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

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

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