gvisor/pkg/sentry/platform/kvm/address_space.go

// Copyright 2018 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package kvm

import (
	"reflect"
	"sync"
	"sync/atomic"

	"gvisor.googlesource.com/gvisor/pkg/atomicbitops"
	"gvisor.googlesource.com/gvisor/pkg/sentry/platform"
	"gvisor.googlesource.com/gvisor/pkg/sentry/platform/filemem"
	"gvisor.googlesource.com/gvisor/pkg/sentry/platform/ring0/pagetables"
	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
)

type vCPUBitArray [(_KVM_NR_VCPUS + 63) / 64]uint64

// dirtySet tracks vCPUs for invalidation.
type dirtySet struct {
	vCPUs vCPUBitArray
}

// forEach iterates over all CPUs in the dirty set.
func (ds *dirtySet) forEach(m *machine, fn func(c *vCPU)) {
	var localSet vCPUBitArray
	for index := 0; index < len(ds.vCPUs); index++ {
		// Clear the dirty set, copy to the local one.
		localSet[index] = atomic.SwapUint64(&ds.vCPUs[index], 0)
	}

	m.mu.RLock()
	defer m.mu.RUnlock()

	for _, c := range m.vCPUs {
		index := uint64(c.id) / 64
		bit := uint64(1) << uint(c.id%64)

		// Call the function if it was set.
		if localSet[index]&bit != 0 {
			fn(c)
		}
	}
}

// mark marks the given vCPU as dirty and returns whether it was previously
// clean. Being previously clean implies that a flush is needed on entry.
func (ds *dirtySet) mark(c *vCPU) bool {
	index := uint64(c.id) / 64
	bit := uint64(1) << uint(c.id%64)

	oldValue := atomic.LoadUint64(&ds.vCPUs[index])
	if oldValue&bit != 0 {
		return false // Not clean.
	}

	// Set the bit unilaterally, and ensure that a flush takes place. Note
	// that it's possible for races to occur here, but since the flush is
	// taking place long after these lines there's no race in practice.
	atomicbitops.OrUint64(&ds.vCPUs[index], bit)
	return true // Previously clean.
}

// addressSpace is a wrapper for PageTables.
type addressSpace struct {
	platform.NoAddressSpaceIO

	// mu is the lock for modifications to the address space.
	//
	// Note that the page tables themselves are not locked.
	mu sync.Mutex

	// filemem is the memory instance.
	filemem *filemem.FileMem

	// machine is the underlying machine.
	machine *machine

	// pageTables are for this particular address space.
	pageTables *pagetables.PageTables

	// dirtySet is the set of dirty vCPUs.
	dirtySet dirtySet

	// files contains files mapped in the host address space.
	//
	// See host_map.go for more information.
	files hostMap
}

// invalidate is the implementation for Invalidate.
func (as *addressSpace) invalidate() {
	as.dirtySet.forEach(as.machine, func(c *vCPU) {
		if c.active.get() == as { // If this happens to be active,
			c.BounceToKernel() // ... force a kernel transition.
		}
	})
}

// Invalidate interrupts all dirty contexts.
func (as *addressSpace) Invalidate() {
	as.mu.Lock()
	defer as.mu.Unlock()
	as.invalidate()
}

// Touch adds the given vCPU to the dirty list.
//
// The return value indicates whether a flush is required.
func (as *addressSpace) Touch(c *vCPU) bool {
	return as.dirtySet.mark(c)
}

func (as *addressSpace) mapHost(addr usermem.Addr, m hostMapEntry, at usermem.AccessType) (inv bool) {
	for m.length > 0 {
		physical, length, ok := translateToPhysical(m.addr)
		if !ok {
			panic("unable to translate segment")
		}
		if length > m.length {
			length = m.length
		}

		// Ensure that this map has physical mappings. If the page does
		// not have physical mappings, the KVM module may inject
		// spurious exceptions when emulation fails (i.e. it tries to
		// emulate because the RIP is pointed at those pages).
		as.machine.mapPhysical(physical, length)

		// Install the page table mappings. Note that the ordering is
		// important; if the pagetable mappings were installed before
		// ensuring the physical pages were available, then some other
		// thread could theoretically access them.
		//
		// Due to the way KVM's shadow paging implementation works,
		// modifications to the page tables while in host mode may not
		// be trapped, leading to the shadow pages being out of sync.
		// Therefore, we need to ensure that we are in guest mode for
		// page table modifications. See the call to bluepill, below.
		as.machine.retryInGuest(func() {
			inv = as.pageTables.Map(addr, length, pagetables.MapOpts{
				AccessType: at,
				User:       true,
			}, physical) || inv
		})
		m.addr += length
		m.length -= length
		addr += usermem.Addr(length)
	}

	return inv
}

func (as *addressSpace) mapHostFile(addr usermem.Addr, fd int, fr platform.FileRange, at usermem.AccessType) error {
	// Create custom host mappings.
	ms, err := as.files.CreateMappings(usermem.AddrRange{
		Start: addr,
		End:   addr + usermem.Addr(fr.End-fr.Start),
	}, at, fd, fr.Start)
	if err != nil {
		return err
	}

	inv := false
	for _, m := range ms {
		// The host mapped slices are guaranteed to be aligned.
		prev := as.mapHost(addr, m, at)
		inv = inv || prev
		addr += usermem.Addr(m.length)
	}
	if inv {
		as.invalidate()
	}

	return nil
}

func (as *addressSpace) mapFilemem(addr usermem.Addr, fr platform.FileRange, at usermem.AccessType, precommit bool) error {
	// TODO: Lock order at the platform level is not sufficiently
	// well-defined to guarantee that the caller (FileMem.MapInto) is not
	// holding any locks that FileMem.MapInternal may take.

	// Retrieve mappings for the underlying filemem. Note that the
	// permissions here are largely irrelevant, since it corresponds to
	// physical memory for the guest. We enforce the given access type
	// below, in the guest page tables.
	bs, err := as.filemem.MapInternal(fr, usermem.AccessType{
		Read:  true,
		Write: true,
	})
	if err != nil {
		return err
	}

	// Save the original range for invalidation.
	orig := usermem.AddrRange{
		Start: addr,
		End:   addr + usermem.Addr(fr.End-fr.Start),
	}

	inv := false
	for !bs.IsEmpty() {
		b := bs.Head()
		bs = bs.Tail()
		// Since fr was page-aligned, b should also be page-aligned. We do the
		// lookup in our host page tables for this translation.
		s := b.ToSlice()
		if precommit {
			for i := 0; i < len(s); i += usermem.PageSize {
				_ = s[i] // Touch to commit.
			}
		}
		prev := as.mapHost(addr, hostMapEntry{
			addr:   reflect.ValueOf(&s[0]).Pointer(),
			length: uintptr(len(s)),
		}, at)
		inv = inv || prev
		addr += usermem.Addr(len(s))
	}
	if inv {
		as.invalidate()
		as.files.DeleteMapping(orig)
	}

	return nil
}

// MapFile implements platform.AddressSpace.MapFile.
func (as *addressSpace) MapFile(addr usermem.Addr, fd int, fr platform.FileRange, at usermem.AccessType, precommit bool) error {
	as.mu.Lock()
	defer as.mu.Unlock()

	// Create an appropriate mapping. If this is filemem, we don't create
	// custom mappings for each in-application mapping. For files however,
	// we create distinct mappings for each address space. Unfortunately,
	// there's not a better way to manage this here. The file underlying
	// this fd can change at any time, so we can't actually index the file
	// and share between address space. Oh well. It's all referring to the
	// same physical pages, hopefully we don't run out of address space.
	if fd != int(as.filemem.File().Fd()) {
		// N.B. precommit is ignored for host files.
		return as.mapHostFile(addr, fd, fr, at)
	}

	return as.mapFilemem(addr, fr, at, precommit)
}

// Unmap unmaps the given range by calling pagetables.PageTables.Unmap.
func (as *addressSpace) Unmap(addr usermem.Addr, length uint64) {
	as.mu.Lock()
	defer as.mu.Unlock()

	// See above re: retryInGuest.
	var prev bool
	as.machine.retryInGuest(func() {
		prev = as.pageTables.Unmap(addr, uintptr(length)) || prev
	})
	if prev {
		as.invalidate()
		as.files.DeleteMapping(usermem.AddrRange{
			Start: addr,
			End:   addr + usermem.Addr(length),
		})

		// Recycle any freed intermediate pages.
		as.pageTables.Allocator.Recycle()
	}
}

// Release releases the page tables.
func (as *addressSpace) Release() {
	as.Unmap(0, ^uint64(0))

	// Free all pages from the allocator.
	as.pageTables.Allocator.(allocator).base.Drain()

	// Drop all cached machine references.
	as.machine.dropPageTables(as.pageTables)
}