shared: add configfs-tsm attestation report helper

Add tsm_report_acquire(), a thin wrapper around the kernel's
/sys/kernel/config/tsm/report/ configfs interface for fetching a
confidential-computing attestation report (SEV-SNP, TDX, ...), including
a caller supplied input.

Signed-off-by: Paul Meyer <katexochen0@gmail.com>

src/shared/meson.build

@@ -208,6 +208,7 @@ shared_sources = files(
         'tomoyo-util.c',
         'tpm2-util.c',
         'tpm2-event-log.c',
+        'tsm-report.c',
         'udev-util.c',
         'unit-file.c',
         'user-record-nss.c',

src/shared/tsm-report.c

@@ -0,0 +1,214 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <fcntl.h>
+#include <unistd.h>
+
+#include "sd-id128.h"
+
+#include "alloc-util.h"
+#include "fd-util.h"
+#include "fileio.h"
+#include "format-util.h"
+#include "fs-util.h"
+#include "io-util.h"
+#include "iovec-util.h"
+#include "log.h"
+#include "parse-util.h"
+#include "process-util.h"
+#include "stdio-util.h"
+#include "tsm-report.h"
+
+#define TSM_REPORT_PATH "/sys/kernel/config/tsm/report"
+
+TsmReport *tsm_report_free(TsmReport *report) {
+        if (!report)
+                return NULL;
+
+        free(report->provider);
+        iovec_done(&report->outblob);
+        iovec_done(&report->auxblob);
+        iovec_done(&report->manifestblob);
+
+        return mfree(report);
+}
+
+static int read_generation(int entry_fd, uint64_t *ret) {
+        _cleanup_free_ char *s = NULL;
+        int r;
+
+        assert(entry_fd >= 0);
+        assert(ret);
+
+        /* The kernel bumps this counter on every option write to the entry. We snapshot it before writing
+         * and re-check it after reading the report to detect a writer racing us on this entry. */
+
+        r = read_one_line_file_at(entry_fd, "generation", &s);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to read 'generation' attribute: %m");
+
+        r = safe_atou64(s, ret);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to parse 'generation' attribute: %m");
+
+        return 0;
+}
+
+static int tsm_report_fill(
+                int entry_fd,
+                const struct iovec *report_data,
+                const TsmReportOptions *options,
+                TsmReport **ret) {
+
+        _cleanup_close_ int inblob_fd = -EBADF;
+        _cleanup_(tsm_report_freep) TsmReport *report = NULL;
+        _cleanup_free_ char *floor = NULL;
+        bool has_privlevel = false, has_floor = false;
+        unsigned privlevel = 0, privlevel_floor = 0;
+        int r;
+
+        assert(entry_fd >= 0);
+        assert(report_data);
+        assert(ret);
+
+        r = read_one_line_file_at(entry_fd, "privlevel_floor", &floor);
+        if (r >= 0) {
+                r = safe_atou(floor, &privlevel_floor);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to parse 'privlevel_floor' attribute: %m");
+                has_floor = true;
+        } else if (r != -ENOENT)
+                return log_debug_errno(r, "Failed to read 'privlevel_floor' attribute: %m");
+        /* -ENOENT: provider has no privlevel concept, leave it unset. */
+
+        if (options && options->privlevel_set) {
+                if (!has_floor)
+                        return log_debug_errno(SYNTHETIC_ERRNO(EINVAL),
+                                               "TSM provider does not support 'privlevel'.");
+
+                if (options->privlevel < privlevel_floor)
+                        return log_debug_errno(SYNTHETIC_ERRNO(EINVAL),
+                                               "Requested privlevel %u is below the provider's floor %u.",
+                                               options->privlevel, privlevel_floor);
+
+                privlevel = options->privlevel;
+                has_privlevel = true;
+        } else if (has_floor) {
+                privlevel = privlevel_floor;
+                has_privlevel = true;
+        }
+
+        /* Snapshot the write-generation counter before touching any input. Note that tracking generations
+         * is defense in-depth, as we are already operating on a private directory per report request. */
+        uint64_t generation;
+        r = read_generation(entry_fd, &generation);
+        if (r < 0)
+                return r;
+
+        /* Write inputs. Each successful option write bumps the kernel's generation by one. */
+
+        if (has_privlevel) {
+                char privlvl_buf[DECIMAL_STR_MAX(unsigned)];
+                xsprintf(privlvl_buf, "%u", privlevel);
+
+                r = write_string_file_at(entry_fd, "privlevel",
+                                         privlvl_buf,
+                                         WRITE_STRING_FILE_DISABLE_BUFFER);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to write 'privlevel' attribute: %m");
+                generation++;
+        }
+
+        inblob_fd = openat(entry_fd, "inblob", O_WRONLY|O_CLOEXEC);
+        if (inblob_fd < 0)
+                return log_debug_errno(errno, "Failed to open 'inblob' attribute: %m");
+        r = loop_write(inblob_fd, report_data->iov_base, report_data->iov_len);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to write 'inblob' attribute: %m");
+        inblob_fd = safe_close(inblob_fd);  /* configfs commits the buffered write only on close. */
+        generation++;
+
+        /* Read output. */
+
+        report = new0(TsmReport, 1);
+        if (!report)
+                return log_oom_debug();
+
+        r = read_full_file_at(entry_fd, "outblob",
+                              (char**) &report->outblob.iov_base, &report->outblob.iov_len);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to read 'outblob' attribute: %m");
+
+        r = read_one_line_file_at(entry_fd, "provider", &report->provider);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to read 'provider' attribute: %m");
+
+        r = read_full_file_at(entry_fd, "auxblob",
+                              (char**) &report->auxblob.iov_base, &report->auxblob.iov_len);
+        if (r < 0 && r != -ENOENT) /* auxblob is optional */
+                return log_debug_errno(r, "Failed to read 'auxblob' attribute: %m");
+
+        r = read_full_file_at(entry_fd, "manifestblob",
+                              (char**) &report->manifestblob.iov_base, &report->manifestblob.iov_len);
+        if (r < 0 && r != -ENOENT) /* manifestblob is optional */
+                return log_debug_errno(r, "Failed to read 'manifestblob' attribute: %m");
+
+        uint64_t generation_now;
+        r = read_generation(entry_fd, &generation_now);
+        if (r < 0)
+                return r;
+        if (generation_now != generation)
+                return log_debug_errno(SYNTHETIC_ERRNO(EBUSY),
+                                       "TSM report generation changed during acquisition (%" PRIu64 " != %" PRIu64 "), concurrent access?",
+                                       generation_now, generation);
+
+        *ret = TAKE_PTR(report);
+        return 0;
+}
+
+int tsm_report_acquire(
+                const struct iovec *report_data,
+                const TsmReportOptions *options,
+                TsmReport **ret) {
+
+        _cleanup_close_ int report_fd = -EBADF, entry_fd = -EBADF;
+        _cleanup_free_ char *name = NULL;
+        sd_id128_t rnd;
+        int r;
+
+        assert(ret);
+
+        if (!report_data || !iovec_is_set(report_data))
+                return log_debug_errno(SYNTHETIC_ERRNO(EINVAL), "Report data can't be empty");
+        if (report_data->iov_len != TSM_REPORT_DATA_SIZE)
+                return log_debug_errno(SYNTHETIC_ERRNO(EINVAL), "Report data must be %u bytes.",
+                                       TSM_REPORT_DATA_SIZE);
+
+        report_fd = open(TSM_REPORT_PATH, O_DIRECTORY|O_CLOEXEC|O_RDONLY);
+        if (report_fd < 0) {
+                if (errno == ENOENT)
+                        return log_debug_errno(SYNTHETIC_ERRNO(EOPNOTSUPP),
+                                               "configfs-tsm interface not available at " TSM_REPORT_PATH ".");
+                return log_debug_errno(errno, "Failed to open " TSM_REPORT_PATH ": %m");
+        }
+
+        /* Private, unique entry name so we don't race with other callers.
+         * PID is included for attribution. */
+        r = sd_id128_randomize(&rnd);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to generate report entry name: %m");
+        r = asprintf(&name, "systemd-report-" PID_FMT "-%s", getpid_cached(), SD_ID128_TO_STRING(rnd));
+        if (r < 0)
+                return log_oom_debug();
+
+        entry_fd = open_mkdir_at(report_fd, name, O_EXCL|O_RDONLY|O_CLOEXEC, 0700);
+        if (entry_fd < 0)
+                return log_debug_errno(entry_fd, "Failed to create TSM report entry: %m");
+
+        r = tsm_report_fill(entry_fd, report_data, options, ret);
+
+        /* Remove the entry regardless of success/failure. */
+        if (unlinkat(report_fd, name, AT_REMOVEDIR) < 0)
+                log_debug_errno(errno, "Failed to remove TSM report entry '%s', ignoring: %m", name);
+
+        return r;
+}

src/shared/tsm-report.h

@@ -0,0 +1,38 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <sys/uio.h>
+
+#include "shared-forward.h"
+
+#define TSM_REPORT_DATA_SIZE 64U
+
+/* Optional knobs. Mirrors the write-only attributes other than inblob.
+ * Zero-initialize ({}) and set only what you need. NULL options == all defaults. */
+typedef struct TsmReportOptions {
+        unsigned privlevel;                  /* e.g. SEV-SNP VMPL */
+        bool privlevel_set;                  /* privlevel_floor is used when unset */
+        /* service_provider currently not supported. */
+} TsmReportOptions;
+
+/* Result. Mirrors the read-only attributes. */
+typedef struct TsmReport {
+        char *provider;             /* e.g. "sev_guest", "tdx_guest" */
+        struct iovec outblob;       /* the attestation report */
+        struct iovec auxblob;       /* optional, unset if empty (e.g. SEV cert_table) */
+        struct iovec manifestblob;  /* optional, unset if empty */
+} TsmReport;
+
+TsmReport *tsm_report_free(TsmReport *report);
+DEFINE_TRIVIAL_CLEANUP_FUNC(TsmReport*, tsm_report_free);
+
+/* Acquire an attestation report via configfs-tsm.
+ *   report_data: mandatory inblob to include in the report, TSM_REPORT_DATA_SIZE bytes
+ *   options:     optional, NULL for defaults
+ *   ret:         result, freed with tsm_report_free()
+ * Returns -EOPNOTSUPP if the configfs-tsm interface is absent, -ENXIO if it is present
+ * but no provider is registered. Other negative errnos are real failures. */
+int tsm_report_acquire(
+                const struct iovec *report_data,
+                const TsmReportOptions *options,
+                TsmReport **ret);