Re: [RFC PATCH v4 2/4] x86/sgx: Implement support for MADV_WILLNEED

Huang, Kai
2023-06-27T11:43:22Z

> > >
> > > You can't build an ABI on assumptions. E.g. even if userspace *intends* to behave,
> > > it wouldn't take much to compute the wrong offset (math is hard).
> >
> > But I don't think we have an well established ABI now? Nothing is documented.
>
> Heh, just because the ABI isn't formally documented doesn't mean it doesn't exist.
> In fact, the most problematic ABIs are the ones that aren't documented.
>
> >

Sure. But just want to make sure, what is the current SGX driver ABI (around
mmap()) from your perspective?

Is it "SGX driver _requires_ pgoff to be 0 for non-ELRANGE-reserve mmap()", or
"SGX driver _ignores_ pgoff"?

See below ...

[...]

> > > if (encl->has_mismatched_offsets) <======
> > > goto unlock;
> >
> > Sorry I am a little bit slow, how do you set "has_mismatched_offsests" to true?
>
> During sgx_mmap(). Though there's a wrinkle I initially missed: if the enclave
> hasn't gone through ECREATE, encl->base is garbage. So either sgx_mmap() needs
> to assume the !CREATED is creating a mismatched offset, or sgx_encl_create() needs
> to iterate over and check all VMAs.
>
> Since there are advantages to usuing mmap(NULL) to find ELRANGE, IMO your best
> option is to do the below. And then that mostly answers the question about
> using mmap(/dev/sgx_enclave) to reserve ELRANGE, i.e. if userspace wants to use
> fallocate(), then it effectively *must not* use mmap(/dev/sgx_enclave).
>
> diff --git a/arch/x86/kernel/cpu/sgx/driver.c b/arch/x86/kernel/cpu/sgx/driver.c
> index 262f5fb18d74..63fb41da35aa 100644
> --- a/arch/x86/kernel/cpu/sgx/driver.c
> +++ b/arch/x86/kernel/cpu/sgx/driver.c
> @@ -94,6 +94,11 @@ static int sgx_mmap(struct file *file, struct vm_area_struct *vma)
> if (ret)
> return ret;
>
> + if (!test_bit(SGX_ENCL_CREATED, &encl->flags) ||
> + vma->vm_pgoff != PFN_DOWN(vma->vm_start - encl->base))
> + encl->has_mismatched_offsests = true;
> +
> +
> vma->vm_ops = &sgx_vm_ops;
> vm_flags_set(vma, VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP | VM_IO);
> vma->vm_private_data = encl;
>

It appears we don't need to mark "has_mismatched_offsets" as true if userspace
uses mmap(/dev/sgx_enclave) to reserve ELRANGE? Enclave's base isn't
established yet, so theoretically, from enclave's perspective, we cannot say it
has a mismatched offset.

However we may want to return -EINVAL if userspace passes a non-zero pgoff in
the mmap(/dev/sgx_enclave) to reserve ELRANGE.

Anyway reserving ELRANGE isn't the big point. The SGX driver ABI is. Let's
forget about reserving ELRANGE in below context, i.e., all mmap() below means
non-ELRANGE-reserve mmap(/dev/sgx_enclave) :)

I think yes marking enclave as "has_mismatched_offsets" works if userspace wants
to use file-based ABIs (fadvice(), etc) for enclave, but this effectively means
Haitao needs to modify _ALL_ existing mmap()s to pass the correct pgoff in order
to use fadvice() for EAUG.

Therefore, it seems more like we are changing ABI (or having a new ABI) to:

Userspace must pass the correct pgoff to all mmaps() in order to use file-based
ABIs for SGX.

( Because I think other drivers/subsystems doesn't have such limitation. For
instance, for a normal file, userspace can have two mmap()s mapping to the same
offset of the same file but with different address, and fadvice() should work
for both mappings.

A VMA-based "has_mismatched_offset" seems more reasonable for SGX but as I
mentioned previously it may not easy to do from SGX driver. )

So I am not sure whether this belongs to "breaking existing ABI"?

If the current ABI is we _require_ pgoff being zero, then the "encl-
>has_mismatched_offsets" seems is breaking this ABI.

If the current ABI is we _ignore_ pgoff, then "encl->has_mismatched_offsets"
isn't breaking existing ABI. But in this case why do we need this at all? SGX
driver just follows the existing file-based ABIs:

If userspace wants to use file-based ABIs for some VMA, it needs to pass the
correct pgoff. If not, file-based ABIs won't work for that VMA, but your
enclave may still work if you don't use file-based ABIs.

Sorry for being noisy, but does above make sense?