bit of the hypervisor spec? If it's not
mandatory, we can not rely on it, so the current path has to be safe.

Alex