condition: Check that subsystem is enabled in ConditionSecurity=tpm2

Instead of succeeding when either the firmware reports a TPM device
or we find a TPM device, let's check that the firmware reports a TPM
device and the TPM subsystem is enabled in the kernel.

To check whether the subsystem enabled, we check if the relevant
subdirectory in /sys exists at all.

man/systemd-creds.xml

@@ -175,8 +175,8 @@
         by the OS kernel drivers and by userspace (i.e. systemd) this prints <literal>yes</literal> and exits
         with exit status zero. If no such device is discovered/supported/used, prints
         <literal>no</literal>. Otherwise prints <literal>partial</literal>. In either of these two cases
-        exits with non-zero exit status. It also shows three lines indicating separately whether drivers,
-        firmware and the system discovered/support/use TPM2.</para>
+        exits with non-zero exit status. It also shows four lines indicating separately whether firmware,
+        drivers, the system and the kernel discovered/support/use TPM2.</para>
 
         <para>Combine with <option>--quiet</option> to suppress the output.</para></listitem>
       </varlistentry>

src/creds/creds.c

@@ -637,10 +637,12 @@ static int verb_has_tpm2(int argc, char **argv, void *userdata) {
 
                 printf("%sfirmware\n"
                        "%sdriver\n"
-                       "%ssystem\n",
+                       "%ssystem\n"
+                       "%ssubsystem\n",
                        plus_minus(s & TPM2_SUPPORT_FIRMWARE),
                        plus_minus(s & TPM2_SUPPORT_DRIVER),
-                       plus_minus(s & TPM2_SUPPORT_SYSTEM));
+                       plus_minus(s & TPM2_SUPPORT_SYSTEM),
+                       plus_minus(s & TPM2_SUPPORT_SUBSYSTEM));
         }
 
         /* Return inverted bit flags. So that TPM2_SUPPORT_FULL becomes EXIT_SUCCESS and the other values

src/shared/condition.c

@@ -664,14 +664,13 @@ static int condition_test_ac_power(Condition *c, char **env) {
 }
 
 static int has_tpm2(void) {
-        /* Checks whether the system has at least one TPM2 resource manager device, i.e. at least one "tpmrm"
-         * class device. Alternatively, we are also happy if the firmware reports support (this is to cover
-         * for cases where we simply haven't loaded the driver for it yet, i.e. during early boot where we
-         * very likely want to use this condition check).
+        /* Checks whether the kernel has the TPM subsystem enabled and the firmware reports support. Note
+         * we don't check for actual TPM devices, since we might not have loaded the driver for it yet, i.e.
+         * during early boot where we very likely want to use this condition check).
          *
          * Note that we don't check if we ourselves are built with TPM2 support here! */
 
-        return (tpm2_support() & (TPM2_SUPPORT_DRIVER|TPM2_SUPPORT_FIRMWARE)) != 0;
+        return FLAGS_SET(tpm2_support(), TPM2_SUPPORT_SUBSYSTEM|TPM2_SUPPORT_FIRMWARE);
 }
 
 static int condition_test_security(Condition *c, char **env) {

src/shared/tpm2-util.c

@@ -2189,7 +2189,11 @@ Tpm2Support tpm2_support(void) {
                         if (r != -ENOENT)
                                 log_debug_errno(r, "Unable to test whether /sys/class/tpmrm/ exists and is populated, assuming it is not: %m");
                 } else if (r == 0) /* populated! */
-                        support |= TPM2_SUPPORT_DRIVER;
+                        support |= TPM2_SUPPORT_SUBSYSTEM|TPM2_SUPPORT_DRIVER;
+                else
+                        /* If the directory exists but is empty, we know the subsystem is enabled but no
+                         * driver has been loaded yet. */
+                        support |= TPM2_SUPPORT_SUBSYSTEM;
         }
 
         if (efi_has_tpm2())

src/shared/tpm2-util.h

@@ -137,11 +137,12 @@ typedef struct {
 typedef enum Tpm2Support {
         /* NOTE! The systemd-creds tool returns these flags 1:1 as exit status. Hence these flags are pretty
          * much ABI! Hence, be extra careful when changing/extending these definitions. */
-        TPM2_SUPPORT_NONE     = 0,       /* no support */
-        TPM2_SUPPORT_FIRMWARE = 1 << 0,  /* firmware reports TPM2 was used */
-        TPM2_SUPPORT_DRIVER   = 1 << 1,  /* the kernel has a driver loaded for it */
-        TPM2_SUPPORT_SYSTEM   = 1 << 2,  /* we support it ourselves */
-        TPM2_SUPPORT_FULL     = TPM2_SUPPORT_FIRMWARE|TPM2_SUPPORT_DRIVER|TPM2_SUPPORT_SYSTEM,
+        TPM2_SUPPORT_NONE      = 0,       /* no support */
+        TPM2_SUPPORT_FIRMWARE  = 1 << 0,  /* firmware reports TPM2 was used */
+        TPM2_SUPPORT_DRIVER    = 1 << 1,  /* the kernel has a driver loaded for it */
+        TPM2_SUPPORT_SYSTEM    = 1 << 2,  /* we support it ourselves */
+        TPM2_SUPPORT_SUBSYSTEM = 1 << 3,  /* the kernel has the tpm subsystem enabled */
+        TPM2_SUPPORT_FULL      = TPM2_SUPPORT_FIRMWARE|TPM2_SUPPORT_DRIVER|TPM2_SUPPORT_SYSTEM|TPM2_SUPPORT_SUBSYSTEM,
 } Tpm2Support;
 
 Tpm2Support tpm2_support(void);