Re: [PATCH v2 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl

[Florent Revest]

On Tue, May 23, 2023 at 6:36 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
> On Wed, May 17, 2023 at 05:03:20PM +0200, Florent Revest wrote:
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index 339fee3eff6a..320eae3b12ab 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -2368,9 +2368,25 @@ static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
> >       if (arg3 || arg4 || arg5)
> >               return -EINVAL;
> >
> > -     if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN))
> > +     if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT))
> >               return -EINVAL;
> >
> > +     /* NO_INHERIT only makes sense with REFUSE_EXEC_GAIN */
> > +     if (bits & PR_MDWE_NO_INHERIT && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
> > +             return -EINVAL;
> > +
> > +     /* Can't gain NO_INHERIT from !NO_INHERIT */
> > +     if (bits & PR_MDWE_NO_INHERIT &&
> > +         test_bit(MMF_HAS_MDWE, &current->mm->flags) &&
> > +         !test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags))
> > +             return -EPERM;
> > +
> > +     if (bits & PR_MDWE_NO_INHERIT)
> > +             set_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags);
> > +     else if (test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags)
> > +              && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
> > +             return -EPERM; /* Cannot unset the flag */

Ugh, I had to proofread this 3 times to figure out what I was trying
to do, not great. :(

> Is this about not unsetting the MMF_HAS_MDWE bit? We already have a
> check further down that covers this case.

Actually, this was about not being able to unset _both_ NO_INHERIT and
HAS_MDWE (this would gain capabilities)... While still being able to
unset NO_INHERIT only (this reduces capabilities)

> Related to this, do we want to allow unsetting MMF_HAS_MDWE_NO_INHERIT?
> It looks like it can't be unset but no error either.

But - sigh - as you point out, that second part wasn't implemented. It
looks like I intended to add an:

else
  clear_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags);

after that block but forgot... The idea was that:
- setting no_inherit is always allowed
- unsetting it is allowed iff we don't also unset REFUSE_EXEC_GAIN

The "consecutive_prctl_flags" selftests in patch 5 were supposed to
make the assumptions here easier to read and verify. This logic was
meant to be covered by  "cant_disable_mdwe_no_inherit" and
"can_lower_privileges" but these tests only check that the "set" prctl
succeeds and not that the end result is the expected one so I missed
this. I'll improve the selftest in the next revision too so we can
catch this more easily next time.

> The above check,
> IIUC, looks more like ensuring we don't clear MMF_HAS_MDWE.

Indeed, without the else, it was useless.

> Maybe we should tighten the logic here a bit and not allow any changes
> after the initial flag setting:
>
> current->mm->flags == 0, we allow:
>         bits == 0 or
>         bits == PR_MDWE_REFUSE_EXEC_GAIN or
>         bits == PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT
>
> current->mm->flags != 0 (some bits were set), we only allow the exactly
> the same bit combination or -EPERM.
>
> So basically build the flags based on the PR_* input bits and compare
> them with current->mm->flags when not 0, return -EPERM if different. I
> think this preserves the ABI as we only have a single bit currently and
> hopefully makes the logic here easier to parse.

On one hand, I thought it would be nice to have the ability to
transition from NO_INHERIT | REFUSE_EXEC_GAIN to REFUSE_EXEC_GAIN
because, conceptually, it seems to me that we shouldn't prevent a
process from dropping further capabilities.

On the other hand, I don't think I have an actual need for that
transition and I agree that if we don't try to handle it, the code
here would become a lot easier to reason about. I'd certainly sleep
better at night with the smaller likelihood of having screwed
something up... :)

Unless someone feels strongly otherwise, I'll do what you suggest in v3

> > @@ -2385,8 +2401,10 @@ static inline int prctl_get_mdwe(unsigned long arg2, unsigned long arg3,
> >       if (arg2 || arg3 || arg4 || arg5)
> >               return -EINVAL;
> >
> > -     return test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
> > -             PR_MDWE_REFUSE_EXEC_GAIN : 0;
> > +     return (test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
> > +             PR_MDWE_REFUSE_EXEC_GAIN : 0) |
> > +             (test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags) ?
> > +             PR_MDWE_NO_INHERIT : 0);
> >  }
>
> Just personal preference, use explicit 'if' blocks and add bits to a
> local variable variable than multiple ternary operators.

Will do!