Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK

[Andy Lutomirski <luto@amacapital.net>]

> On Nov 5, 2019, at 9:02 AM, Daniel Colascione <dancol@google.com> wrote:
> 
> On Tue, Nov 5, 2019 at 8:56 AM Andrea Arcangeli <aarcange@redhat.com> wrote:
>> 
>>> On Tue, Nov 05, 2019 at 08:39:26AM -0800, Daniel Colascione wrote:
>>> I'm not suggesting that we fail userfaultfd(2) without CAP_SYS_PTRACE.
>>> That would, as you point out, break things. I'm talking about
>>> recording *whether* we had CAP_SYS_PTRACE in an internal flag in the
>>> uffd context when we create the thing --- and then, at ioctl time,
>>> checking that flag, not the caller's CAP_SYS_PTRACE, to see whether
>>> UFFD_FEATURE_EVENT_FORK should be made available. This way, the
>>> security check hinges on whether the caller *at create time* was
>>> privileged.
>> 
>> Until now it wasn't clear to me you still wanted to do the permission
>> check in UFFDIO_API time, and you only intended to move the
>> "measurement" of the capability to the syscall.
>> 
>> So you're suggesting to add more kernel complexity to code pending for
>> removal to achieve a theoretically more pure solution in the band-aid
>> required to defer the removal of the posix-breaking read
>> implementation of the uffd fork feature?
> 
> And you're suggesting making a security check work weirdly unlike most
> other security checks because you hope it'll get removed one day?
> Temporary solutions aren't, and if something goes into the kernel at
> all, it's worth getting right. The general rule is that access checks
> happen at open time. The kernel has already been bitten by UFFD
> exempting itself from the normal rules (e.g., the
> read(2)-makes-a-file-descriptor thing) in the name of expediency.
> There shouldn't be any more exceptions.

I don’t think ioctl() checking permission is particularly unusual. In principle, it’s better than open for a retrofit — open didn’t capture this permission in the past, so adding it makes an existing capability stronger than it was, which isn’t fantastic.