devicefilter.go

/*
   Copyright The containerd Authors.

   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.
*/

// Devicefilter containes eBPF device filter program
//
// The implementation is based on https://github.com/containers/crun/blob/0.10.2/src/libcrun/ebpf.c
//
// Although ebpf.c is originally licensed under LGPL-3.0-or-later, the author (Giuseppe Scrivano)
// agreed to relicense the file in Apache License 2.0: https://github.com/opencontainers/runc/issues/2144#issuecomment-543116397
//
// This particular Go implementation based on runc version
// https://github.com/opencontainers/runc/blob/master/libcontainer/cgroups/ebpf/devicefilter/devicefilter.go

package cgroup2

import (
	"errors"
	"fmt"
	"math"

	"github.com/cilium/ebpf/asm"
	"github.com/opencontainers/runtime-spec/specs-go"
	"golang.org/x/sys/unix"
)

const (
	// license string format is same as kernel MODULE_LICENSE macro
	license = "Apache"
)

// DeviceFilter returns eBPF device filter program and its license string
func DeviceFilter(devices []specs.LinuxDeviceCgroup) (asm.Instructions, string, error) {
	p := &program{}
	p.init()
	for i := len(devices) - 1; i >= 0; i-- {
		if err := p.appendDevice(devices[i]); err != nil {
			return nil, "", err
		}
	}
	insts, err := p.finalize()
	return insts, license, err
}

type program struct {
	insts       asm.Instructions
	hasWildCard bool
	blockID     int
}

func (p *program) init() {
	// struct bpf_cgroup_dev_ctx: https://elixir.bootlin.com/linux/v5.3.6/source/include/uapi/linux/bpf.h#L3423
	/*
		u32 access_type
		u32 major
		u32 minor
	*/
	// R2 <- type (lower 16 bit of u32 access_type at R1[0])
	p.insts = append(p.insts,
		asm.LoadMem(asm.R2, asm.R1, 0, asm.Half))

	// R3 <- access (upper 16 bit of u32 access_type at R1[0])
	p.insts = append(p.insts,
		asm.LoadMem(asm.R3, asm.R1, 0, asm.Word),
		// RSh: bitwise shift right
		asm.RSh.Imm32(asm.R3, 16))

	// R4 <- major (u32 major at R1[4])
	p.insts = append(p.insts,
		asm.LoadMem(asm.R4, asm.R1, 4, asm.Word))

	// R5 <- minor (u32 minor at R1[8])
	p.insts = append(p.insts,
		asm.LoadMem(asm.R5, asm.R1, 8, asm.Word))
}

// appendDevice needs to be called from the last element of OCI linux.resources.devices to the head element.
func (p *program) appendDevice(dev specs.LinuxDeviceCgroup) error {
	if p.blockID < 0 {
		return errors.New("the program is finalized")
	}
	if p.hasWildCard {
		// All entries after wildcard entry are ignored
		return nil
	}

	bpfType := int32(-1)
	hasType := true
	switch dev.Type {
	case string('c'):
		bpfType = int32(unix.BPF_DEVCG_DEV_CHAR)
	case string('b'):
		bpfType = int32(unix.BPF_DEVCG_DEV_BLOCK)
	case string('a'):
		hasType = false
	default:
		// if not specified in OCI json, typ is set to DeviceTypeAll
		return fmt.Errorf("invalid DeviceType %q", dev.Type)
	}
	if *dev.Major > math.MaxUint32 {
		return fmt.Errorf("invalid major %d", *dev.Major)
	}
	if *dev.Minor > math.MaxUint32 {
		return fmt.Errorf("invalid minor %d", *dev.Major)
	}
	hasMajor := *dev.Major >= 0 // if not specified in OCI json, major is set to -1
	hasMinor := *dev.Minor >= 0
	bpfAccess := int32(0)
	for _, r := range dev.Access {
		switch r {
		case 'r':
			bpfAccess |= unix.BPF_DEVCG_ACC_READ
		case 'w':
			bpfAccess |= unix.BPF_DEVCG_ACC_WRITE
		case 'm':
			bpfAccess |= unix.BPF_DEVCG_ACC_MKNOD
		default:
			return fmt.Errorf("unknown device access %v", r)
		}
	}
	// If the access is rwm, skip the check.
	hasAccess := bpfAccess != (unix.BPF_DEVCG_ACC_READ | unix.BPF_DEVCG_ACC_WRITE | unix.BPF_DEVCG_ACC_MKNOD)

	blockSym := fmt.Sprintf("block-%d", p.blockID)
	nextBlockSym := fmt.Sprintf("block-%d", p.blockID+1)
	prevBlockLastIdx := len(p.insts) - 1
	if hasType {
		p.insts = append(p.insts,
			// if (R2 != bpfType) goto next
			asm.JNE.Imm(asm.R2, bpfType, nextBlockSym),
		)
	}
	if hasAccess {
		p.insts = append(p.insts,
			// if (R3 & bpfAccess == 0 /* use R1 as a temp var */) goto next
			asm.Mov.Reg32(asm.R1, asm.R3),
			asm.And.Imm32(asm.R1, bpfAccess),
			asm.JEq.Imm(asm.R1, 0, nextBlockSym),
		)
	}
	if hasMajor {
		p.insts = append(p.insts,
			// if (R4 != major) goto next
			asm.JNE.Imm(asm.R4, int32(*dev.Major), nextBlockSym),
		)
	}
	if hasMinor {
		p.insts = append(p.insts,
			// if (R5 != minor) goto next
			asm.JNE.Imm(asm.R5, int32(*dev.Minor), nextBlockSym),
		)
	}
	if !hasType && !hasAccess && !hasMajor && !hasMinor {
		p.hasWildCard = true
	}
	p.insts = append(p.insts, acceptBlock(dev.Allow)...)
	// set blockSym to the first instruction we added in this iteration
	p.insts[prevBlockLastIdx+1] = p.insts[prevBlockLastIdx+1].WithSymbol(blockSym)
	p.blockID++
	return nil
}

func (p *program) finalize() (asm.Instructions, error) {
	if p.hasWildCard {
		// acceptBlock with asm.Return() is already inserted
		return p.insts, nil
	}
	blockSym := fmt.Sprintf("block-%d", p.blockID)
	p.insts = append(p.insts,
		// R0 <- 0
		asm.Mov.Imm32(asm.R0, 0).WithSymbol(blockSym),
		asm.Return(),
	)
	p.blockID = -1
	return p.insts, nil
}

func acceptBlock(accept bool) asm.Instructions {
	v := int32(0)
	if accept {
		v = 1
	}
	return []asm.Instruction{
		// R0 <- v
		asm.Mov.Imm32(asm.R0, v),
		asm.Return(),
	}
}