Re: [RFC PATCH v2 7/8] mseal:Check seal flag for mmap(2)

[Jeff Xu]

Hi Linus,

On Tue, Oct 17, 2023 at 10:43 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, 17 Oct 2023 at 10:04, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Honestly, there is only two kinds of sealing that makes sense:
> >
> >  - you cannot change the permissions of some area
> >
> >  - you cannot unmap an area
>
> Actually, I guess at least theoretically, there could be three different things:
>
>  - you cannot move an area
>
Yes.

Actually, the newly added selftest covers some of the above:
1. can't change the permission of some areas.
test_seal_mprotect
test_seal_mmap_overwrite_prot

2. can't unmap an area (thus mmap() to the same address later)
test_seal_munmap

3. can't move to an area:
test_seal_mremap_move         //can't move from a sealed area:
test_seal_mremap_move_fixed_zero //can't move from a sealed area to a
fixed address
test_seal_mremap_move_fixed   //can't move to a sealed area.

4 can't expand or shrink the area:
test_seal_mremap_shrink
test_seal_mremap_expand

> although I do think that maybe just saying "you cannot unmap" might
> also include "you cannot move".
>
> But I guess it depends on whether you feel it's the virtual _address_
> you are protecting, or whether it's the concept of mapping something.
>
> I personally think that from a security perspective, what you want to
> protect is a particular address. That implies that "seal from
> unmapping" would thus also include "you can't move this area
> elsewhere".
>
> But at least conceptually, splitting "unmap" and "move" apart might
> make some sense. I would like to hear a practical reason for it,
> though.
>
> Without that practical reason, I think the only two sane sealing operations are:
>
>  - SEAL_MUNMAP: "don't allow this mapping address to go away"
>
>    IOW no unmap, no shrinking, no moving mremap
>
>  - SEAL_MPROTECT: "don't allow any mapping permission changes"
>
I agree with the concept in general. The separation of two seal types
is easy to understand.

For mmap(MAP_FIXED), I know for a fact that it can modify permission of
an existing mapping, (as in selftest:test_seal_mmap_overwrite_prot).
I think it can also expand an existing VMA. This is not a problem, code-wise,
I mention it here, because it needs extra care when coding mmap() change.

> Again, that permission case might end up being "don't allow
> _additional_ permissions" and "don't allow taking permissions away".
> Or it could be split by operation (ie "don't allow permission changes
> to writability / readability / executability respectively").
>
Yes. If the application desires this, it can also be done.
i.e. seal of X bit, or seal of W bit, this will be similar to file sealing.
I discussed this with Stephan before, at this point of time,  Chrome
doesn't have a use case.

> I suspect there isn't a real-life example of splitting the
> SEAL_MPROTECT (the same way I doubt there's a real-life example for
> splitting the UNMAP into "unmap vs move"), so unless there is some
> real reason, I'd keep the sealing minimal and to just those two flags.
>
I think two seal-type (permission and unmap/move/expand/shrink)
will work for the Chrome case. Stephen Röttger is an expert in Chrome,
on vacation/ be back soon. I will wait for Stephen to confirm.

> We could always add more flags later, if there is a real use case
> (IOW, if we start with "don't allow any permission changes", we could
> add a flag later that just says "don't allow writability changes").
>
Agreed 100%, thanks for understanding.

-Jeff


>                Linus