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I'm not sure why mount is installed with SUID bit on many distributions.

In graphical environment, it doesn't require user to type the mount command to mount a partition. Normally udisks will communicate with another daemon to mount the drive.

And on a server, I don't see the necessity as well.

So, is there any reason to have SUID bit set?

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2 Answers 2

up vote 13 down vote accepted

mount() requires root (or CAP_SYS_ADMIN on Linux), but it is possible to specify a mountpoint in /etc/fstab that is allowed to be mounted by a user by using the users option. To facilitate this, they need to elevate to the superuser account to be able to execute mount() successfully.

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It will depend on the options used when the drives are mounted.

Have a look at /etc/fstab, if the entry for the partitions in question has defaults set as an option then this implies that suid is set, from man mount[1]...

defaults
              Use default options: rw, suid, dev, exec, auto, nouser, and async.

You can over-ride this by specifying alternative options in /etc/fstab, for example you can specify users as an option (although I was caught out by the fact that this implies noexec so I couldn't run any scripts and had to over-ride this), so have this in my /etc/fstab:

LABEL=home      /home       ext4    noatime,users,exec  0 4

[1] I was reading man mount and man fstab yesterday after having a problem with default options myself.

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As far as I can tell, the question is about the mount binary having the setuid bit set, not about allowing suid files on the mounted filesystem, which is what the suid option does. –  Chris Down Jan 2 at 8:58
    
I agree, and your post (which was made whilst I was typing out mine) answers this directly (and is a depth of knowledge I don't have). I thought that what I wrote would be useful to leave up as it provides some sort of practical work around (including the same solution of adding users in /etc/fstab but also highlights something which bit me yesterday which was the default options that users implies prevent execution and can over-ridden with exec). Happy for it to be deleted though. –  slackline Jan 2 at 11:17
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