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

[Fares Mehanna]

>> 
>> 
>>> On 11. Oct 2024, at 14:36, Mediouni, Mohamed <mediou@xxxxxxxxx> wrote:
>>>
>>>
>>>
>>>> On 11. Oct 2024, at 14:04, David Hildenbrand <david@xxxxxxxxxx> 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.
>> typo, read kernel
> 
> 
> Okay, thanks. So going into that direction makes more sense.
> 
> I do wonder if we really have to deal with fork() ... if the primary 
> users don't really have meaning in the forked child (e.g., just like 
> fork() with KVM IIRC) we might just get away by "losing" these 
> allocations in the child process.
> 
> Happy to learn why fork() must be supported.

It really depends on the use cases of the kernel secret allocation, but in my
mind a troubling scenario:
1. Process A had a resource X.
2. Kernel decided to keep some data related to resource X in process A secret
   memory.
3. Process A decided to fork, now process B share the resource X.
4. Process B started using resource X. <-- This will crash the kernel as the
   used kernel page table on process B has no mapping for the secret memory used
   in resource X.

I haven't tried to trigger this crash myself though.

I didn't think in depth about this issue yet, but I need to because duplicating
the secret memory mappings in the new forked process is easy (To give kernel
access on the secret memory), but tearing them down across all forked processes
is a bit complicated (To clean stale mappings on parent/child processes). Right
now tearing down the mapping will only happen on mm_struct which allocated the
secret memory.

Thanks!
Fares.



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