fusermount: bail out on transient config read failure

If an attacker wishes to use the default configuration instead of the
system's actual configuration, they can attempt to trigger a failure in
read_conf(). This only permits increasing mount_max if it is lower than the
default, so it's not particularly interesting. Still, this should probably
be prevented robustly; bail out if funny stuff happens when we're trying to
read the config.

Note that the classic attack trick of opening so many files that the
system-wide limit is reached won't work here - because fusermount only
drops the fsuid, not the euid, the process is running with euid=0 and
CAP_SYS_ADMIN, so it bypasses the number-of-globally-open-files check in
get_empty_filp() (unless you're inside a user namespace).

diff --git a/util/fusermount.c b/util/fusermount.c
index 5175c0115a053f781df47181157ff2412e9be253..012affb16d7689e334d916829feab463cbfd01d2 100644 (file)
--- a/util/fusermount.c
+++ b/util/fusermount.c
@@ -566,10 +566,19 @@ static void read_conf(void)
                        fprintf(stderr, "%s: reading %s: missing newline at end of file\n", progname, FUSE_CONF);
 
                }
+               if (ferror(fp)) {
+                       fprintf(stderr, "%s: reading %s: read failed\n", progname, FUSE_CONF);
+                       exit(1);
+               }
                fclose(fp);
        } else if (errno != ENOENT) {
+               bool fatal = (errno != EACCES && errno != ELOOP &&
+                             errno != ENAMETOOLONG && errno != ENOTDIR &&
+                             errno != EOVERFLOW);
                fprintf(stderr, "%s: failed to open %s: %s\n",
                        progname, FUSE_CONF, strerror(errno));
+               if (fatal)
+                       exit(1);
        }
 }