sd-device: do no allocate strings of unknown length on the stack

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=33881.

Not only we would duplicate unknown input on the stack, we would do it
over and over. So let's first check that the input has reasonable length,
but also allocate just one fixed size buffer.

1 files changed, 27 insertions, 27 deletions

diff --git a/src/libsystemd/sd-device/sd-device.c b/src/libsystemd/sd-device/sd-device.c
index f2e8c2112c..da1a7a3181 100644
--- a/src/libsystemd/sd-device/sd-device.c
+++ b/src/libsystemd/sd-device/sd-device.c
@@ -245,29 +245,31 @@ _public_ int sd_device_new_from_devnum(sd_device **ret, char type, dev_t devnum)
 }
 
 _public_ int sd_device_new_from_subsystem_sysname(sd_device **ret, const char *subsystem, const char *sysname) {
-        char *name, *syspath;
-        size_t len = 0;
+        char syspath[PATH_MAX], *name;
 
         assert_return(ret, -EINVAL);
         assert_return(subsystem, -EINVAL);
         assert_return(sysname, -EINVAL);
+        assert_return(strlen(sysname) < PATH_MAX - strlen("/sys/bus/"), -ENAMETOOLONG);
 
         if (streq(subsystem, "subsystem")) {
-                syspath = strjoina("/sys/subsystem/", sysname);
-                if (access(syspath, F_OK) >= 0)
+                if (snprintf_ok(syspath, sizeof syspath, "/sys/subsystem/%s", sysname) &&
+                    access(syspath, F_OK) >= 0)
                         return sd_device_new_from_syspath(ret, syspath);
 
-                syspath = strjoina("/sys/bus/", sysname);
-                if (access(syspath, F_OK) >= 0)
+                if (snprintf_ok(syspath, sizeof syspath, "/sys/bus/%s", sysname) &&
+                    access(syspath, F_OK) >= 0)
                         return sd_device_new_from_syspath(ret, syspath);
 
-                syspath = strjoina("/sys/class/", sysname);
-                if (access(syspath, F_OK) >= 0)
+                if (snprintf_ok(syspath, sizeof syspath, "/sys/class/%s", sysname) &&
+                    access(syspath, F_OK) >= 0)
                         return sd_device_new_from_syspath(ret, syspath);
+
         } else  if (streq(subsystem, "module")) {
-                syspath = strjoina("/sys/module/", sysname);
-                if (access(syspath, F_OK) >= 0)
+                if (snprintf_ok(syspath, sizeof syspath, "/sys/module/%s", sysname) &&
+                    access(syspath, F_OK) >= 0)
                         return sd_device_new_from_syspath(ret, syspath);
+
         } else if (streq(subsystem, "drivers")) {
                 char subsys[PATH_MAX];
                 char *driver;
@@ -278,39 +280,37 @@ _public_ int sd_device_new_from_subsystem_sysname(sd_device **ret, const char *s
                         driver[0] = '\0';
                         driver++;
 
-                        syspath = strjoina("/sys/subsystem/", subsys, "/drivers/", driver);
-                        if (access(syspath, F_OK) >= 0)
+                        if (snprintf_ok(syspath, sizeof syspath, "/sys/subsystem/%s/drivers/%s", subsys, driver) &&
+                            access(syspath, F_OK) >= 0)
                                 return sd_device_new_from_syspath(ret, syspath);
 
-                        syspath = strjoina("/sys/bus/", subsys, "/drivers/", driver);
-                        if (access(syspath, F_OK) >= 0)
+                        if (snprintf_ok(syspath, sizeof syspath, "/sys/bus/%s/drivers/%s", subsys, driver) &&
+                            access(syspath, F_OK) >= 0)
                                 return sd_device_new_from_syspath(ret, syspath);
                 }
         }
 
         /* translate sysname back to sysfs filename */
         name = strdupa(sysname);
-        while (name[len] != '\0') {
-                if (name[len] == '/')
-                        name[len] = '!';
 
-                len++;
-        }
+        for (size_t i = 0; name[i]; i++)
+                if (name[i] == '/')
+                        name[i] = '!';
 
-        syspath = strjoina("/sys/subsystem/", subsystem, "/devices/", name);
-        if (access(syspath, F_OK) >= 0)
+        if (snprintf_ok(syspath, sizeof syspath, "/sys/subsystem/%s/devices/%s", subsystem, name) &&
+            access(syspath, F_OK) >= 0)
                 return sd_device_new_from_syspath(ret, syspath);
 
-        syspath = strjoina("/sys/bus/", subsystem, "/devices/", name);
-        if (access(syspath, F_OK) >= 0)
+        if (snprintf_ok(syspath, sizeof syspath, "/sys/bus/%s/devices/%s", subsystem, name) &&
+            access(syspath, F_OK) >= 0)
                 return sd_device_new_from_syspath(ret, syspath);
 
-        syspath = strjoina("/sys/class/", subsystem, "/", name);
-        if (access(syspath, F_OK) >= 0)
+        if (snprintf_ok(syspath, sizeof syspath, "/sys/class/%s/%s", subsystem, name) &&
+            access(syspath, F_OK) >= 0)
                 return sd_device_new_from_syspath(ret, syspath);
 
-        syspath = strjoina("/sys/firmware/", subsystem, "/", sysname);
-        if (access(syspath, F_OK) >= 0)
+        if (snprintf_ok(syspath, sizeof syspath, "/sys/firmware/%s/%s", subsystem, sysname) &&
+            access(syspath, F_OK) >= 0)
                 return sd_device_new_from_syspath(ret, syspath);
 
         return -ENODEV;