On 09/08/17 13:05, Dave Martin wrote:
> Currently, a guest kernel sees the true CPU feature registers
> (ID_*_EL1) when it reads them using MRS instructions.  This means
> that the guest will observe features that are present in the
> hardware but the host doesn't understand or doesn't provide support
> for.  A guest may legimitately try to use such a feature as per the
> architecture, but use of the feature may trap instead of working
> normally, triggering undef injection into the guest.
> 
> This is not a problem for the host, but the guest may go wrong when
> running on newer hardware than the host knows about.
> 
> This patch hides from guest VMs any AArch64-specific CPU features
> that the host doesn't support, by exposing to the guest the
> sanitised versions of the registers computed by the cpufeatures
> framework, instead of the true hardware registers.  To achieve
> this, HCR_EL2.TID3 is now set for AArch64 guests, and emulation
> code is added to KVM to report the sanitised versions of the
> affected registers in response to MRS and register reads from
> userspace.
> 
> The affected registers are removed from invariant_sys_regs[] (since
> the invariant_sys_regs handling is no longer quite correct for
> them) and added to sys_reg_desgs[], with appropriate access(),
> get_user() and set_user() methods.  No runtime vcpu storage is
> allocated for the registers: instead, they are read on demand from
> the cpufeatures framework.  This may need modification in the
> future if there is a need for userspace to customise the features
> visible to the guest.
> 
> Attempts by userspace to write the registers are handled similarly
> to the current invariant_sys_regs handling: writes are permitted,
> but only if they don't attempt to change the value.  This is
> sufficient to support VM snapshot/restore from userspace.
> 
> Because of the additional registers, restoring a VM on an older
> kernel may not work unless userspace knows how to handle the extra
> VM registers exposed to the KVM user ABI by this patch.
> 
> Under the principle of least damage, this patch makes no attempt to
> handle any of the other registers currently in
> invariant_sys_regs[], or to emulate registers for AArch32: however,
> these could be handled in a similar way in future, as necessary.
> 
> Signed-off-by: Dave Martin <dave.mar...@arm.com>
> ---
>  arch/arm64/kvm/hyp/switch.c |   6 ++
>  arch/arm64/kvm/sys_regs.c   | 224 
> +++++++++++++++++++++++++++++++++++---------
>  2 files changed, 185 insertions(+), 45 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 945e79c..35a90b8 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -81,11 +81,17 @@ static void __hyp_text __activate_traps(struct kvm_vcpu 
> *vcpu)
>        * it will cause an exception.
>        */
>       val = vcpu->arch.hcr_el2;
> +
>       if (!(val & HCR_RW) && system_supports_fpsimd()) {
>               write_sysreg(1 << 30, fpexc32_el2);
>               isb();
>       }
> +
> +     if (val & HCR_RW) /* for AArch64 only: */
> +             val |= HCR_TID3; /* TID3: trap feature register accesses */
> +
>       write_sysreg(val, hcr_el2);
> +
>       /* Trap on AArch32 cp15 c15 accesses (EL1 or EL0) */
>       write_sysreg(1 << 15, hstr_el2);
>       /*
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 2e070d3..6583dd7 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -892,6 +892,135 @@ static bool access_cntp_cval(struct kvm_vcpu *vcpu,
>       return true;
>  }
>  
> +/* Read a sanitised cpufeature ID register by sys_reg_desc */
> +static u64 read_id_reg(struct sys_reg_desc const *r, bool raz)
> +{
> +     u32 id = sys_reg((u32)r->Op0, (u32)r->Op1,
> +                      (u32)r->CRn, (u32)r->CRm, (u32)r->Op2);
> +
> +     return raz ? 0 : read_sanitised_ftr_reg(id);
> +}
> +
> +/* cpufeature ID register access trap handlers */
> +
> +static bool __access_id_reg(struct kvm_vcpu *vcpu,
> +                         struct sys_reg_params *p,
> +                         const struct sys_reg_desc const *r,
> +                         bool raz)
> +{
> +     if (p->is_write) {
> +             kvm_inject_undefined(vcpu);
> +             return false;
> +     }

I don't think this is supposed to happen (should have UNDEF-ed at EL1).
You can call write_to_read_only() in that case, which will spit out a
warning and inject the exception.

> +
> +     p->regval = read_id_reg(r, raz);
> +     return true;
> +}
> +
> +static bool access_id_reg(struct kvm_vcpu *vcpu,
> +                       struct sys_reg_params *p,
> +                       const struct sys_reg_desc *r)
> +{
> +     return __access_id_reg(vcpu, p, r, false);
> +}
> +
> +static bool access_raz_id_reg(struct kvm_vcpu *vcpu,
> +                           struct sys_reg_params *p,
> +                           const struct sys_reg_desc *r)
> +{
> +     return __access_id_reg(vcpu, p, r, true);
> +}
> +
> +static int reg_from_user(u64 *val, const void __user *uaddr, u64 id);
> +static int reg_to_user(void __user *uaddr, const u64 *val, u64 id);
> +static u64 sys_reg_to_index(const struct sys_reg_desc *reg);
> +
> +/*
> + * cpufeature ID register user accessors
> + *
> + * For now, these registers are immutable for userspace, so no values
> + * are stored, and for set_id_reg() we don't allow the effective value
> + * to be changed.
> + */
> +static int __get_id_reg(const struct sys_reg_desc *rd, void __user *uaddr,
> +                     bool raz)
> +{
> +     const u64 id = sys_reg_to_index(rd);
> +     const u64 val = read_id_reg(rd, raz);
> +
> +     BUG_ON(KVM_REG_SIZE(id) != sizeof(val));
> +     return reg_to_user(uaddr, &val, id);
> +}
> +
> +static int __set_id_reg(const struct sys_reg_desc *rd, void __user *uaddr,
> +                     bool raz)
> +{
> +     const u64 id = sys_reg_to_index(rd);
> +     int err;
> +     u64 val;
> +
> +     BUG_ON(KVM_REG_SIZE(id) != sizeof(val));
> +     err = reg_from_user(&val, uaddr, id);
> +     if (err)
> +             return err;
> +
> +     /* This is what we mean by invariant: you can't change it. */
> +     if (val != read_id_reg(rd, raz))
> +             return -EINVAL;
> +
> +     return 0;
> +}
> +
> +static int get_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> +                   const struct kvm_one_reg *reg, void __user *uaddr)
> +{
> +     return __get_id_reg(rd, uaddr, false);
> +}
> +
> +static int set_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> +                   const struct kvm_one_reg *reg, void __user *uaddr)
> +{
> +     return __set_id_reg(rd, uaddr, false);
> +}
> +
> +static int get_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc 
> *rd,
> +                       const struct kvm_one_reg *reg, void __user *uaddr)
> +{
> +     return __get_id_reg(rd, uaddr, true);
> +}
> +
> +static int set_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc 
> *rd,
> +                       const struct kvm_one_reg *reg, void __user *uaddr)
> +{
> +     return __set_id_reg(rd, uaddr, true);
> +}
> +
> +/* sys_reg_desc initialiser for cpufeature ID register name_EL1 */
> +#define _ID(name) {                  \
> +     SYS_DESC(SYS_##name##_EL1),     \
> +     .access = access_id_reg,        \
> +     .get_user = get_id_reg,         \
> +     .set_user = set_id_reg,         \
> +}
> +
> +/*
> + * sys_reg_desc initialiser for cpufeature ID register ID_name_EL1
> + * (So we can get 4 regs to 1 line.)
> + */
> +#define ID(name) _ID(ID_##name)
> +
> +/*
> + * sys_reg_desc initialiser for unknown (RAZ) cpufeature ID register
> + * Op0=3, Op1=0, CRn=0, CRm=crm, Op2=op2
> + * (1 <= crm < 8, 0 <= Op2 < 8).
> + */
> +#define _ID_RAZ(crm, op2) {                          \
> +     Op0(3), Op1(0), CRn(0), CRm(crm), Op2(op2),     \
> +     .access = access_raz_id_reg,                    \
> +     .get_user = get_raz_id_reg,                     \
> +     .set_user = set_raz_id_reg,                     \
> +}
> +
>  /*
>   * Architected system registers.
>   * Important: Must be sorted ascending by Op0, Op1, CRn, CRm, Op2
> @@ -944,6 +1073,32 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>       { SYS_DESC(SYS_DBGVCR32_EL2), NULL, reset_val, DBGVCR32_EL2, 0 },
>  
>       { SYS_DESC(SYS_MPIDR_EL1), NULL, reset_mpidr, MPIDR_EL1 },
> +
> +     /*
> +      * All non-RAZ feature registers listed here must also be
> +      * present in arm64_ftr_regs[].
> +      */
> +
> +     /* AArch64 mappings of the AArch32 ID registers */
> +     /* ID_AFR0_EL1 not exposed to guests for now */
> +     ID(PFR0),       ID(PFR1),       ID(DFR0),       _ID_RAZ(1,3),
> +     ID(MMFR0),      ID(MMFR1),      ID(MMFR2),      ID(MMFR3),
> +     ID(ISAR0),      ID(ISAR1),      ID(ISAR2),      ID(ISAR3),
> +     ID(ISAR4),      ID(ISAR5),      ID(MMFR4),      _ID_RAZ(2,7),
> +     _ID(MVFR0),     _ID(MVFR1),     _ID(MVFR2),     _ID_RAZ(3,3),
> +     _ID_RAZ(3,4),   _ID_RAZ(3,5),   _ID_RAZ(3,6),   _ID_RAZ(3,7),

#bikeshed:

OK, this is giving me a headache. Too many variants with similar names.
ID and _ID
I'm also slightly perplexed with the amalgamation of RAZ because the
register is not defined yet in the architecture, and RAZ because we
don't expose it (like ID_AFR0_EL1). Yes, there is a number of comments
to document that, but the code should aim to be be self-documenting. How
about IDRAZ() for those we want to "hide", and IDRSV for encodings that
are not allocated yet? It would look like this:

        IDREG(ID_PFR0),         IDREG(ID_PFR1),         IDREG(ID_DFR0),
        IDRAZ(ID_AFR0),         IDREG(ID_MMFR0),        IDREG(ID_MMFR1),
        IDREG(ID_MMFR2),        IDREG(ID_MMFR3),        IDREG(ID_ISAR0),
        IDREG(ID_ISAR1),        IDREG(ID_ISAR2),        IDREG(ID_ISAR3),
        IDREG(ID_ISAR4),        IDREG(ID_ISAR5),        IDREG(ID_MMFR4),
        IDRSV(2,7),             IDREG(MVFR0),           IDREG(MVFR1),
        IDREG(MVFR2),           IDRSV(3,3),             IDRSV(3,4),     
        IDRSV(3,5),             IDRSV(3,6),             IDRSV(3,7),

Yes, only 3 a line. Lines are cheap. And yes, they also have similar
names, but I said #bikeshed.

> +
> +     /* AArch64 ID registers */
> +     ID(AA64PFR0),   ID(AA64PFR1),   _ID_RAZ(4,2),   _ID_RAZ(4,3),
> +     _ID_RAZ(4,4),   _ID_RAZ(4,5),   _ID_RAZ(4,6),   _ID_RAZ(4,7),
> +     ID(AA64DFR0),   ID(AA64DFR1),   _ID_RAZ(5,2),   _ID_RAZ(5,3),
> +     /* ID_AA64AFR0_EL1 and ID_AA64AFR0_EL1 not exposed to guests for now */
> +     _ID_RAZ(5,4),   _ID_RAZ(5,5),   _ID_RAZ(5,6),   _ID_RAZ(5,7),
> +     ID(AA64ISAR0),  ID(AA64ISAR1),  _ID_RAZ(6,2),   _ID_RAZ(6,3),
> +     _ID_RAZ(6,4),   _ID_RAZ(6,5),   _ID_RAZ(6,6),   _ID_RAZ(6,7),
> +     ID(AA64MMFR0),  ID(AA64MMFR1),  ID(AA64MMFR2),  _ID_RAZ(7,3),
> +     _ID_RAZ(7,4),   _ID_RAZ(7,5),   _ID_RAZ(7,6),   _ID_RAZ(7,7),
> +
>       { SYS_DESC(SYS_SCTLR_EL1), access_vm_reg, reset_val, SCTLR_EL1, 
> 0x00C50078 },
>       { SYS_DESC(SYS_CPACR_EL1), NULL, reset_val, CPACR_EL1, 0 },
>       { SYS_DESC(SYS_TTBR0_EL1), access_vm_reg, reset_unknown, TTBR0_EL1 },
> @@ -1790,8 +1945,8 @@ static const struct sys_reg_desc 
> *index_to_sys_reg_desc(struct kvm_vcpu *vcpu,
>       if (!r)
>               r = find_reg(&params, sys_reg_descs, ARRAY_SIZE(sys_reg_descs));
>  
> -     /* Not saved in the sys_reg array? */
> -     if (r && !r->reg)
> +     /* Not saved in the sys_reg array and not otherwise accessible? */
> +     if (r && !(r->reg || r->get_user))
>               r = NULL;
>  
>       return r;
> @@ -1815,20 +1970,6 @@ static const struct sys_reg_desc 
> *index_to_sys_reg_desc(struct kvm_vcpu *vcpu,
>  FUNCTION_INVARIANT(midr_el1)
>  FUNCTION_INVARIANT(ctr_el0)
>  FUNCTION_INVARIANT(revidr_el1)
> -FUNCTION_INVARIANT(id_pfr0_el1)
> -FUNCTION_INVARIANT(id_pfr1_el1)
> -FUNCTION_INVARIANT(id_dfr0_el1)
> -FUNCTION_INVARIANT(id_afr0_el1)
> -FUNCTION_INVARIANT(id_mmfr0_el1)
> -FUNCTION_INVARIANT(id_mmfr1_el1)
> -FUNCTION_INVARIANT(id_mmfr2_el1)
> -FUNCTION_INVARIANT(id_mmfr3_el1)
> -FUNCTION_INVARIANT(id_isar0_el1)
> -FUNCTION_INVARIANT(id_isar1_el1)
> -FUNCTION_INVARIANT(id_isar2_el1)
> -FUNCTION_INVARIANT(id_isar3_el1)
> -FUNCTION_INVARIANT(id_isar4_el1)
> -FUNCTION_INVARIANT(id_isar5_el1)
>  FUNCTION_INVARIANT(clidr_el1)
>  FUNCTION_INVARIANT(aidr_el1)
>  
> @@ -1836,20 +1977,6 @@ FUNCTION_INVARIANT(aidr_el1)
>  static struct sys_reg_desc invariant_sys_regs[] = {
>       { SYS_DESC(SYS_MIDR_EL1), NULL, get_midr_el1 },
>       { SYS_DESC(SYS_REVIDR_EL1), NULL, get_revidr_el1 },
> -     { SYS_DESC(SYS_ID_PFR0_EL1), NULL, get_id_pfr0_el1 },
> -     { SYS_DESC(SYS_ID_PFR1_EL1), NULL, get_id_pfr1_el1 },
> -     { SYS_DESC(SYS_ID_DFR0_EL1), NULL, get_id_dfr0_el1 },
> -     { SYS_DESC(SYS_ID_AFR0_EL1), NULL, get_id_afr0_el1 },
> -     { SYS_DESC(SYS_ID_MMFR0_EL1), NULL, get_id_mmfr0_el1 },
> -     { SYS_DESC(SYS_ID_MMFR1_EL1), NULL, get_id_mmfr1_el1 },
> -     { SYS_DESC(SYS_ID_MMFR2_EL1), NULL, get_id_mmfr2_el1 },
> -     { SYS_DESC(SYS_ID_MMFR3_EL1), NULL, get_id_mmfr3_el1 },
> -     { SYS_DESC(SYS_ID_ISAR0_EL1), NULL, get_id_isar0_el1 },
> -     { SYS_DESC(SYS_ID_ISAR1_EL1), NULL, get_id_isar1_el1 },
> -     { SYS_DESC(SYS_ID_ISAR2_EL1), NULL, get_id_isar2_el1 },
> -     { SYS_DESC(SYS_ID_ISAR3_EL1), NULL, get_id_isar3_el1 },
> -     { SYS_DESC(SYS_ID_ISAR4_EL1), NULL, get_id_isar4_el1 },
> -     { SYS_DESC(SYS_ID_ISAR5_EL1), NULL, get_id_isar5_el1 },
>       { SYS_DESC(SYS_CLIDR_EL1), NULL, get_clidr_el1 },
>       { SYS_DESC(SYS_AIDR_EL1), NULL, get_aidr_el1 },
>       { SYS_DESC(SYS_CTR_EL0), NULL, get_ctr_el0 },
> @@ -2079,12 +2206,31 @@ static bool copy_reg_to_user(const struct 
> sys_reg_desc *reg, u64 __user **uind)
>       return true;
>  }
>  
> +static int walk_one_sys_reg(const struct sys_reg_desc *rd,
> +                         u64 __user **uind,
> +                         unsigned int *total)
> +{
> +     /*
> +      * Ignore registers we trap but don't save,
> +      * and for which no custom user accessor is provided.
> +      */
> +     if (!(rd->reg || rd->get_user))
> +             return 0;
> +
> +     if (!copy_reg_to_user(rd, uind))
> +             return -EFAULT;
> +
> +     (*total)++;
> +     return 0;
> +}
> +
>  /* Assumed ordered tables, see kvm_sys_reg_table_init. */
>  static int walk_sys_regs(struct kvm_vcpu *vcpu, u64 __user *uind)
>  {
>       const struct sys_reg_desc *i1, *i2, *end1, *end2;
>       unsigned int total = 0;
>       size_t num;
> +     int err;
>  
>       /* We check for duplicates here, to allow arch-specific overrides. */
>       i1 = get_target_table(vcpu->arch.target, true, &num);
> @@ -2098,21 +2244,9 @@ static int walk_sys_regs(struct kvm_vcpu *vcpu, u64 
> __user *uind)
>       while (i1 || i2) {
>               int cmp = cmp_sys_reg(i1, i2);
>               /* target-specific overrides generic entry. */
> -             if (cmp <= 0) {
> -                     /* Ignore registers we trap but don't save. */
> -                     if (i1->reg) {
> -                             if (!copy_reg_to_user(i1, &uind))
> -                                     return -EFAULT;
> -                             total++;
> -                     }
> -             } else {
> -                     /* Ignore registers we trap but don't save. */
> -                     if (i2->reg) {
> -                             if (!copy_reg_to_user(i2, &uind))
> -                                     return -EFAULT;
> -                             total++;
> -                     }
> -             }
> +             err = walk_one_sys_reg(cmp <= 0 ? i1 : i2, &uind, &total);

Please move this ternary operator out of the function parameters, as
that code is hairy enough. Or use the new function twice within the "if"
statement.

> +             if (err)
> +                     return err;
>  
>               if (cmp <= 0 && ++i1 == end1)
>                       i1 = NULL;
> 

Thanks,

        M.
-- 
Jazz is not dead. It just smells funny...
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