dissect-image: let's lock down fstypes a bit

When we dissect images automatically, let's be a bit more conservative
with the file system types we are willing to mount: only mount common
file systems automatically.

Explicit mounts requested by admins should always be OK, but when we do
automatic mounts, let's not permit barely maintained, possibly legacy
file systems.

The list for now covers the four common writable and two common
read-only file systems. Sooner or later we might want to add more to the
list.

Also, it might make sense to eventually make this configurable via the
image dissection policy logic.

3 files changed, 51 insertions, 0 deletions

diff --git a/src/gpt-auto-generator/gpt-auto-generator.c b/src/gpt-auto-generator/gpt-auto-generator.c
index d697a9b362..1d626f2a5a 100644
--- a/src/gpt-auto-generator/gpt-auto-generator.c
+++ b/src/gpt-auto-generator/gpt-auto-generator.c
@@ -170,6 +170,15 @@ static int add_mount(
 
                 what = crypto_what;
                 fstype = NULL;
+        } else if (fstype) {
+                r = dissect_fstype_ok(fstype);
+                if (r < 0)
+                        return log_error_errno(r, "Unable to determine of dissected file system type '%s' is permitted: %m", fstype);
+                if (!r)
+                        return log_error_errno(
+                                        SYNTHETIC_ERRNO(EIDRM),
+                                        "Refusing to automatically mount uncommon file system '%s' to '%s'.",
+                                        fstype, where);
         }
 
         r = unit_name_from_path(where, ".mount", &unit);
diff --git a/src/shared/dissect-image.c b/src/shared/dissect-image.c
index 4351f185cd..100b8cc8f1 100644
--- a/src/shared/dissect-image.c
+++ b/src/shared/dissect-image.c
@@ -74,6 +74,38 @@
 /* how many times to wait for the device nodes to appear */
 #define N_DEVICE_NODE_LIST_ATTEMPTS 10
 
+int dissect_fstype_ok(const char *fstype) {
+        const char *e;
+        bool b;
+
+        /* When we automatically mount file systems, be a bit conservative by default what we are willing to
+         * mount, just as an extra safety net to not mount with badly maintained legacy file system
+         * drivers. */
+
+        e = secure_getenv("SYSTEMD_DISSECT_FILE_SYSTEMS");
+        if (e) {
+                _cleanup_strv_free_ char **l = NULL;
+
+                l = strv_split(e, ":");
+                if (!l)
+                        return -ENOMEM;
+
+                b = strv_contains(l, fstype);
+        } else
+                b = STR_IN_SET(fstype,
+                               "btrfs",
+                               "erofs",
+                               "ext4",
+                               "squashfs",
+                               "vfat",
+                               "xfs");
+        if (b)
+                return true;
+
+        log_debug("File system type '%s' is not allowed to be mounted as result of automatic dissection.", fstype);
+        return false;
+}
+
 int probe_filesystem_full(
                 int fd,
                 const char *path,
@@ -1339,6 +1371,11 @@ static int mount_partition(
 
         if (!fstype)
                 return -EAFNOSUPPORT;
+        r = dissect_fstype_ok(fstype);
+        if (r < 0)
+                return r;
+        if (!r)
+                return -EIDRM; /* Recognizable error */
 
         /* We are looking at an encrypted partition? This either means stacked encryption, or the caller
          * didn't call dissected_image_decrypt() beforehand. Let's return a recognizable error for this
@@ -1488,6 +1525,7 @@ int dissected_image_mount(
          *  -EUCLEAN      → fsck for file system failed
          *  -EBUSY        → File system already mounted/used elsewhere (kernel)
          *  -EAFNOSUPPORT → File system type not supported or not known
+         *  -EIDRM        → File system is not among allowlisted "common" file systems
          */
 
         if (!(m->partitions[PARTITION_ROOT].found ||
@@ -1625,6 +1663,8 @@ int dissected_image_mount_and_warn(
                 return log_error_errno(r, "File system already mounted elsewhere.");
         if (r == -EAFNOSUPPORT)
                 return log_error_errno(r, "File system type not supported or not known.");
+        if (r == -EIDRM)
+                return log_error_errno(r, "File system is too uncommon, refused.");
         if (r < 0)
                 return log_error_errno(r, "Failed to mount image: %m");
 
diff --git a/src/shared/dissect-image.h b/src/shared/dissect-image.h
index 37a41e050a..b65193f887 100644
--- a/src/shared/dissect-image.h
+++ b/src/shared/dissect-image.h
@@ -183,3 +183,5 @@ bool dissected_image_verity_sig_ready(const DissectedImage *image, PartitionDesi
 int mount_image_privately_interactively(const char *path, DissectImageFlags flags, char **ret_directory, LoopDevice **ret_loop_device);
 
 int verity_dissect_and_mount(int src_fd, const char *src, const char *dest, const MountOptions *options, const char *required_host_os_release_id, const char *required_host_os_release_version_id, const char *required_host_os_release_sysext_level, const char *required_sysext_scope);
+
+int dissect_fstype_ok(const char *fstype);