Re: [RFC PATCH 0/7] support for mm-local memory allocations and use it

Alexander Graf
2024-09-20T13:19:44Z

On 11.09.24 16:33, Fares Mehanna wrote:
> In a series posted a few years ago [1], a proposal was put forward to allow the
> kernel to allocate memory local to a mm and thus push it out of reach for
> current and future speculation-based cross-process attacks. We still believe
> this is a nice thing to have.
>
> However, in the time passed since that post Linux mm has grown quite a few new
> goodies, so we'd like to explore possibilities to implement this functionality
> with less effort and churn leveraging the now available facilities.
>
> An RFC was posted few months back [2] to show the proof of concept and a simple
> test driver.
>
> In this RFC, we're using the same approach of implementing mm-local allocations
> piggy-backing on memfd_secret(), using regular user addresses but pinning the
> pages and flipping the user/supervisor flag on the respective PTEs to make them
> directly accessible from kernel.
> In addition to that we are submitting 5 patches to use the secret memory to hide
> the vCPU gp-regs and fp-regs on arm64 VHE systems.
>
> The generic drawbacks of using user virtual addresses mentioned in the previous
> RFC [2] still hold, in addition to a more specific one:
>
> - While the user virtual addresses allocated for kernel secret memory are not
> directly accessible by userspace as the PTEs restrict that, copy_from_user()
> and copy_to_user() can operate on those ranges, so that e.g. the usermode can
> guess the address and pass it as the target buffer for read(), making the
> kernel overwrite it with the user-controlled content. Effectively making the
> secret memory in the current implementation missing confidentiality and
> integrity guarantees.
>
> In the specific case of vCPU registers, this is fine because the owner process
> can read and write to them using KVM IOCTLs anyway. But in the general case this
> represents a security concern and needs to be addressed.
>
> A possible way forward for the arch-agnostic implementation is to limit the user
> virtual addresses used for kernel to specific range that can be checked against
> in copy_from_user() and copy_to_user().
>
> For arch specific implementation, using separate PGD is the way to go.
>
> [1] https://lore.kernel.org/lkml/20190612170834.14855-1-mhillenb@amazon.de/
> [2] https://lore.kernel.org/lkml/20240621201501.1059948-1-rkagan@amazon.de/

Hey Mark and Mike,

We talked at LPC about mm-local memory and you had some inputs. It would
be amazing to write them down here so I don't end up playing game of
telephone :)

Thanks!