Re: [PATCH v4 2/5] userfaultfd: add /dev/userfaultfd for fine grained access control

[Nadav Amit <namit@vmware.com>]

On Jul 19, 2022, at 12:56 PM, Axel Rasmussen <axelrasmussen@google.com> wrote:

> Historically, it has been shown that intercepting kernel faults with
> userfaultfd (thereby forcing the kernel to wait for an arbitrary amount
> of time) can be exploited, or at least can make some kinds of exploits
> easier. So, in 37cd0575b8 "userfaultfd: add UFFD_USER_MODE_ONLY" we
> changed things so, in order for kernel faults to be handled by
> userfaultfd, either the process needs CAP_SYS_PTRACE, or this sysctl
> must be configured so that any unprivileged user can do it.
> 
> In a typical implementation of a hypervisor with live migration (take
> QEMU/KVM as one such example), we do indeed need to be able to handle
> kernel faults. But, both options above are less than ideal:
> 
> - Toggling the sysctl increases attack surface by allowing any
>  unprivileged user to do it.
> 
> - Granting the live migration process CAP_SYS_PTRACE gives it this
>  ability, but *also* the ability to "observe and control the
>  execution of another process [...], and examine and change [its]
>  memory and registers" (from ptrace(2)). This isn't something we need
>  or want to be able to do, so granting this permission violates the
>  "principle of least privilege".
> 
> This is all a long winded way to say: we want a more fine-grained way to
> grant access to userfaultfd, without granting other additional
> permissions at the same time.
> 
> To achieve this, add a /dev/userfaultfd misc device. This device
> provides an alternative to the userfaultfd(2) syscall for the creation
> of new userfaultfds. The idea is, any userfaultfds created this way will
> be able to handle kernel faults, without the caller having any special
> capabilities. Access to this mechanism is instead restricted using e.g.
> standard filesystem permissions.

Are there any other “devices" that when opened by different processes
provide such isolated interfaces in each process? I.e., devices that if you
read from them in different processes you get completely unrelated data?
(putting aside namespaces).

It all sounds so wrong to me, that I am going to try again to pushback
(sorry).

From a semantic point of view - userfaultfd is process specific. It is
therefore similar to /proc/[pid]/mem (or /proc/[pid]/pagemap and so on).

So why can’t we put it there? I saw that you argued against it in your
cover-letter, and I think that your argument is you would need
CAP_SYS_PTRACE if you want to access userfaultfd of other processes. But
this is EXACTLY the way opening /proc/[pid]/mem is performed - see
proc_mem_open().

So instead of having some strange device that behaves differently in the
context of each process, you can just have /proc/[pid]/userfaultfd and then
use mm_access() to check if you have permissions to access userfaultfd (just
like proc_mem_open() does). This would be more intuitive for users as it is
similar to other /proc/[pid]/X, and would cover both local and remote
use-cases.