hw/uefi: add var-service-pkcs7.c

This implements pkcs7 signature verification using gnutls.
Needed to check authenticated variable updates.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-ID: <20250225163031.1409078-12-kraxel@redhat.com>

hw/uefi/var-service-pkcs7.c

@@ -0,0 +1,436 @@
+/*
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ *
+ * uefi vars device - pkcs7 verification
+ */
+#include "qemu/osdep.h"
+#include "qemu/error-report.h"
+#include "system/dma.h"
+
+#include <gnutls/gnutls.h>
+#include <gnutls/pkcs7.h>
+#include <gnutls/crypto.h>
+
+#include "hw/uefi/var-service.h"
+
+#define AUTHVAR_DIGEST_ALGO GNUTLS_DIG_SHA256
+#define AUTHVAR_DIGEST_SIZE 32
+
+/*
+ * Replicate the signed data for signature verification.
+ */
+static gnutls_datum_t *build_signed_data(mm_variable_access *va, void *data)
+{
+    variable_auth_2 *auth = data;
+    uint64_t data_offset = sizeof(efi_time) + auth->hdr_length;
+    uint16_t *name = (void *)va + sizeof(mm_variable_access);
+    gnutls_datum_t *sdata;
+    uint64_t pos = 0;
+
+    sdata = g_new(gnutls_datum_t, 1);
+    sdata->size = (va->name_size - 2
+                   + sizeof(QemuUUID)
+                   + sizeof(va->attributes)
+                   + sizeof(auth->timestamp)
+                   + va->data_size - data_offset);
+    sdata->data = g_malloc(sdata->size);
+
+    /* Variable Name (without terminating \0) */
+    memcpy(sdata->data + pos, name, va->name_size - 2);
+    pos += va->name_size - 2;
+
+    /* Variable Namespace Guid */
+    memcpy(sdata->data + pos, &va->guid, sizeof(va->guid));
+    pos += sizeof(va->guid);
+
+    /* Attributes */
+    memcpy(sdata->data + pos, &va->attributes, sizeof(va->attributes));
+    pos += sizeof(va->attributes);
+
+    /* TimeStamp */
+    memcpy(sdata->data + pos, &auth->timestamp, sizeof(auth->timestamp));
+    pos += sizeof(auth->timestamp);
+
+    /* Variable Content */
+    memcpy(sdata->data + pos, data + data_offset, va->data_size - data_offset);
+    pos += va->data_size - data_offset;
+
+    assert(pos == sdata->size);
+    return sdata;
+}
+
+/*
+ * See WrapPkcs7Data() in edk2.
+ *
+ * UEFI spec allows pkcs7 signatures being used without the envelope which
+ * identifies them as pkcs7 signatures.  openssl and gnutls will not parse them
+ * without the envelope though.  So add it if needed.
+ */
+static void wrap_pkcs7(gnutls_datum_t *pkcs7)
+{
+    static uint8_t signed_data_oid[9] = {
+        0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x07, 0x02
+    };
+    gnutls_datum_t wrap;
+
+    if (pkcs7->data[4] == 0x06 &&
+        pkcs7->data[5] == 0x09 &&
+        memcmp(pkcs7->data + 6, signed_data_oid, sizeof(signed_data_oid)) == 0 &&
+        pkcs7->data[15] == 0x0a &&
+        pkcs7->data[16] == 0x82) {
+        return;
+    }
+
+    wrap.size = pkcs7->size + 19;
+    wrap.data = g_malloc(wrap.size);
+
+    wrap.data[0] = 0x30;
+    wrap.data[1] = 0x82;
+    wrap.data[2] = (wrap.size - 4) >> 8;
+    wrap.data[3] = (wrap.size - 4) & 0xff;
+    wrap.data[4] = 0x06;
+    wrap.data[5] = 0x09;
+    memcpy(wrap.data + 6, signed_data_oid, sizeof(signed_data_oid));
+
+    wrap.data[15] = 0xa0;
+    wrap.data[16] = 0x82;
+    wrap.data[17] = pkcs7->size >> 8;
+    wrap.data[18] = pkcs7->size & 0xff;
+    memcpy(wrap.data + 19, pkcs7->data, pkcs7->size);
+
+    g_free(pkcs7->data);
+    *pkcs7 = wrap;
+}
+
+static gnutls_datum_t *build_pkcs7(void *data)
+{
+    variable_auth_2 *auth = data;
+    gnutls_datum_t *pkcs7;
+
+    pkcs7 = g_new(gnutls_datum_t, 1);
+    pkcs7->size = auth->hdr_length - 24;
+    pkcs7->data = g_malloc(pkcs7->size);
+    memcpy(pkcs7->data, data + 16 + 24, pkcs7->size);
+
+    wrap_pkcs7(pkcs7);
+
+    return pkcs7;
+}
+
+/*
+ * Read UEFI signature database, store x509 all certificates found in
+ * gnutls_x509_trust_list_t.
+ */
+static gnutls_x509_trust_list_t build_trust_list_sb(uefi_variable *var)
+{
+    gnutls_x509_trust_list_t tlist;
+    gnutls_datum_t cert_data;
+    gnutls_x509_crt_t cert;
+    uefi_vars_siglist siglist;
+    uefi_vars_cert *c;
+    int rc;
+
+    rc = gnutls_x509_trust_list_init(&tlist, 0);
+    if (rc < 0) {
+        warn_report("gnutls_x509_trust_list_init error: %s",
+                     gnutls_strerror(rc));
+        return NULL;
+    }
+
+    uefi_vars_siglist_init(&siglist);
+    uefi_vars_siglist_parse(&siglist, var->data, var->data_size);
+
+    QTAILQ_FOREACH(c, &siglist.x509, next) {
+        cert_data.size = c->size;
+        cert_data.data = c->data;
+
+        rc = gnutls_x509_crt_init(&cert);
+        if (rc < 0) {
+            warn_report("gnutls_x509_crt_init error: %s", gnutls_strerror(rc));
+            break;
+        }
+        rc = gnutls_x509_crt_import(cert, &cert_data, GNUTLS_X509_FMT_DER);
+        if (rc < 0) {
+            warn_report("gnutls_x509_crt_import error: %s",
+                        gnutls_strerror(rc));
+            gnutls_x509_crt_deinit(cert);
+            break;
+        }
+        rc = gnutls_x509_trust_list_add_cas(tlist, &cert, 1, 0);
+        if (rc < 0) {
+            warn_report("gnutls_x509_crt_import error: %s",
+                        gnutls_strerror(rc));
+            gnutls_x509_crt_deinit(cert);
+            break;
+        }
+    }
+
+    uefi_vars_siglist_free(&siglist);
+
+    return tlist;
+}
+
+static int build_digest_authvar(gnutls_x509_crt_t signer,
+                                gnutls_x509_crt_t root,
+                                uint8_t *hash_digest)
+{
+    char *cn;
+    size_t cn_size = 0;
+    uint8_t fp[AUTHVAR_DIGEST_SIZE];
+    size_t fp_size = sizeof(fp);
+    gnutls_hash_hd_t hash;
+    int rc;
+
+    /* get signer CN */
+    rc = gnutls_x509_crt_get_dn_by_oid(signer, GNUTLS_OID_X520_COMMON_NAME,
+                                       0, 0, NULL, &cn_size);
+    if (rc != GNUTLS_E_SHORT_MEMORY_BUFFER) {
+        warn_report("gnutls_x509_crt_get_dn_by_oid error #1: %s",
+                    gnutls_strerror(rc));
+        return rc;
+    }
+
+    cn = g_malloc(cn_size);
+    rc = gnutls_x509_crt_get_dn_by_oid(signer, GNUTLS_OID_X520_COMMON_NAME,
+                                       0, 0, cn, &cn_size);
+    if (rc < 0) {
+        warn_report("gnutls_x509_crt_get_dn_by_oid error #2: %s",
+                    gnutls_strerror(rc));
+        goto err;
+    }
+
+    /* get root certificate fingerprint */
+    rc = gnutls_x509_crt_get_fingerprint(root, AUTHVAR_DIGEST_ALGO,
+                                         fp, &fp_size);
+    if (rc < 0) {
+        warn_report("gnutls_x509_crt_get_fingerprint error: %s",
+                    gnutls_strerror(rc));
+        goto err;
+    }
+
+    /* digest both items */
+    rc = gnutls_hash_init(&hash, AUTHVAR_DIGEST_ALGO);
+    if (rc < 0) {
+        warn_report("gnutls_hash_init error: %s",
+                    gnutls_strerror(rc));
+        goto err;
+    }
+    rc = gnutls_hash(hash, cn, cn_size);
+    if (rc < 0) {
+        warn_report("gnutls_hash error: %s",
+                    gnutls_strerror(rc));
+        goto err;
+    }
+    rc = gnutls_hash(hash, fp, fp_size);
+    if (rc < 0) {
+        warn_report("gnutls_hash error: %s",
+                    gnutls_strerror(rc));
+        goto err;
+    }
+    gnutls_hash_deinit(hash, hash_digest);
+
+    return 0;
+
+err:
+    g_free(cn);
+    return rc;
+}
+
+/*
+ * uefi spec 2.9, section 8.2.2
+ *
+ * For EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS variables which are
+ * NOT secure boot variables we should track the root certificate of the trust
+ * chain, and the subject CN of the signer certificate.
+ *
+ * So we'll go store a digest of these two items so we can verify this.  Also
+ * create a gnutls_x509_trust_list_t with the root certificate, so
+ * gnutls_pkcs7_verify() will pass (assuming the signature is otherwise
+ * correct).
+ */
+static gnutls_x509_trust_list_t build_trust_list_authvar(gnutls_pkcs7_t pkcs7,
+                                                         uint8_t *hash_digest)
+{
+    gnutls_datum_t signer_data = { 0 };
+    gnutls_datum_t root_data = { 0 };
+    gnutls_x509_crt_t signer = NULL;
+    gnutls_x509_crt_t root = NULL;
+    gnutls_x509_trust_list_t tlist = NULL;
+    int n, rc;
+
+    n = gnutls_pkcs7_get_crt_count(pkcs7);
+
+    /* first is signer certificate */
+    rc = gnutls_pkcs7_get_crt_raw2(pkcs7, 0, &signer_data);
+    if (rc < 0) {
+        warn_report("gnutls_pkcs7_get_crt_raw2(0) error: %s",
+                    gnutls_strerror(rc));
+        goto done;
+    }
+    rc = gnutls_x509_crt_init(&signer);
+    if (rc < 0) {
+        warn_report("gnutls_x509_crt_init error: %s", gnutls_strerror(rc));
+        goto done;
+    }
+    rc = gnutls_x509_crt_import(signer, &signer_data, GNUTLS_X509_FMT_DER);
+    if (rc < 0) {
+        warn_report("gnutls_x509_crt_import error: %s",
+                    gnutls_strerror(rc));
+        gnutls_x509_crt_deinit(signer);
+        goto done;
+    }
+
+    /* last is root-of-trust certificate (can be identical to signer) */
+    rc = gnutls_pkcs7_get_crt_raw2(pkcs7, n - 1, &root_data);
+    if (rc < 0) {
+        warn_report("gnutls_pkcs7_get_crt_raw2(%d) error: %s",
+                    n - 1, gnutls_strerror(rc));
+        goto done;
+    }
+    rc = gnutls_x509_crt_init(&root);
+    if (rc < 0) {
+        warn_report("gnutls_x509_crt_init error: %s", gnutls_strerror(rc));
+        goto done;
+    }
+    rc = gnutls_x509_crt_import(root, &root_data, GNUTLS_X509_FMT_DER);
+    if (rc < 0) {
+        warn_report("gnutls_x509_crt_import error: %s",
+                    gnutls_strerror(rc));
+        goto done;
+    }
+
+    /* calc digest for signer CN + root cert */
+    rc = build_digest_authvar(signer, root, hash_digest);
+    if (rc < 0) {
+        goto done;
+    }
+
+    /* add root to trust list */
+    rc = gnutls_x509_trust_list_init(&tlist, 0);
+    if (rc < 0) {
+        warn_report("gnutls_x509_trust_list_init error: %s",
+                    gnutls_strerror(rc));
+        goto done;
+    }
+    rc = gnutls_x509_trust_list_add_cas(tlist, &root, 1, 0);
+    if (rc < 0) {
+        warn_report("gnutls_x509_crt_import error: %s",
+                    gnutls_strerror(rc));
+        gnutls_x509_trust_list_deinit(tlist, 1);
+        tlist = NULL;
+        goto done;
+    } else {
+        /* ownership passed to tlist */
+        root = NULL;
+    }
+
+done:
+    if (signer_data.data) {
+        gnutls_free(signer_data.data);
+    }
+    if (root_data.data) {
+        gnutls_free(root_data.data);
+    }
+    if (signer) {
+        gnutls_x509_crt_deinit(signer);
+    }
+    if (root) {
+        gnutls_x509_crt_deinit(root);
+    }
+    return tlist;
+}
+
+static void free_datum(gnutls_datum_t *ptr)
+{
+    if (!ptr) {
+        return;
+    }
+    g_free(ptr->data);
+    g_free(ptr);
+}
+
+static void gnutls_log_stderr(int level, const char *msg)
+{
+    if (strncmp(msg, "ASSERT:", 7) == 0) {
+        return;
+    }
+    fprintf(stderr, "    %d: %s", level, msg);
+}
+
+/*
+ * pkcs7 signature verification (EFI_VARIABLE_AUTHENTICATION_2).
+ */
+efi_status uefi_vars_check_pkcs7_2(uefi_variable *siglist,
+                                   void **digest, uint32_t *digest_size,
+                                   mm_variable_access *va, void *data)
+{
+    gnutls_x509_trust_list_t tlist = NULL;
+    gnutls_datum_t *signed_data = NULL;
+    gnutls_datum_t *pkcs7_data = NULL;
+    gnutls_pkcs7_t pkcs7 = NULL;
+    efi_status status = EFI_SECURITY_VIOLATION;
+    int rc;
+
+    if (0) {
+        /* gnutls debug logging */
+        static bool first = true;
+
+        if (first) {
+            first = false;
+            gnutls_global_set_log_function(gnutls_log_stderr);
+            gnutls_global_set_log_level(99);
+        }
+    }
+
+    signed_data = build_signed_data(va, data);
+    pkcs7_data = build_pkcs7(data);
+
+    rc = gnutls_pkcs7_init(&pkcs7);
+    if (rc < 0) {
+        warn_report("gnutls_pkcs7_init error: %s", gnutls_strerror(rc));
+        goto out;
+    }
+
+    rc = gnutls_pkcs7_import(pkcs7, pkcs7_data, GNUTLS_X509_FMT_DER);
+    if (rc < 0) {
+        warn_report("gnutls_pkcs7_import error: %s", gnutls_strerror(rc));
+        goto out;
+    }
+
+    if (siglist) {
+        /* secure boot variables */
+        tlist = build_trust_list_sb(siglist);
+    } else if (digest && digest_size) {
+        /* other authenticated variables */
+        *digest_size = AUTHVAR_DIGEST_SIZE;
+        *digest = g_malloc(*digest_size);
+        tlist = build_trust_list_authvar(pkcs7, *digest);
+    } else {
+        /* should not happen */
+        goto out;
+    }
+
+    rc = gnutls_pkcs7_verify(pkcs7, tlist,
+                             NULL, 0,
+                             0, signed_data,
+                             GNUTLS_VERIFY_DISABLE_TIME_CHECKS |
+                             GNUTLS_VERIFY_DISABLE_TRUSTED_TIME_CHECKS);
+    if (rc < 0) {
+        warn_report("gnutls_pkcs7_verify error: %s", gnutls_strerror(rc));
+        goto out;
+    }
+
+    /* check passed */
+    status = EFI_SUCCESS;
+
+out:
+    free_datum(signed_data);
+    free_datum(pkcs7_data);
+    if (tlist) {
+        gnutls_x509_trust_list_deinit(tlist, 1);
+    }
+    if (pkcs7) {
+        gnutls_pkcs7_deinit(pkcs7);
+    }
+    return status;
+}