Re: [PATCH v3 09/21] KVM: X86: Don't track dirty for KVM_SET_[TSS_ADDR|IDENTITY_MAP_ADDR]

[Sean Christopherson]

On Fri, Jan 31, 2020 at 10:08:32AM -0500, Peter Xu wrote:
> On Tue, Jan 28, 2020 at 10:24:03AM -0800, Sean Christopherson wrote:
> > On Tue, Jan 28, 2020 at 01:50:05PM +0800, Peter Xu wrote:
> > > On Tue, Jan 21, 2020 at 07:56:57AM -0800, Sean Christopherson wrote:
> > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > > index c4d3972dcd14..ff97782b3919 100644
> > > > > --- a/arch/x86/kvm/x86.c
> > > > > +++ b/arch/x86/kvm/x86.c
> > > > > @@ -9584,7 +9584,15 @@ void kvm_arch_sync_events(struct kvm *kvm)
> > > > >  	kvm_free_pit(kvm);
> > > > >  }
> > > > >  
> > > > > -int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
> > > > > +/*
> > > > > + * If `uaddr' is specified, `*uaddr' will be returned with the
> > > > > + * userspace address that was just allocated.  `uaddr' is only
> > > > > + * meaningful if the function returns zero, and `uaddr' will only be
> > > > > + * valid when with either the slots_lock or with the SRCU read lock
> > > > > + * held.  After we release the lock, the returned `uaddr' will be invalid.
> > > > 
> > > > This is all incorrect.  Neither of those locks has any bearing on the
> > > > validity of the hva.  slots_lock does as the name suggests and prevents
> > > > concurrent writes to the memslots.  The SRCU lock ensures the implicit
> > > > memslots lookup in kvm_clear_guest_page() won't result in a use-after-free
> > > > due to derefencing old memslots.
> > > > 
> > > > Neither of those has anything to do with the userspace address, they're
> > > > both fully tied to KVM's gfn->hva lookup.  As Paolo pointed out, KVM's
> > > > mapping is instead tied to the lifecycle of the VM.  Note, even *that* has
> > > > no bearing on the validity of the mapping or address as KVM only increments
> > > > mm_count, not mm_users, i.e. guarantees the mm struct itself won't be freed
> > > > but doesn't ensure the vmas or associated pages tables are valid.
> > > > 
> > > > Which is the entire point of using __copy_{to,from}_user(), as they
> > > > gracefully handle the scenario where the process has not valid mapping
> > > > and/or translation for the address.
> > > 
> > > Sorry I don't understand.
> > > 
> > > I do think either the slots_lock or SRCU would protect at least the
> > > existing kvm.memslots, and if so at least the previous vm_mmap()
> > > return value should still be valid.
> > 
> > Nope.  kvm->slots_lock only protects gfn->hva lookups, e.g. userspace can
> > munmap() the range at any time.
> 
> Do we need to consider that?  If the userspace did this then it'll
> corrupt itself, and imho private memory slot is not anything special
> here comparing to the user memory slots.  For example, the userspace
> can unmap any region after KVM_SET_USER_MEMORY_REGION ioctl even if
> the region is filled into some of the userspace_addr of
> kvm_userspace_memory_region, so the cached userspace_addr can be
> invalid, then kvm_write_guest_page() can fail too with the same
> reason.  IMHO kvm only need to make sure it handles the failure path
> then it's perfectly fine.

Yes?  No?  My point is that your original comment's assertion that "'uaddr'
will only be valid when with either the slots_lock or with the SRCU read
lock held." is wrong and misleading.

> > > I agree that __copy_to_user() will protect us from many cases from process
> > > mm pov (which allows page faults inside), but again if the kvm.memslots is
> > > changed underneath us then it's another story, IMHO, and that's why we need
> > > either the lock or SRCU.
> > 
> > No, again, slots_lock and SRCU only protect gfn->hva lookups.
> 
> Yes, then could you further explain why do you think we don't need the
> slot lock?  

For the same reason we don't take mmap_sem, it gains us nothing, i.e. KVM
still has to use copy_{to,from}_user().

In the proposed __x86_set_memory_region() refactor, vmx_set_tss_addr()
would be provided the hva of the memory region.  Since slots_lock and SRCU
only protect gfn->hva, why would KVM take slots_lock since it already has
the hva?

> > > Or are you assuming that (1) __x86_set_memory_region() is only for the
> > > 3 private kvm memslots, 
> > 
> > It's not an assumption, the entire purpose of __x86_set_memory_region()
> > is to provide support for private KVM memslots.
> > 
> > > and (2) currently the kvm private memory slots will never change after VM
> > > is created and before VM is destroyed?
> > 
> > No, I'm not assuming the private memslots are constant, e.g. the flow in
> > question, vmx_set_tss_addr() is directly tied to an unprotected ioctl().
> 
> Why it's unprotected?

Because it doesn't need to be protected.

> Now vmx_set_tss_add() is protected by the slots lock so concurrent operation
> is safe, also it'll return -EEXIST if called for more than once.

Returning -EEXIST is an ABI change, e.g. userspace can currently call
KVM_SET_TSS_ADDR any number of times, it just needs to ensure proper
serialization between calls.

If you want to change the ABI, then submit a patch to do exactly that.
But don't bury an ABI change under the pretense that it's a bug fix.

> [1]
> 
> > 
> > KVM's sole responsible for vmx_set_tss_addr() is to not crash the kernel.
> > Userspace is responsible for ensuring it doesn't break its guests, e.g.
> > that multiple calls to KVM_SET_TSS_ADDR are properly serialized.
> > 
> > In the existing code, KVM ensures it doesn't crash by holding the SRCU lock
> > for the duration of init_rmode_tss() so that the gfn->hva lookups in
> > kvm_clear_guest_page() don't dereference a stale memslots array.
> 
> Here in the current master branch we have both the RCU lock and the
> slot lock held, that's why I think we can safely remove the RCU lock
> as long as we're still holding the slots lock.  We can't do the
> reverse because otherwise multiple KVM_SET_TSS_ADDR could race.

Your wording is all messed up.  "we have both the RCU lock and the slot
lock held" is wrong.  KVM holds slot_lock around __x86_set_memory_region(),
because changing the memslots must be mutually exclusive.  It then *drops*
slots_lock because it's done writing the memslots and grabs the SRCU lock
in order to protect the gfn->hva lookups done by init_rmode_tss().  It
*intentionally* drops slots_lock because writing init_rmode_tss() does not
need to be a mutually exclusive operation, per KVM's existing ABI.

If KVM held both slots_lock and SRCU then __x86_set_memory_region() would
deadlock on synchronize_srcu().

> > In no way
> > does that ensure the validity of the resulting hva,
> 
> Yes, but as I mentioned, I don't think it's an issue to be considered
> by KVM, otherwise we should have the same issue all over the places
> when we fetch the cached userspace_addr from any user slots.

Huh?  Of course it's an issue that needs to be considered by KVM, e.g.
kvm_{read,write}_guest_cached() aren't using __copy_{to,}from_user() for
giggles.

> > e.g. multiple calls to
> > KVM_SET_TSS_ADDR would race to set vmx->tss_addr and so init_rmode_tss()
> > could be operating on a stale gpa.
> 
> Please refer to [1].
> 
> I just want to double-confirm on what we're discussing now. Are you
> sure you're suggesting that we should remove the slot lock in
> init_rmode_tss()?  Asked because you discussed quite a bit on how the
> slot lock should protect GPA->HVA, about concurrency and so on, then
> I'm even more comfused...

Yes, if init_rmode_tss() is provided the hva then it does not need to
grab srcu_read_lock(&kvm->srcu) because it can directly call
__copy_{to,from}_user() instead of bouncing through the KVM helpers that
translate a gfn to hva.

The code can look like this.  That being said, I've completely lost track
of why __x86_set_memory_region() needs to provide the hva, i.e. have no
idea if we *should* do this, or it would be better to keep the current
code, which would be slower, but less custom.

static int init_rmode_tss(void __user *hva)
{
	const void *zero_page = (const void *)__va(page_to_phys(ZERO_PAGE(0)));
	u16 data = TSS_BASE_SIZE + TSS_REDIRECTION_SIZE;
	int r;

	r = __copy_to_user(hva, zero_page, PAGE_SIZE);
	if (r)
		return -EFAULT;

	r = __copy_to_user(hva + TSS_IOPB_BASE_OFFSET, &data, sizeof(u16))
	if (r)
		return -EFAULT;

	hva += PAGE_SIZE;
	r = __copy_to_user(hva + PAGE_SIZE, zero_page, PAGE_SIZE);
	if (r)
		return -EFAULT;

	hva += PAGE_SIZE;
	r = __copy_to_user(hva + PAGE_SIZE, zero_page, PAGE_SIZE);
	if (r)
		return -EFAULT;

	data = ~0;
	hva += RMODE_TSS_SIZE - 2 * PAGE_SIZE - 1;
	r = __copy_to_user(hva, &data, sizeof(u16))
	if (r)
		return -EFAULT;
}

static int vmx_set_tss_addr(struct kvm *kvm, unsigned int addr)
{
	void __user *hva;

	if (enable_unrestricted_guest)
		return 0;

	mutex_lock(&kvm->slots_lock);
	hva = __x86_set_memory_region(kvm, TSS_PRIVATE_MEMSLOT, addr,
				      PAGE_SIZE * 3);
	mutex_unlock(&kvm->slots_lock);

	if (IS_ERR(hva))
		return PTR_ERR(hva);

	to_kvm_vmx(kvm)->tss_addr = addr;
	return init_rmode_tss(hva);
}

Yes, userspace can corrupt its VM by invoking KVM_SET_TSS_ADDR multiple
times without serializing the calls, but that's already true today.