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List:       fedora-devel-list
Subject:    Re: Restricting automounting of uncommon filesystems?
From:       Demi Marie Obenour <demiobenour () gmail ! com>
Date:       2023-07-24 14:08:50
Message-ID: 2cfef8e9-5aa1-cd9f-cc96-f1d7b06a3acf () gmail ! com
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On 7/24/23 08:47, Richard W.M. Jones wrote:
> On Sun, Jul 23, 2023 at 11:18:45PM -0400, Demi Marie Obenour wrote:
>> On 7/23/23 12:10, Solomon Peachy via devel wrote:
>>> On Sun, Jul 23, 2023 at 11:25:12AM -0400, Neal Gompa wrote:
>>>>> If the system administrator wants to mount $UNCOMMONFS, they should be
>>>>> able to do so without hassle, but that doesn't mean that a normal user
>>>>> who got handed a sketchy USB stick at a conference should be able to do
>>>>> so with no restrictions at all.
>>>>>
>>>>
>>>> So then some kind of configuration to udisks2 to have a similar effect?
>>>
>>> And we're right back at square one, with the *overwhelmingly* common case 
>>> of a single-user system whose "admin" is sitting in front of the system.
>>>
>>> Of _course_ they want to mount the disk.  It's why they plugged it in, 
>>> and they don't give two hoots if it's a "common filesystem" or not.
>>>
>>> (FFS, most of the stuff I personally plug in these days is ext4 or ntfs, 
>>> because fat32 sucks and I can't rely on exfat on all systems I need to 
>>> interoperate with)
>>>
>>> And let's be realistic here -- the overwhelmingly common threat model 
>>> here is that there are untrusted files on a correctly-formed filesystem.  
>>> Bad guys rarely need or care to get root on the system; what they're 
>>> after requires normal, non-elevated user permissions.
>>>
>>> Prompting users 'are you sure you want to use this device' will turn a 
>>> "yes" into an automatic reflex.  Not automounting by default will just 
>>> add another thing to the "things to change on default fedora 
>>> installations" lists out there (ie right after the "enable freshrpms and 
>>> install modern video codecs" step), becuase it's a usability nightmare.
>>>
>>> In the "usability vs security" tradeoff, usability/convenience *always* 
>>> wins unless you're at a place that has armed guards at the door with 
>>> instructions to shoot first.
>>>
>>>  - Solomon
>>
>> Then the mount needs to be done in a sandbox, such as a KVM guest or
>> sandboxed userspace process.
> 
> This is what libguestfs does (KVM guest).
> 
> Rich.

I saw that libguestfs has a guestmount(1) tool, and I think this could be
a potential solution.  An exploit against the kernel FS driver would only
grant access to a KVM guest, and the QEMU process can be tightly sandboxed
by means such as seccomp and SELinux.

I still believe that mounting should _not_ be automatic, though, because
it could have side-effects (such as replaying the FS journal) that might
not be wanted.  To prevent prompt fatigue, Fedora could offer to remember
the user's choice.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
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