Re: Why is /run/vm/by-id/${VM} writable by the VMM?

[Alyssa Ross <hi@alyssa.is>]

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Demi Marie Obenour <demiobenour@gmail.com> writes:

> On 7/21/26 12:05, Alyssa Ross wrote:
>> Demi Marie Obenour <demiobenour@gmail.com> writes:
>> 
>>> On 7/15/26 14:54, Alyssa Ross wrote:
>>>> Demi Marie Obenour <demiobenour@gmail.com> writes:
>>>>
>>>>> On 7/13/26 05:04, Alyssa Ross wrote:
>>>>>> Demi Marie Obenour <demiobenour@gmail.com> writes:
>>>>>>
>>>>>>> This seems to be a fairly significant hole in the VMM's sandbox.
>>>>>>> Various code operates in that directory and isn't secure against
>>>>>>> symlink attacks.
>>>>>>>
>>>>>>> Does the VMM really need write access to that directory?  Could it
>>>>>>> be given access to a subdirectory that is mounted "nosymfollow"?
>>>>>>
>>>>>> I think it's just so it can create e.g. the serial file.  That could
>>>>>> indeed be moved to a subdirectory.  The ideal solution is of course for
>>>>>> Cloud Hypervisor to be able to accept every file it would otherwise
>>>>>> create as an open file descriptor, which I expect will happen
>>>>>> eventually.
>>>>>
>>>>> Should it be moved to a subdirectory for now?  That would make the
>>>>> sandbox meaningful in practice.
>>>>
>>>> I suppose it could be.  Might make more sense to dedicate efforts
>>>> towards fixing Cloud Hypervisor to accept file descriptors for the ~2
>>>> paths it currently needs to create, though.  (That's obviously more
>>>> work, but also more useful.)  If we were to work around this in
>>>> Spectrum, we could keep symlinks outside the directory pointing into it,
>>>> to keep paths nice and avoid churn once we can put them back in the
>>>> parent directory.
>>>
>>> Do you happen to know which paths these are?
>> 
>> I think it might just be "serial"?  Not sure.
>
> How difficult would it be to modify Cloud Hypervisor so this is passed
> as a file descriptor?

I don't expect it would be very hard.  It's recently been done for
e.g. VFIO FDs, but I also don't think I have any special knowledge here
that you don't?

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