Re: [PATCH v4 0/5] userfaultfd: add /dev/userfaultfd for fine grained access control

[Nadav Amit <namit@vmware.com>]

On Aug 1, 2022, at 10:13 AM, Axel Rasmussen <axelrasmussen@google.com> wrote:

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> 
> I finished up some other work and got around to writing a v5 today,
> but I ran into a problem with /proc/[pid]/userfaultfd.
> 
> Files in /proc/[pid]/* are owned by the user/group which started the
> process, and they don't support being chmod'ed.
> 
> For the userfaultfd device, I think we want the following semantics:
> - For UFFDs created via the device, we want to always allow handling
> kernel mode faults
> - For security, the device should be owned by root:root by default, so
> unprivileged users don't have default access to handle kernel faults
> - But, the system administrator should be able to chown/chmod it, to
> grant access to handling kernel faults for this process more widely.
> 
> It could be made to work like that but I think it would involve at least:
> 
> - Special casing userfaultfd in proc_pid_make_inode
> - Updating setattr/getattr for /proc/[pid] to meaningfully store and
> then retrieve uid/gid different from the task's, again probably
> special cased for userfautlfd since we don't want this behavior for
> other files
> 
> It seems to me such a change might raise eyebrows among procfs folks.
> Before I spend the time to write this up, does this seem like
> something that would obviously be nack'ed?

[ Please avoid top-posting in the future ]

I have no interest in making your life harder than it should be. If you
cannot find a suitable alternative, I will not fight against it.

How about this alternative: how about following KVM usage-model?

IOW: You open /dev/userfaultfd, but this is not the file-descriptor that you
use for most operations. Instead you first issue an ioctl - similarly to
KVM_CREATE_VM - to get a file-descriptor for your specific process. You then
use this new file-descriptor to perform your operations (read/ioctl/etc).

This would make the fact that ioctls/reads from different processes refer to
different contexts (i.e., file-descriptors) much more natural.

Does it sound better?