Re: [PATCH v5 12/29] s390x/diag: Implement DIAG 508 subcode 1 for signature verification

[Farhan Ali <alifm@linux.ibm.com>]

On 8/18/2025 2:43 PM, Zhuoying Cai wrote:
> From: Collin Walling <walling@linux.ibm.com>
>
> DIAG 508 subcode 1 performs signature-verification on signed components.
> A signed component may be a Linux kernel image, or any other signed
> binary. **Verification of initrd is not supported.**
>
> The instruction call expects two item-pairs: an address of a device
> component, an address of the analogous signature file (in PKCS#7 DER format),
> and their respective lengths. All of this data should be encapsulated
> within a Diag508SignatureVerificationBlock, with the CertificateStoreInfo
> fields ignored. The DIAG handler will read from the provided addresses
> to retrieve the necessary data, parse the signature file, then
> perform the signature-verification. Because there is no way to
> correlate a specific certificate to a component, each certificate
> in the store is tried until either verification succeeds, or all
> certs have been exhausted.
>
> The subcode value is denoted by setting the second-to-left-most bit of
> a 2-byte field.
>
> A return code of 1 indicates success, and the index and length of the
> corresponding certificate will be set in the CertificateStoreInfo
> portion of the SigVerifBlock. The following values indicate failure:
>
> 	0x0102: certificate not available
> 	0x0202: component data is invalid
> 	0x0302: signature is not in PKCS#7 format
> 	0x0402: signature-verification failed
>
> Signed-off-by: Collin Walling <walling@linux.ibm.com>
> Signed-off-by: Zhuoying Cai <zycai@linux.ibm.com>
> ---
>   docs/specs/s390x-secure-ipl.rst |   5 ++
>   include/hw/s390x/ipl/diag508.h  |  23 +++++++
>   target/s390x/diag.c             | 112 +++++++++++++++++++++++++++++++-
>   3 files changed, 139 insertions(+), 1 deletion(-)
>
> diff --git a/docs/specs/s390x-secure-ipl.rst b/docs/specs/s390x-secure-ipl.rst
> index 6b3249173f..385f8d85a8 100644
> --- a/docs/specs/s390x-secure-ipl.rst
> +++ b/docs/specs/s390x-secure-ipl.rst
> @@ -64,3 +64,8 @@ that requires assistance from QEMU.
>   
>   Subcode 0 - query installed subcodes
>       Returns a 64-bit mask indicating which subcodes are supported.
> +
> +Subcode 1 - perform signature verification
> +    Perform signature-verification on a signed component, using certificates
> +    from the certificate store and leveraging qcrypto libraries to perform
> +    this operation.
> diff --git a/include/hw/s390x/ipl/diag508.h b/include/hw/s390x/ipl/diag508.h
> index 6281ad8299..c99c6705c0 100644
> --- a/include/hw/s390x/ipl/diag508.h
> +++ b/include/hw/s390x/ipl/diag508.h
> @@ -11,5 +11,28 @@
>   #define S390X_DIAG508_H
>   
>   #define DIAG_508_SUBC_QUERY_SUBC    0x0000
> +#define DIAG_508_SUBC_SIG_VERIF     0x8000
> +
> +#define DIAG_508_RC_OK              0x0001
> +#define DIAG_508_RC_NO_CERTS        0x0102
> +#define DIAG_508_RC_INVAL_COMP_DATA 0x0202
> +#define DIAG_508_RC_INVAL_PKCS7_SIG 0x0302
> +#define DIAG_508_RC_FAIL_VERIF      0x0402
> +
> +struct Diag508CertificateStoreInfo {
> +    uint8_t  idx;
> +    uint8_t  reserved[7];
> +    uint64_t len;
> +};
> +typedef struct Diag508CertificateStoreInfo Diag508CertificateStoreInfo;
> +
> +struct Diag508SignatureVerificationBlock {
> +    Diag508CertificateStoreInfo csi;
> +    uint64_t comp_len;
> +    uint64_t comp_addr;
> +    uint64_t sig_len;
> +    uint64_t sig_addr;
> +};
> +typedef struct Diag508SignatureVerificationBlock Diag508SignatureVerificationBlock;
>   
>   #endif
> diff --git a/target/s390x/diag.c b/target/s390x/diag.c
> index 6519a3cedc..2fe25a2c66 100644
> --- a/target/s390x/diag.c
> +++ b/target/s390x/diag.c
> @@ -573,9 +573,107 @@ void handle_diag_320(CPUS390XState *env, uint64_t r1, uint64_t r3, uintptr_t ra)
>       }
>   }
>   
> +static int diag_508_verify_sig(uint8_t *cert, size_t cert_size,
> +                              uint8_t *comp, size_t comp_size,
> +                              uint8_t *sig, size_t sig_size)
> +{
> +    g_autofree uint8_t *sig_pem = NULL;
> +    size_t sig_size_pem;
> +    int rc;
> +
> +    /*
> +     * PKCS#7 signature with DER format
> +     * Convert to PEM format for signature verification
> +     */
> +    rc = qcrypto_pkcs7_convert_sig_pem(sig, sig_size, &sig_pem, &sig_size_pem, NULL);
> +    if (rc < 0) {
> +        return -1;
> +    }
> +
> +    /*
> +     * Ignore errors from signature format convertion and verification,
> +     * because currently in the certificate lookup process.
> +     *
> +     * Any error is treated as a verification failure,
> +     * and the final result (verified or not) will be reported later.
> +     */

The comment here seems misleading as we are checking the return code 
from qcrypto_x509_verify_sig and not ignoring the error here. Is it 
meant for the handle_diag508_sig_verif function where we are looping 
through the certs?

Thanks Farhan

> +    rc = qcrypto_x509_verify_sig(cert, cert_size,
> +                                 comp, comp_size,
> +                                 sig_pem, sig_size_pem, NULL);
> +    if (rc < 0) {
> +        return -1;
> +    }
> +
> +    return 0;
> +}
> +
> +static int handle_diag508_sig_verif(uint64_t addr, size_t csi_size, size_t svb_size,
> +                                    S390IPLCertificateStore *qcs)
> +{
> +    int rc;
> +    int verified;
> +    uint64_t comp_len, comp_addr;
> +    uint64_t sig_len, sig_addr;
> +    g_autofree uint8_t *svb_comp = NULL;
> +    g_autofree uint8_t *svb_sig = NULL;
> +    g_autofree Diag508SignatureVerificationBlock *svb = NULL;
> +
> +    if (!qcs || !qcs->count) {
> +        return DIAG_508_RC_NO_CERTS;
> +    }
> +
> +    svb = g_new0(Diag508SignatureVerificationBlock, 1);
> +    cpu_physical_memory_read(addr, svb, svb_size);
> +
> +    comp_len = be64_to_cpu(svb->comp_len);
> +    comp_addr = be64_to_cpu(svb->comp_addr);
> +    sig_len = be64_to_cpu(svb->sig_len);
> +    sig_addr = be64_to_cpu(svb->sig_addr);
> +
> +    if (!comp_len || !comp_addr) {
> +        return DIAG_508_RC_INVAL_COMP_DATA;
> +    }
> +
> +    if (!sig_len || !sig_addr) {
> +        return DIAG_508_RC_INVAL_PKCS7_SIG;
> +    }
> +
> +    svb_comp = g_malloc0(comp_len);
> +    cpu_physical_memory_read(comp_addr, svb_comp, comp_len);
> +
> +    svb_sig = g_malloc0(sig_len);
> +    cpu_physical_memory_read(sig_addr, svb_sig, sig_len);
> +
> +    rc = DIAG_508_RC_FAIL_VERIF;
> +    /*
> +     * It is uncertain which certificate contains
> +     * the analogous key to verify the signed data
> +     */
> +    for (int i = 0; i < qcs->count; i++) {
> +        verified = diag_508_verify_sig(qcs->certs[i].raw,
> +                                       qcs->certs[i].size,
> +                                       svb_comp, comp_len,
> +                                       svb_sig, sig_len);
> +        if (verified == 0) {
> +            svb->csi.idx = i;
> +            svb->csi.len = cpu_to_be64(qcs->certs[i].der_size);
> +            cpu_physical_memory_write(addr, &svb->csi, be32_to_cpu(csi_size));
> +            rc = DIAG_508_RC_OK;
> +            break;
> +       }
> +    }
> +
> +    return rc;
> +}
> +
> +QEMU_BUILD_BUG_MSG(sizeof(Diag508SignatureVerificationBlock) != 48,
> +                   "size of Diag508SignatureVerificationBlock is wrong");
> +
>   void handle_diag_508(CPUS390XState *env, uint64_t r1, uint64_t r3, uintptr_t ra)
>   {
> +    S390IPLCertificateStore *qcs = s390_ipl_get_certificate_store();
>       uint64_t subcode = env->regs[r3];
> +    uint64_t addr = env->regs[r1];
>       int rc;
>   
>       if (env->psw.mask & PSW_MASK_PSTATE) {
> @@ -590,7 +688,19 @@ void handle_diag_508(CPUS390XState *env, uint64_t r1, uint64_t r3, uintptr_t ra)
>   
>       switch (subcode) {
>       case DIAG_508_SUBC_QUERY_SUBC:
> -        rc = 0;
> +        rc = DIAG_508_SUBC_SIG_VERIF;
> +        break;
> +    case DIAG_508_SUBC_SIG_VERIF:
> +        size_t csi_size = sizeof(Diag508CertificateStoreInfo);
> +        size_t svb_size = sizeof(Diag508SignatureVerificationBlock);
> +
> +        if (!diag_parm_addr_valid(addr, svb_size, false) ||
> +            !diag_parm_addr_valid(addr, csi_size, true)) {
> +            s390_program_interrupt(env, PGM_ADDRESSING, ra);
> +            return;
> +        }
> +
> +        rc = handle_diag508_sig_verif(addr, csi_size, svb_size, qcs);
>           break;
>       default:
>           s390_program_interrupt(env, PGM_SPECIFICATION, ra);