Re: [PATCH v6 12/28] s390x/diag: Implement DIAG 508 subcode 1 for signature verification

[Thomas Huth <thuth@redhat.com>]

On 18/09/2025 01.21, 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 Diag508SigVerifBlock.
> 
> 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 Diag508SigVerifBlock.
> 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
> 	0x0502: length of Diag508SigVerifBlock is invalid
> 
> 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             | 115 +++++++++++++++++++++++++++++++-
>   3 files changed, 142 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/specs/s390x-secure-ipl.rst b/docs/specs/s390x-secure-ipl.rst
> index 0919425e9a..eec368d17b 100644
> --- a/docs/specs/s390x-secure-ipl.rst
> +++ b/docs/specs/s390x-secure-ipl.rst
> @@ -66,3 +66,8 @@ 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..ad401cc867 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
> +#define DIAG_508_RC_INVAL_LEN       0x0502
> +
> +struct Diag508SigVerifBlock {
> +    uint32_t length;
> +    uint8_t reserved0[3];
> +    uint8_t version;
> +    uint32_t reserved[2];
> +    uint8_t cert_store_index;
> +    uint8_t reserved1[7];
> +    uint64_t cert_len;
> +    uint64_t comp_len;
> +    uint64_t comp_addr;
> +    uint64_t sig_len;
> +    uint64_t sig_addr;
> +};
> +typedef struct Diag508SigVerifBlock Diag508SigVerifBlock;
>   
>   #endif
> diff --git a/target/s390x/diag.c b/target/s390x/diag.c
> index ee64257dbc..379fb8f2b4 100644
> --- a/target/s390x/diag.c
> +++ b/target/s390x/diag.c
> @@ -602,9 +602,112 @@ 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;
> +    }
> +
> +    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 svb_size,
> +                                    S390IPLCertificateStore *qcs)
> +{
> +    int rc;
> +    int verified;
> +    uint32_t svb_len;
> +    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 Diag508SigVerifBlock *svb = NULL;
> +
> +    if (!qcs || !qcs->count) {
> +        return DIAG_508_RC_NO_CERTS;
> +    }
> +
> +    svb = g_new0(Diag508SigVerifBlock, 1);
> +    cpu_physical_memory_read(addr, svb, svb_size);
> +
> +    svb_len = be32_to_cpu(svb->length);
> +    if (svb_len != svb_size) {
> +        return DIAG_508_RC_INVAL_LEN;
> +    }
> +
> +    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;
> +    }

I think there should also be something like an upper limit for comp_len and 
sign_len here. Otherwise a malicious guest could force QEMU into allocating 
giga- or terabytes of memory here to cause out-of-memory situations in the host.

> +    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
> +     *
> +     * Ignore errors from signature format convertion and verification,
> +     * because currently in the certificate lookup process.

The second half of above sentence looks incomplete?

> +     *
> +     * Any error is treated as a verification failure,
> +     * and the final result (verified or not) will be reported later.
> +     */
> +    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->cert_store_index = i;
> +            svb->cert_len = cpu_to_be64(qcs->certs[i].der_size);
> +            cpu_physical_memory_write(addr, svb, be32_to_cpu(svb_size));
> +            rc = DIAG_508_RC_OK;
> +            break;
> +       }
> +    }
> +
> +    return rc;
> +}

  Thomas