basic/unit-name: do not use strdupa() on a path

The path may have unbounded length, for example through a fuse mount.

CVE-2021-33910: attacked controlled alloca() leads to crash in systemd and
ultimately a kernel panic. Systemd parses the content of /proc/self/mountinfo
and each mountpoint is passed to mount_setup_unit(), which calls
unit_name_path_escape() underneath. A local attacker who is able to mount a
filesystem with a very long path can crash systemd and the whole system.

https://bugzilla.redhat.com/show_bug.cgi?id=1970887

The resulting string length is bounded by UNIT_NAME_MAX, which is 256. But we
can't easily check the length after simplification before doing the
simplification, which in turns uses a copy of the string we can write to.
So we can't reject paths that are too long before doing the duplication.
Hence the most obvious solution is to switch back to strdup(), as before
7410616cd9dbbec97cf98d75324da5cda2b2f7a2.

diff --git a/src/basic/unit-name.c b/src/basic/unit-name.c
index 284a773483165875eca5878d1345f64f613125b7..a22763443fdd5f70c2e25cec0d1d9d0196338cbe 100644 (file)
--- a/src/basic/unit-name.c
+++ b/src/basic/unit-name.c
@@ -378,12 +378,13 @@ int unit_name_unescape(const char *f, char **ret) {
 }
 
 int unit_name_path_escape(const char *f, char **ret) {
-        char *p, *s;
+        _cleanup_free_ char *p = NULL;
+        char *s;
 
         assert(f);
         assert(ret);
 
-        p = strdupa(f);
+        p = strdup(f);
         if (!p)
                 return -ENOMEM;
 
@@ -395,13 +396,9 @@ int unit_name_path_escape(const char *f, char **ret) {
                 if (!path_is_normalized(p))
                         return -EINVAL;
 
-                /* Truncate trailing slashes */
+                /* Truncate trailing slashes and skip leading slashes */
                 delete_trailing_chars(p, "/");
-
-                /* Truncate leading slashes */
-                p = skip_leading_chars(p, "/");
-
-                s = unit_name_escape(p);
+                s = unit_name_escape(skip_leading_chars(p, "/"));
         }
         if (!s)
                 return -ENOMEM;