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// 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 seccomp provides basic seccomp filters.
package seccomp
import (
"fmt"
"sort"
"gvisor.googlesource.com/gvisor/pkg/abi/linux"
"gvisor.googlesource.com/gvisor/pkg/bpf"
"gvisor.googlesource.com/gvisor/pkg/log"
)
const (
// violationLabel is added to the program to take action on a violation.
violationLabel = "violation"
// allowLabel is added to the program to allow the syscall to take place.
allowLabel = "allow"
)
// Install generates BPF code based on the set of syscalls provided. It only
// allows syscalls that conform to the specification (*) and generates SIGSYS
// trap unless kill is set.
//
// (*) The current implementation only checks the syscall number. It does NOT
// validate any of the arguments.
func Install(syscalls []uintptr, kill bool) error {
// Sort syscalls and remove duplicates to build the BST.
sort.Slice(syscalls, func(i, j int) bool { return syscalls[i] < syscalls[j] })
syscalls = filterUnique(syscalls)
log.Infof("Installing seccomp filters for %d syscalls (kill=%t)", len(syscalls), kill)
for _, s := range syscalls {
log.Infof("syscall filter: %v", s)
}
instrs, err := buildProgram(syscalls, kill)
if err != nil {
return err
}
if log.IsLogging(log.Debug) {
programStr, err := bpf.DecodeProgram(instrs)
if err != nil {
programStr = fmt.Sprintf("Error: %v\n%s", err, programStr)
}
log.Debugf("Seccomp program dump:\n%s", programStr)
}
if err := seccomp(instrs); err != nil {
return err
}
log.Infof("Seccomp filters installed.")
return nil
}
// buildProgram builds a BPF program that whitelists all given syscalls.
//
// Precondition: syscalls must be sorted and unique.
func buildProgram(syscalls []uintptr, kill bool) ([]linux.BPFInstruction, error) {
const archOffset = 4 // offsetof(seccomp_data, arch)
program := bpf.NewProgramBuilder()
violationAction := uint32(linux.SECCOMP_RET_KILL)
if !kill {
violationAction = linux.SECCOMP_RET_TRAP
}
// Be paranoid and check that syscall is done in the expected architecture.
//
// A = seccomp_data.arch
// if (A != AUDIT_ARCH_X86_64) goto violation
program.AddStmt(bpf.Ld|bpf.Abs|bpf.W, archOffset)
program.AddJumpFalseLabel(bpf.Jmp|bpf.Jeq|bpf.K, linux.AUDIT_ARCH_X86_64, 0, violationLabel)
if err := buildIndex(syscalls, program); err != nil {
return nil, err
}
// violation: return violationAction
if err := program.AddLabel(violationLabel); err != nil {
return nil, err
}
program.AddStmt(bpf.Ret|bpf.K, violationAction)
// allow: return SECCOMP_RET_ALLOW
if err := program.AddLabel(allowLabel); err != nil {
return nil, err
}
program.AddStmt(bpf.Ret|bpf.K, linux.SECCOMP_RET_ALLOW)
return program.Instructions()
}
// filterUnique filters unique system calls.
//
// Precondition: syscalls must be sorted.
func filterUnique(syscalls []uintptr) []uintptr {
filtered := make([]uintptr, 0, len(syscalls))
for i := 0; i < len(syscalls); i++ {
if len(filtered) > 0 && syscalls[i] == filtered[len(filtered)-1] {
// This call has already been inserted, skip.
continue
}
filtered = append(filtered, syscalls[i])
}
return filtered
}
// buildIndex builds a BST to quickly search through all syscalls that are whitelisted.
//
// Precondition: syscalls must be sorted and unique.
func buildIndex(syscalls []uintptr, program *bpf.ProgramBuilder) error {
root := createBST(syscalls)
// Load syscall number into A and run through BST.
//
// A = seccomp_data.nr
program.AddStmt(bpf.Ld|bpf.Abs|bpf.W, 0)
return root.buildBSTProgram(program, true)
}
// createBST converts sorted syscall slice into a balanced BST.
// Panics if syscalls is empty.
func createBST(syscalls []uintptr) *node {
i := len(syscalls) / 2
parent := node{value: syscalls[i]}
if i > 0 {
parent.left = createBST(syscalls[:i])
}
if i+1 < len(syscalls) {
parent.right = createBST(syscalls[i+1:])
}
return &parent
}
// node represents a tree node.
type node struct {
value uintptr
left *node
right *node
}
// label returns the label corresponding to this node. If node is nil (syscall not present),
// violationLabel is returned for convenience.
func (n *node) label() string {
if n == nil {
return violationLabel
}
return fmt.Sprintf("index_%v", n.value)
}
// buildBSTProgram converts a binary tree started in 'root' into BPF code. The ouline of the code
// is as follows:
//
// // SYS_PIPE(22), root
// (A == 22) ? goto allow : continue
// (A > 22) ? goto index_35 : goto index_9
//
// index_9: // SYS_MMAP(9), leaf
// (A == 9) ? goto allow : goto violation
//
// index_35: // SYS_NANOSLEEP(35), single child
// (A == 35) ? goto allow : continue
// (A > 35) ? goto index_50 : goto violation
//
// index_50: // SYS_LISTEN(50), leaf
// (A == 50) ? goto allow : goto violation
//
func (n *node) buildBSTProgram(program *bpf.ProgramBuilder, root bool) error {
if n == nil {
return nil
}
// Root node is never referenced by label, skip it.
if !root {
if err := program.AddLabel(n.label()); err != nil {
return err
}
}
// Leaf nodes don't require extra check, they either allow or violate!
if n.left == nil && n.right == nil {
program.AddJumpLabels(bpf.Jmp|bpf.Jeq|bpf.K, uint32(n.value), allowLabel, violationLabel)
return nil
}
// Non-leaf node. Allows syscall if it matches, check which turn to take otherwise. Note
// that 'violationLabel' is returned for nil children.
program.AddJumpTrueLabel(bpf.Jmp|bpf.Jeq|bpf.K, uint32(n.value), allowLabel, 0)
program.AddJumpLabels(bpf.Jmp|bpf.Jgt|bpf.K, uint32(n.value), n.right.label(), n.left.label())
if err := n.left.buildBSTProgram(program, false); err != nil {
return err
}
return n.right.buildBSTProgram(program, false)
}
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