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

Huang, Kai
2023-06-22T23:22:08Z

On Thu, 2023-06-22 at 15:01 -0700, Sean Christopherson wrote:
> On Mon, Jun 19, 2023, Kai Huang wrote:
> > On Fri, 2023-06-16 at 15:05 -0700, Sean Christopherson wrote:
> > > But I think this is unrelated to what you really care about, e.g. a userspace that
> > > tightly controls its virtual memory could hardcode enclave placement (IIRC graphene
> > > did/does do that). I.e. the alignment issue is a completely different discussion.
> > >
> > > [*] https://lore.kernel.org/all/20190522153836.GA24833@linux.intel.com
> >
> > Right. For the sake of making progress of this Haitao's series, we want to
> > understand where did "SGX driver requires mmap(/dev/sgx_enclave) to use
> > MAP_ANONYMOUS semantics" come from.
> >
> > But for this topic (how to reserve ELRANGE). I am not sure the reason that we
> > prefer mmap(MAP_ANONYMOUS) still stands. For instance, the discussion in your
> > above link was old implementation/assumption -- e.g., MAP_FIXED wasn't even
> > used/supported for mmap()ing enclave chunks.
> >
> > So I am wondering now whether we suggest user to use mmap(MAP_ANONYMOUS) to get
> > a size-aligned address still stands? The current SGX driver's
> > get_unmapped_area:
> >
> > static unsigned long sgx_get_unmapped_area(struct file *file,
> > unsigned long addr,
> > unsigned long len,
> > unsigned long pgoff,
> > unsigned long flags)
> > {
> > if ((flags & MAP_TYPE) == MAP_PRIVATE)
> > return -EINVAL;
> >
> > if (flags & MAP_FIXED)
> > return addr;
> >
> > return current->mm->get_unmapped_area(file, addr, len, pgoff, flags);
> > }
> >
> >
> > As you can see if we don't pass MAP_FIXED, which is the case for mmap() to
> > reserve ELRANGE, looks there's no difference between mmap(MAP_ANONYMOUS) and
> > mmap(/dev/sgx_enclave)?
>
> In practice, there's *probably* no significant difference. Using an anonymous
> mapping is advantageous because there's no additional overhead, e.g. for locking
> the file, it can be done in advance of actually opening /dev/sgx_enclave, helps
> document (in userspace) that the code isn't actually creating an enclave, just
> finding a naturally aligned area in virtual memory (which isn't SGX specific), etc.

Sure, and I agree this perhaps is cleaner and simpler for SGX. But for this
purpose I think we should just enforce this policy in SGX driver, i.e., we
should disable mmap(/dev/sgx_enclave) before ECREATE.

To me at least the SGX driver should be clear whether it supports userspace to
use mmap(/dev/sgx_enclave) to reserve ELRANGE. There should be no ambiguity
here.

The current driver only disallows mmap(/dev/sgx_enclave) outside of enclave
range after EINIT:

int sgx_encl_may_map(struct sgx_encl *encl, unsigned long start,
unsigned long end, unsigned long vm_flags)
{
...

/* Disallow mapping outside enclave's address range. */
if (test_bit(SGX_ENCL_INITIALIZED, &encl->flags) &&
(start < encl->base || end > encl->base + encl->size))
return -EACCES;
}

which has what value btw?

>
> There are definitely differences, e.g. an LSM could restrict access to
> /dev/sgx_enclave.  That particular one is obviously a "don't care",
>

I think you meant "RW->RX" requires EXECMOD, etc?

Yeah I am not sure whether this matters to reserving ELRANGE. That part should
apply to the mmap(/dev/sgx_enclave) after ECREATE.

> but it's quite
> difficult to say that mmap(/dev/sgx_enclave) and mmap(NULL) are equivalent due to
> the amount of code that's involved in handling mmap().

Sure. I was talking about only from "allocating size-aligned" part. Arguably,
in terms of reserving ELRANGE, userspace only cares about:

1) The ELRANGE is big enough to hold the enclave
2) The ELRANGE meets alignment requirement
3) Usperspace can mmap(/dev/sgx_enclave) and/or mprotect() small areas to get
the correct enclave addr within ELRANGE with desired permission

>
> > > > Also, if I am not missing something, the current driver doesn't prevent using
> > > > mmap(/dev/sgx_enclave, PROT_NONE) to reserve ELANGE. So having clear
> > > > documentation is helpful for SGX users to choose how to write their apps.
> > > >
> > > > Go back to the "SGX driver uses MAP_ANONYMOUS semantics for mmap()", I believe
> > > > this just is "SGX driver requires mmap() after ECREATE/EINIT to use MAP_SHARED |
> > > > MAP_FIXED and pgoff is ignored". Or more precisely, pgoff is "not _used_ by SGX
> > > > driver".
> > > >
> > > > In fact, I think "pgoff is ignored/not used" is technically wrong for enclave.
> > >
> > > Yeah, it's wrong. It works because, until now apparently, there was never a reason
> > > a need to care about the file offset since ELRANGE base always provided the necessary
> > > information. It wasn't a deliberate design choice, we simply overlooked that detail
> > > (unless Jarkko was holding back on me all those years ago :-) ).
> >
> > Yeah. But I am not sure whether there are corner cases that we can have
> > potential bug around here, since those VMA's are not aligned between the core-mm
> > and the driver.
> >
> > I haven't thought carefully though. I guess from core-mm's perspective there
> > are multi-VMAs mapping to the same/overlapping part of the enclave, while the
> > SGX driver treats them as mapping to different non-overlapping parts. Perhaps
> > it's OK as long as SGX driver doesn't use vma->pgoff to do something???
>
> Ya, exactly.
>
> > > > IMHO we should stop saying SGX driver uses MAP_ANONYMOUS semantics, because the
> > > > truth is it just takes advantage of MAP_FIXED and carelessly ignores the pgoff
> > > > due to the nature of SGX w/o considering from core-MM's perspective.
> > > >
> > > > And IMHO there are two ways to fix:
> > > >
> > > > 1) From now on, we ask SGX apps to use the correct pgoff in their
> > > > mmap(/dev/sgx_enclave). This shouldn't impact the existing SGX apps because SGX
> > > > driver doesn't use vma->pgoff anyway.
> > >
> > > Heh, just "asking" won't help. And breaking userspace, i.e. requiring all apps
> > > to update, will just get you yelled at :-)
> >
> > We can document properly and assume the new apps will follow. As I mentioned
> > above, the old apps, which doesn't/shouldn't have been using fadvice() anyway,
> > doesn't need to be changed, i.e., they should continue to work. :)
> >
> > No?
>
> 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.

No?

>
> > > > 2) For the sake of avoiding having to ask existing SGX apps to change their
> > > > mmap()s, we _officially_ say that userspace isn't required to pass a correct
> > > > pgoff to mmap() (i.e. passing 0 as did in existing apps), but the kernel should
> > > > fix the vma->pgoff internally.
> > >
> > > I recommend you don't do this. Overwriting vm_pgoff would probably work, but it's
> > > going to make a flawed API even messier. E.g. you'll have painted SGX into a
> > > corner if you ever want to decouple vma->start/end from encl->base. I highly
> > > doubt that will ever happen given how ELRANGE works, but I don't think a hack-a-fix
> > > buys you enough to justify any more ugliness.
> >
> > I also found it's not feasible to cleanly fix the pgoff from userspace (I
> > thought the pgoff could be fixed at very early stage of do_mmap() so everything
> > later just worked, but it's not the case). In do_mmap() the pgoff from
> > userspace is already used to VMA merging/splitting etc before creating the
> > target VMA.
> >
> > Hmm.. Now I think we shouldn't silently fix pgoff in SGX driver as it may
> > surprise the core-mm later because the core-mm has already done some job around
> > VMAs before vma->pgoff is changed.
> >
> > >
> > > > I do prefer option 2) because it has no harm to anyone: 1) No changes to
> > > > existing SGX apps; 2) It aligns with the core-MM to so all existing mmap()
> > > > operations should work as expected, meaning no surprise; 3) And this patchset
> > > > from Haitao to use fadvice() to accelerate EAUG flow just works.
> > >
> > > I think you can have your cake and eat it too. IIUC, the goal is to get fadvise()
> > > working, and to do that without creating a truly heinous uAPI, you need an accurate
> > > vm_pgoff. So, use a carrot and stick approach.
> > >
> > > If userspace properly defines vm_pgoff during mmap() and doesn't specify MAP_ANONYMOUS,
> > > then they get to use fadvise() (give 'em a carrot). But if *any* mmap() on the
> > > enclave doesn't followo those rules, mark the enclave as tainted or whatever and
> > > disallow fadvise() (hit 'em with a stick).
> >
> > If we are supposed to use mmap(MAP_ANONYMOUS) to reserve ELRANGE, then I think
> > userspace will just use MAP_FIXED for all mmap()s to /dev/sgx_enclave.
>
> There is no "supposed" to. Using mmap(NULL) is purely a suggestion to avoid
> running into overheads and checks that are ultimately unnecessary. The only
> requirement is that userspace provide a compatible chunk for virtual memory,
> *how* userspace finds that chunk can be done in a number of ways. mmap(NULL)
> just happens to be the most convenient one (IMO).

Even this, I think we should document it IMO, at least whether
mmap(/dev/sgx_enclave) can be used to reserve ELRANGE.

>
> > To detect whether a VMA has the correct pgoff, we need to somehow mark it in
> > sgx_mmap(), but currently we don't have facility to do that. Even we can do,
> > the core madvice() -> vfs_fadvice() cannot be aware of such VMA either, and
> > in sgx_fadvice() we may have problem locating the correct VMA to do EAUG
> > (because the vfs_fadvice() uses 0 pgoff to calculate the file offset).
> >
> > So, now I think we should just let userspace itself to pass the correct pgoff
> > when it wants to use fadvice(), which is the option 1) I mentioned above. The
> > kernel doesn't/shouldn't need to fix vma->pgoff.
> >
> > In another words, I think:
> >
> > - For the old apps, doesn't matter, continue to work;
> > - For new apps which want to use fadvice() to accelerate EAUG, it should passï¿½
> > the correct pgoff in mmap(/dev/sgx_enclave) even with MAP_FIXED.
> >
> > And we don't say "SGX driver uses MAP_ANONYMOUS semantics for
> > mmap(/dev/sgx_enclave) any more".
> >
> > Does this make sense?
>
> Yes, but as above, IMO the kernel needs to enforce that userspace has properly
> set the offset, otherwise userspace will end up with completely nonsensical
> behavior if it fails to set the correct offset. The proposed sgx_fadvise() already
> takes mmap_lock for read, i.e. is mutually exclusive with sgx_mmap(), so encforcing
> the requirement should be quite straightforward, e.g.
>
> mmap_read_lock(current->mm);
>
> vma = find_vma(current->mm, start);

The "start" here may already be wrong because before vfs_fadvice() uses VMA's pgoff
to calculate the start before passing it to sgx_fadvice().

> if (!vma)
> goto unlock;
> if (vma->vm_private_data != encl)
> goto unlock;
> if (encl->has_mismatched_offsets) <======
> goto unlock;

Sorry I am a little bit slow, how do you set "has_mismatched_offsests" to true?

>
> pos = start;
> if (pos < vma->vm_start || end > vma->vm_end) {
> /* Don't allow any gaps */
> goto unlock;
> }