sysext: validate that they do not contain os-release

sysexts are not supposed to ship os-release files, enforce this
when loading them

src/shared/discover-image.c

@@ -22,6 +22,7 @@
 #include "dissect-image.h"
 #include "env-file.h"
 #include "env-util.h"
+#include "extension-util.h"
 #include "fd-util.h"
 #include "fs-util.h"
 #include "hashmap.h"
@@ -1151,6 +1152,16 @@ int image_read_metadata(Image *i) {
                 _cleanup_free_ char *hostname = NULL;
                 _cleanup_free_ char *path = NULL;
 
+                if (i->class == IMAGE_EXTENSION) {
+                        r = extension_has_forbidden_content(i->path);
+                        if (r < 0)
+                                return r;
+                        if (r > 0)
+                                return log_debug_errno(SYNTHETIC_ERRNO(ENOMEDIUM),
+                                                       "Conflicting content found in image %s, refusing.",
+                                                       i->name);
+                }
+
                 r = chase("/etc/hostname", i->path, CHASE_PREFIX_ROOT|CHASE_TRAIL_SLASH, &path, NULL);
                 if (r < 0 && r != -ENOENT)
                         log_debug_errno(r, "Failed to chase /etc/hostname in image %s: %m", i->name);

src/shared/dissect-image.c

@@ -1787,11 +1787,16 @@ int dissected_image_mount(
                                         ok = true;
                         }
                         if (!ok && FLAGS_SET(flags, DISSECT_IMAGE_VALIDATE_OS_EXT)) {
-                                r = path_is_extension_tree(where, m->image_name, FLAGS_SET(flags, DISSECT_IMAGE_RELAX_SYSEXT_CHECK));
+                                r = extension_has_forbidden_content(where);
                                 if (r < 0)
                                         return r;
-                                if (r > 0)
-                                        ok = true;
+                                if (r == 0) {
+                                        r = path_is_extension_tree(where, m->image_name, FLAGS_SET(flags, DISSECT_IMAGE_RELAX_SYSEXT_CHECK));
+                                        if (r < 0)
+                                                return r;
+                                        if (r > 0)
+                                                ok = true;
+                                }
                         }
 
                         if (!ok)

src/shared/extension-util.c

@@ -2,6 +2,7 @@
 
 #include "alloc-util.h"
 #include "architecture.h"
+#include "chase.h"
 #include "env-util.h"
 #include "extension-util.h"
 #include "log.h"
@@ -135,3 +136,20 @@ int parse_env_extension_hierarchies(char ***ret_hierarchies) {
         *ret_hierarchies = TAKE_PTR(l);
         return 0;
 }
+
+int extension_has_forbidden_content(const char *root) {
+        int r;
+
+        /* Insist that extension images do not overwrite the underlying OS release file (it's fine if
+         * they place one in /etc/os-release, i.e. where things don't matter, as they aren't
+         * merged.) */
+        r = chase("/usr/lib/os-release", root, CHASE_PREFIX_ROOT, NULL, NULL);
+        if (r > 0) {
+                log_debug("Extension contains '/usr/lib/os-release', which is not allowed, refusing.");
+                return 1;
+        }
+        if (r < 0 && r != -ENOENT)
+                return log_debug_errno(r, "Failed to determine whether '/usr/lib/os-release' exists in the extension: %m");
+
+        return 0;
+}

src/shared/extension-util.h

@@ -14,3 +14,7 @@ int extension_release_validate(
 
 /* Parse SYSTEMD_SYSEXT_HIERARCHIES and if not set, return "/usr /opt" */
 int parse_env_extension_hierarchies(char ***ret_hierarchies);
+
+/* Insist that extension images do not overwrite the underlying OS release file (it's fine if they place one
+ * in /etc/os-release, i.e. where things don't matter, as they aren't merged.) */
+int extension_has_forbidden_content(const char *root);