Re: [RFC PATCH v4 00/12] hardening: statically allocated protected memory

[Igor Stoppa]

On 12/02/2019 02:09, Kees Cook wrote:
> On Mon, Feb 11, 2019 at 3:28 PM Igor Stoppa <igor.stoppa@xxxxxxxxx> wrote:

[...]

> > Patch-set implementing write-rare memory protection for statically
> > allocated data.
>
> It seems like this could be expanded in the future to cover dynamic
> memory too (i.e. just a separate base range in the mm).

Indeed. And part of the code refactoring is also geared in that 
direction. I am working on that part, but it was agreed that I would 
first provide this subset of features covering statically allocated 
memory. So I'm sticking to the plan. But this is roughly 1/3 of the 
basic infra I have in mind.

> > Its purpose is to keep write protected the kernel data which is seldom
> > modified, especially if altering it can be exploited during an attack.
> >
> > There is no read overhead, however writing requires special operations that
> > are probably unsuitable for often-changing data.
> > The use is opt-in, by applying the modifier __wr_after_init to a variable
> > declaration.
> >
> > As the name implies, the write protection kicks in only after init() is
> > completed; before that moment, the data is modifiable in the usual way.
> >
> > Current Limitations:
> > * supports only data which is allocated statically, at build time.
> > * supports only x86_64 and arm64;other architectures need to provide own
> >    backend
>
> It looked like only the memset() needed architecture support. Is there
> a reason for not being able to implement memset() in terms of an
> inefficient put_user() loop instead? That would eliminate the need for
> per-arch support, yes?

So far, yes, however from previous discussion about power arch, I 
understood this implementation would not be so easy to adapt.
Lacking other examples where the extra mapping could be used, I did not 
want to add code without a use case.

Probably both arm and x86 32 bit could do, but I would like to first get 
to the bitter end with memory protection (the other 2 thirds).

Mostly, I hated having just one arch and I also really wanted to have arm64.

But eventually, yes, a generic put_user() loop could do, provided that 
there are other arch where the extra mapping to user space would be a 
good way to limit write access. This last part is what I'm not sure of.

> > - I've added a simple example: the protection of ima_policy_flags
>
> You'd also looked at SELinux too, yes? What other things could be
> targeted for protection? (It seems we can't yet protect page tables
> themselves with this...)

Yes, I have. See the "1/3" explanation above. I'm also trying to get 
away with as small example as possible, to get the basic infra merged.
SELinux is not going to be a small patch set. I'd rather move to it once 
at least some of the framework is merged. It might be a good use case 
for dynamic allocation, if I do not find something smaller.
But for static write rare, going after IMA was easier, and it is still a 
good target for protection, imho, as flipping this variable should be 
sufficient for turning IMA off.

For the page tables, I have in mind a little bit different approach, 
that I hope to explain better once I get to do the dynamic allocation.

> > - the x86_64 user space address range is double the size of the kernel
> >    address space, so it's possible to randomize the beginning of the
> >    mapping of the kernel address space, but on arm64 they have the same
> >    size, so it's not possible to do the same
>
> Only the wr_rare section needs mapping, though, yes?

Yup, however, once more, I'm not so keen to do what seems as premature 
optimization, before I have addressed the framework in its entirety, as 
the dynamic allocation will need similar treatment.

> > - I'm not sure if it's correct, since it doesn't seem to be that common in
> >    kernel sources, but instead of using #defines for overriding default
> >    function calls, I'm using "weak" for the default functions.
>
> The tradition is to use #defines for easier readability, but "weak"
> continues to be a thing. *shrug*

Yes, I wasn't so sure about it, but I kinda liked the fact that, by 
using "weak", the arch header becomes optional, unless one has to 
redefine the struct wr_state.

> This will be a nice addition to protect more of the kernel's static
> data from write-what-where attacks. :)

I hope so :-)

--
thanks, igor