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When I plug in a USB drive it is automatically mounted on /run/media/user/fslabel. This is done, I guess by some udev/dbus/thunar/nautilus/gvfs or other system, and I find this convenient and do not want to revert to manual mounting by root. However, I have a problem with the default mount options: vfat drives are mounted such that the executable flag ist set on all regular files. This is a nuissance and a security problem and I wish to disable it.

How do I set system-wide options for mounting, like using the noexec flag for all vfat partitions and disabling mounting of ext4 partitions by user-space programs/daemons?

A few years ago I tried something very time-consuming on a different system, like editing some udev or dbus rules (quite apparently not files designed to be edited by hand), which was a great effort due to lack of proper documentation and great variation between distros. Is this the intended and only way? If so, could someone please tell me what to change where?

I am using Arch Linux, CentOS and openSUSE with the XFCE Desktop. Automount may be performed by one of nautilus, thunar or dolphin, running in the background (or possibly, a service started by these?!). I am not sure because it happens in the background.

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  • On what unix variant/distribution? What desktop environment do you use? Do note that setting the executable flag is not a security problem except in extremely rare scenarios with restricted accounts or strange configurations that would somehow automatically execute executable files, but not read readable files, on removable media. – Gilles Jun 14 '15 at 13:34
  • Thanks @Gilles - I mostly use Arch Linux with XFCE, but also openSUSE and CentOS. I run instances of nautilus and/or dolphin and/or thunar just to use their automount function (I am not patient enough to copy or open files using these programs as navigation is so slow). Potential security problems arise due to rsyncing / cp -ping files to my own filesystem so that suddenly I have thousands of executable files which may even be writable to group/others (again due to mount options I did not set). I do consider this a security risk - but then this question was no question on security. – Ned64 Jun 14 '15 at 15:37
  • The Arch wiki suggests that when a partition of type VFAT is mounted the /usr/bin/mount.vfat script is executed. By the looks of it, you could manually edit that file to add whichever options you like. – Bridgey Jun 18 '15 at 16:48
  • Thanks @Bridgey , but I think that would have consequences on manual mounting (as root) of these drives. It's probably better to configure the calling process. – Ned64 Jun 18 '15 at 17:03
  • @don_crissti Thanks, I think you are on to something. Indeed a /usr/lib/udisks2/udisksd is running as root. This in turn is apparently automatically started by dbus-daemon. I am pretty sure one of them can be configured (e.g. with entries in /lib/udev/rules.d/, I think that is what I did last time -- /lib/udev/rules.d/80-udisks.rules?). wiki.archlinux.org/index.php/Udisks also has some hints, and I will dig deeper - unless someone knows the next steps? - - - - As to the second question, no, I meant external ext[34] and LUKS partitions (the user gets a password prompt?!). – Ned64 Jun 23 '15 at 18:24
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External media/drives mounting is handled by udisks2 on most modern distros. I don't think there's any trivial way to change the default mount options as they are hard-coded (see FSMountOptions in udiskslinuxfilesystem.c) that is, they're not configurable (at least not yet1). Your options are quite limited: unmount the partition and remount it with different mount options (unless you're willing to patch the source code or write your own automount tool).

As to your other question:

I think one may be able to disallow mounting by type, though, by the looks of the rules?! When I insert a USB (3.0) thumbdrive or HDD all ext[34] partitions get mounted (I wish they weren't) and the user gets a graphical prompt for any LUKS partition to unlock. I wish to disable both. A user may have FAT drives but others may only be mounted by root.

You could use a udev rule to ignore all USB thumbdrive partitions except vfat ones. Create a new rule file e.g. /usr/lib/udev/rules.d/90-ignore-nonvfat.rules with the following content:

SUBSYSTEM=="block", ENV{DEVTYPE}=="partition", ENV{ID_BUS}=="usb", ENV{ID_FS_TYPE}!="vfat", ENV{UDISKS_IGNORE}="1"

(replace UDISKS_IGNORE with UDISKS_PRESENTATION_HIDE if your distro uses udisks1).

1: see FreeDesktop ML for a proposed patch (and a long discussion).

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