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

["Mediouni, Mohamed" <mediou@amazon.de>]

> On 11. Oct 2024, at 14:04, David Hildenbrand <david@redhat.com> wrote:
> 
> On 10.10.24 17:52, 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.
>>> 
>>> I'm a bit lost on what exactly we want to achieve. The point where we
>>> start flipping user/supervisor flags confuses me :)
>>> 
>>> With secretmem, you'd get memory allocated that
>>> (a) Is accessible by user space -- mapped into user space.
>>> (b) Is inaccessible by kernel space -- not mapped into the direct map
>>> (c) GUP will fail, but copy_from / copy_to user will work.
>>> 
>>> 
>>> Another way, without secretmem, would be to consider these "secrets"
>>> kernel allocations that can be mapped into user space using mmap() of a
>>> special fd. That is, they wouldn't have their origin in secretmem, but
>>> in KVM as a kernel allocation. It could be achieved by using VM_MIXEDMAP
>>> with vm_insert_pages(), manually removing them from the directmap.
>>> 
>>> But, I am not sure who is supposed to access what. Let's explore the
>>> requirements. I assume we want:
>>> 
>>> (a) Pages accessible by user space -- mapped into user space.
>>> (b) Pages inaccessible by kernel space -- not mapped into the direct map
>>> (c) GUP to fail (no direct map).
>>> (d) copy_from / copy_to user to fail?
>>> 
>>> And on top of that, some way to access these pages on demand from kernel
>>> space? (temporary CPU-local mapping?)
>>> 
>>> Or how would the kernel make use of these allocations?
>>> 
>>> -- 
>>> Cheers,
>>> 
>>> David / dhildenb
>> Hi David,
> 
> Hi Fares!
> 
>> Thanks for taking a look at the patches!
>> We're trying to allocate a kernel memory that is accessible to the kernel but
>> only when the context of the process is loaded.
>> So this is a kernel memory that is not needed to operate the kernel itself, it
>> is to store & process data on behalf of a process. The requirement for this
>> memory is that it would never be touched unless the process is scheduled on this
>> core. otherwise any other access will crash the kernel.
>> So this memory should only be directly readable and writable by the kernel, but
>> only when the process context is loaded. The memory shouldn't be readable or
>> writable by the owner process at all.
>> This is basically done by removing those pages from kernel linear address and
>> attaching them only in the process mm_struct. So during context switching the
>> kernel loses access to the secret memory scheduled out and gain access to the
>> new process secret memory.
>> This generally protects against speculation attacks, and if other process managed
>> to trick the kernel to leak data from memory. In this case the kernel will crash
>> if it tries to access other processes secret memory.
>> Since this memory is special in the sense that it is kernel memory but only make
>> sense in the term of the owner process, I tried in this patch series to explore
>> the possibility of reusing memfd_secret() to allocate this memory in user virtual
>> address space, manage it in a VMA, flipping the permissions while keeping the
>> control of the mapping exclusively with the kernel.
>> Right now it is:
>> (a) Pages not accessible by user space -- even though they are mapped into user
>>     space, the PTEs are marked for kernel usage.
> 
> Ah, that is the detail I was missing, now I see what you are trying to achieve, thanks!
> 
> It is a bit architecture specific, because ... imagine architectures that have separate kernel+user space page table hierarchies, and not a simple PTE flag to change access permissions between kernel/user space.
> 
> IIRC s390 is one such architecture that uses separate page tables for the user-space + kernel-space portions.
> 
>> (b) Pages accessible by kernel space -- even though they are not mapped into the
>>     direct map, the PTEs in uvaddr are marked for kernel usage.
>> (c) copy_from / copy_to user won't fail -- because it is in the user range, but
>>     this can be fixed by allocating specific range in user vaddr to this feature
>>     and check against this range there.
>> (d) The secret memory vaddr is guessable by the owner process -- that can also
>>     be fixed by allocating bigger chunk of user vaddr for this feature and
>>     randomly placing the secret memory there.
>> (e) Mapping is off-limits to the owner process by marking the VMA as locked,
>>     sealed and special.
> 
> Okay, so in this RFC you are jumping through quite some hoops to have a kernel allocation unmapped from the direct map but mapped into a per-process page table only accessible by kernel space. :)
> 
> So you really don't want this mapped into user space at all (consequently, no GUP, no access, no copy_from_user ...). In this RFC it's mapped but turned inaccessible by flipping the "kernel vs. user" switch.
> 
>> Other alternative (that was implemented in the first submission) is to track those
>> allocations in a non-shared kernel PGD per process, then handle creating, forking
>> and context-switching this PGD.
> 
> That sounds like a better approach. So we would remove the pages from the shared kernel direct map and map them into a separate kernel-portion in the per-MM page tables?
> 
> Can you envision that would also work with architectures like s390x? I assume we would not only need the per-MM user space page table hierarchy, but also a per-MM kernel space page table hierarchy, into which we also map the common/shared-among-all-processes kernel space page tables (e.g., directmap).
Yes, that’s also applicable to arm64. There’s currently no separate per-mm user space page hierarchy there.
>> What I like about the memfd_secret() approach is the simplicity and being arch
>> agnostic, what I don't like is the increased attack surface by using VMAs to
>> track those allocations.
> 
> Yes, but memfd_secret() was really design for user space to hold secrets. But I can see how you came to this solution.
> 
>> I'm thinking of working on a PoC to implement the first approach of using a
>> non-shared kernel PGD for secret memory allocations on arm64. This includes
>> adding kernel page table per process where all PGDs are shared but one which
>> will be used for secret allocations mapping. And handle the fork & context
>> switching (TTBR1 switching(?)) correctly for the secret memory PGD.
>> What do you think? I'd really appreciate opinions and possible ways forward.
> 
> Naive question: does arm64 rather resemble the s390x model or the x86-64 model?
arm64 has separate page tables for kernel and user-mode. Except for the KPTI case, the kernel page tables aren’t swapped per-process and stay the same all the time.

Thanks,
-Mohamed
> -- 
> Cheers,
> 
> David / dhildenb
> 




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