Re: [PATCH v2 24/25] KVM: x86: Introduce KVM_TDX_GET_CPUID

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 10/31/2024 3:00 AM, Rick Edgecombe wrote:
> From: Xiaoyao Li <xiaoyao.li@intel.com>
> 
> Implement an IOCTL to allow userspace to read the CPUID bit values for a
> configured TD.
> 
> The TDX module doesn't provide the ability to set all CPUID bits. Instead
> some are configured indirectly, or have fixed values. But it does allow
> for the final resulting CPUID bits to be read. This information will be
> useful for userspace to understand the configuration of the TD, and set
> KVM's copy via KVM_SET_CPUID2.
> 
> To prevent userspace from starting to use features that might not have KVM
> support yet, filter the reported values by KVM's support CPUID bits.

This sentence is not implemented, we need drop it.

> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Co-developed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
> Signed-off-by: Tony Lindgren <tony.lindgren@linux.intel.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> ---
> uAPI breakout v2:
>   - Improve error path for tdx_vcpu_get_cpuid() (Xu)
>   - Drop unused cpuid in struct kvm_tdx (Xu)
>   - Rip out cpuid bit filtering
>   - Fixup SEAMCALL call sites due to function parameter changes to SEAMCALL
>     wrappers (Kai)
>   - Add mmu.h for kvm_gfn_direct_bits() (Binbin)
>   - Drop unused nr_premapped (Tao)
>   - Fix formatting for tdx_vcpu_get_cpuid_leaf() (Tony)
>   - Use helpers for phys_addr_bits (Paolo)
> 
> uAPI breakout v1:
>   - New patch
> ---
>   arch/x86/include/uapi/asm/kvm.h |   1 +
>   arch/x86/kvm/vmx/tdx.c          | 167 ++++++++++++++++++++++++++++++++
>   arch/x86/kvm/vmx/tdx_arch.h     |   5 +
>   arch/x86/kvm/vmx/tdx_errno.h    |   1 +
>   4 files changed, 174 insertions(+)
> 
> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> index 2cfec4b42b9d..36fa03376581 100644
> --- a/arch/x86/include/uapi/asm/kvm.h
> +++ b/arch/x86/include/uapi/asm/kvm.h
> @@ -931,6 +931,7 @@ enum kvm_tdx_cmd_id {
>   	KVM_TDX_CAPABILITIES = 0,
>   	KVM_TDX_INIT_VM,
>   	KVM_TDX_INIT_VCPU,
> +	KVM_TDX_GET_CPUID,
>   
>   	KVM_TDX_CMD_NR_MAX,
>   };
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 9008db6cf3b4..1feb3307fd70 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -2,6 +2,7 @@
>   #include <linux/cpu.h>
>   #include <asm/tdx.h>
>   #include "capabilities.h"
> +#include "mmu.h"
>   #include "x86_ops.h"
>   #include "tdx.h"
>   
> @@ -857,6 +858,94 @@ static int __tdx_td_init(struct kvm *kvm, struct td_params *td_params,
>   	return ret;
>   }
>   
> +static u64 tdx_td_metadata_field_read(struct kvm_tdx *tdx, u64 field_id,
> +				      u64 *data)
> +{
> +	u64 err;
> +
> +	err = tdh_mng_rd(tdx->tdr_pa, field_id, data);
> +
> +	return err;
> +}
> +
> +#define TDX_MD_UNREADABLE_LEAF_MASK	GENMASK(30, 7)
> +#define TDX_MD_UNREADABLE_SUBLEAF_MASK	GENMASK(31, 7)
> +
> +static int tdx_read_cpuid(struct kvm_vcpu *vcpu, u32 leaf, u32 sub_leaf,
> +			  bool sub_leaf_set, struct kvm_cpuid_entry2 *out)
> +{
> +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm);
> +	u64 field_id = TD_MD_FIELD_ID_CPUID_VALUES;
> +	u64 ebx_eax, edx_ecx;
> +	u64 err = 0;
> +
> +	if (sub_leaf & TDX_MD_UNREADABLE_LEAF_MASK ||
> +	    sub_leaf_set & TDX_MD_UNREADABLE_SUBLEAF_MASK)
> +		return -EINVAL;
> +
> +	/*
> +	 * bit 23:17, REVSERVED: reserved, must be 0;
> +	 * bit 16,    LEAF_31: leaf number bit 31;
> +	 * bit 15:9,  LEAF_6_0: leaf number bits 6:0, leaf bits 30:7 are
> +	 *                      implicitly 0;
> +	 * bit 8,     SUBLEAF_NA: sub-leaf not applicable flag;
> +	 * bit 7:1,   SUBLEAF_6_0: sub-leaf number bits 6:0. If SUBLEAF_NA is 1,
> +	 *                         the SUBLEAF_6_0 is all-1.
> +	 *                         sub-leaf bits 31:7 are implicitly 0;
> +	 * bit 0,     ELEMENT_I: Element index within field;
> +	 */
> +	field_id |= ((leaf & 0x80000000) ? 1 : 0) << 16;
> +	field_id |= (leaf & 0x7f) << 9;
> +	if (sub_leaf_set)
> +		field_id |= (sub_leaf & 0x7f) << 1;
> +	else
> +		field_id |= 0x1fe;
> +
> +	err = tdx_td_metadata_field_read(kvm_tdx, field_id, &ebx_eax);
> +	if (err) //TODO check for specific errors
> +		goto err_out;
> +
> +	out->eax = (u32) ebx_eax;
> +	out->ebx = (u32) (ebx_eax >> 32);
> +
> +	field_id++;
> +	err = tdx_td_metadata_field_read(kvm_tdx, field_id, &edx_ecx);
> +	/*
> +	 * It's weird that reading edx_ecx fails while reading ebx_eax
> +	 * succeeded.
> +	 */
> +	if (WARN_ON_ONCE(err))
> +		goto err_out;
> +
> +	out->ecx = (u32) edx_ecx;
> +	out->edx = (u32) (edx_ecx >> 32);
> +
> +	out->function = leaf;
> +	out->index = sub_leaf;
> +	out->flags |= sub_leaf_set ? KVM_CPUID_FLAG_SIGNIFCANT_INDEX : 0;
> +
> +	/*
> +	 * Work around missing support on old TDX modules, fetch
> +	 * guest maxpa from gfn_direct_bits.
> +	 */
> +	if (leaf == 0x80000008) {
> +		gpa_t gpa_bits = gfn_to_gpa(kvm_gfn_direct_bits(vcpu->kvm));
> +		unsigned int g_maxpa = __ffs(gpa_bits) + 1;
> +
> +		out->eax = tdx_set_guest_phys_addr_bits(out->eax, g_maxpa);
> +	}
> +
> +	return 0;
> +
> +err_out:
> +	out->eax = 0;
> +	out->ebx = 0;
> +	out->ecx = 0;
> +	out->edx = 0;
> +
> +	return -EIO;
> +}
> +
>   static int tdx_td_init(struct kvm *kvm, struct kvm_tdx_cmd *cmd)
>   {
>   	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> @@ -1055,6 +1144,81 @@ static int tdx_td_vcpu_init(struct kvm_vcpu *vcpu, u64 vcpu_rcx)
>   	return ret;
>   }
>   
> +/* Sometimes reads multipple subleafs. Return how many enties were written. */
> +static int tdx_vcpu_get_cpuid_leaf(struct kvm_vcpu *vcpu, u32 leaf, int max_cnt,
> +				   struct kvm_cpuid_entry2 *output_e)
> +{
> +	int i;
> +
> +	if (!max_cnt)
> +		return 0;
> +
> +	/* First try without a subleaf */
> +	if (!tdx_read_cpuid(vcpu, leaf, 0, false, output_e))
> +		return 1;
> +
> +	/*
> +	 * If the try without a subleaf failed, try reading subleafs until
> +	 * failure. The TDX module only supports 6 bits of subleaf index.
> +	 */
> +	for (i = 0; i < 0b111111; i++) {
> +		if (i > max_cnt)
> +			goto out;
> +
> +		/* Keep reading subleafs until there is a failure. */
> +		if (tdx_read_cpuid(vcpu, leaf, i, true, output_e))
> +			return i;
> +
> +		output_e++;
> +	}
> +
> +out:
> +	return i;
> +}
> +
> +static int tdx_vcpu_get_cpuid(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *cmd)
> +{
> +	struct kvm_cpuid2 __user *output, *td_cpuid;
> +	struct kvm_cpuid_entry2 *output_e;
> +	int r = 0, i = 0, leaf;
> +
> +	output = u64_to_user_ptr(cmd->data);
> +	td_cpuid = kzalloc(sizeof(*td_cpuid) +
> +			sizeof(output->entries[0]) * KVM_MAX_CPUID_ENTRIES,
> +			GFP_KERNEL);
> +	if (!td_cpuid)
> +		return -ENOMEM;
> +
> +	for (leaf = 0; leaf <= 0x1f; leaf++) {

0x1f needs clarification here.

If it's going to use the maximum leaf KVM can support, it should be 0x24 
to align with __do_cpuid_func().

alternatively, it can use the EAX value of leaf 0 returned by TDX 
module. That is the value TDX module presents to the TD guest.

> +		output_e = &td_cpuid->entries[i];
> +		i += tdx_vcpu_get_cpuid_leaf(vcpu, leaf,
> +					     KVM_MAX_CPUID_ENTRIES - i - 1,
> +					     output_e);
> +	}
> +
> +	for (leaf = 0x80000000; leaf <= 0x80000008; leaf++) {
> +		output_e = &td_cpuid->entries[i];
> +		i += tdx_vcpu_get_cpuid_leaf(vcpu, leaf,
> +					     KVM_MAX_CPUID_ENTRIES - i - 1,
> +					     output_e);

Though what gets passed in for max_cnt is

   KVM_MAX_CPUID_ENTRIES - i - 1

tdx_vcpu_get_cpuid_leaf() can return "max_cnt+1", i.e., 
KVM_MAX_CPUID_ENTRIES - i.

Then, it makes next round i to be KVM_MAX_CPUID_ENTRIES, and

   output_e = &td_cpuid->entries[i];

will overflow the buffer and access illegal memory.

Similar issue inside tdx_vcpu_get_cpuid_leaf() as I replied in [*]

[*] 
https://lore.kernel.org/all/7574968a-f0e2-49d5-b740-2454a0f70bb6@intel.com/

> +	}
> +
> +	td_cpuid->nent = i;
> +
> +	if (copy_to_user(output, td_cpuid, sizeof(*output))) {
> +		r = -EFAULT;
> +		goto out;
> +	}
> +	if (copy_to_user(output->entries, td_cpuid->entries,
> +			 td_cpuid->nent * sizeof(struct kvm_cpuid_entry2)))
> +		r = -EFAULT;
> +
> +out:
> +	kfree(td_cpuid);
> +
> +	return r;
> +}
> +
>   static int tdx_vcpu_init(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *cmd)
>   {
>   	struct msr_data apic_base_msr;
> @@ -1108,6 +1272,9 @@ int tdx_vcpu_ioctl(struct kvm_vcpu *vcpu, void __user *argp)
>   	case KVM_TDX_INIT_VCPU:
>   		ret = tdx_vcpu_init(vcpu, &cmd);
>   		break;
> +	case KVM_TDX_GET_CPUID:
> +		ret = tdx_vcpu_get_cpuid(vcpu, &cmd);
> +		break;
>   	default:
>   		ret = -EINVAL;
>   		break;
> diff --git a/arch/x86/kvm/vmx/tdx_arch.h b/arch/x86/kvm/vmx/tdx_arch.h
> index 9d41699e66a2..d80ec118834e 100644
> --- a/arch/x86/kvm/vmx/tdx_arch.h
> +++ b/arch/x86/kvm/vmx/tdx_arch.h
> @@ -157,4 +157,9 @@ struct td_params {
>   
>   #define MD_FIELD_ID_FEATURES0_TOPOLOGY_ENUM	BIT_ULL(20)
>   
> +/*
> + * TD scope metadata field ID.
> + */
> +#define TD_MD_FIELD_ID_CPUID_VALUES		0x9410000300000000ULL
> +
>   #endif /* __KVM_X86_TDX_ARCH_H */
> diff --git a/arch/x86/kvm/vmx/tdx_errno.h b/arch/x86/kvm/vmx/tdx_errno.h
> index dc3fa2a58c2c..f9dbb3a065cc 100644
> --- a/arch/x86/kvm/vmx/tdx_errno.h
> +++ b/arch/x86/kvm/vmx/tdx_errno.h
> @@ -23,6 +23,7 @@
>   #define TDX_FLUSHVP_NOT_DONE			0x8000082400000000ULL
>   #define TDX_EPT_WALK_FAILED			0xC0000B0000000000ULL
>   #define TDX_EPT_ENTRY_STATE_INCORRECT		0xC0000B0D00000000ULL
> +#define TDX_METADATA_FIELD_NOT_READABLE		0xC0000C0200000000ULL
>   
>   /*
>    * TDX module operand ID, appears in 31:0 part of error code as