tpm2: add some extra validation of device string before using it

Let's add some extra validation before constructing and using the .so
name to load. This isn't really security sensitive, given that we
used secure_getenv() to get the device string (and it thus should have
been come from a trusted source) but let's better be safe than sorry.

(cherry picked from commit 50a0851)

src/shared/tpm2-util.c

@@ -174,15 +174,27 @@ int tpm2_context_init(const char *device, struct tpm2_context *ret) {
 
                 param = strchr(device, ':');
                 if (param) {
+                        /* Syntax #1: Pair of driver string and arbitrary parameter */
                         driver = strndupa_safe(device, param - device);
+                        if (isempty(driver))
+                                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "TPM2 driver name is empty, refusing.");
+
                         param++;
-                } else {
+                } else if (path_is_absolute(device) && path_is_valid(device)) {
+                        /* Syntax #2: TPM device node */
                         driver = "device";
                         param = device;
-                }
+                } else
+                        return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Invalid TPM2 driver string, refusing.");
+
+                log_debug("Using TPM2 TCTI driver '%s' with device '%s'.", driver, param);
 
                 fn = strjoina("libtss2-tcti-", driver, ".so.0");
 
+                /* Better safe than sorry, let's refuse strings that cannot possibly be valid driver early, before going to disk. */
+                if (!filename_is_valid(fn))
+                        return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "TPM2 driver name '%s' not valid, refusing.", driver);
+
                 dl = dlopen(fn, RTLD_NOW);
                 if (!dl)
                         return log_error_errno(SYNTHETIC_ERRNO(ENOTRECOVERABLE), "Failed to load %s: %s", fn, dlerror());