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// Copyright 2019 The gVisor 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.
package boot
import (
"bufio"
"io"
"strconv"
"strings"
"gvisor.googlesource.com/gvisor/pkg/abi/linux"
"gvisor.googlesource.com/gvisor/pkg/sentry/context"
"gvisor.googlesource.com/gvisor/pkg/sentry/fs"
"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
)
type fileReader struct {
// Ctx is the context for the file reader.
Ctx context.Context
// File is the file to read from.
File *fs.File
}
// Read implements io.Reader.Read.
func (r *fileReader) Read(buf []byte) (int, error) {
n, err := r.File.Readv(r.Ctx, usermem.BytesIOSequence(buf))
return int(n), err
}
// getExecUserHome returns the home directory of the executing user read from
// /etc/passwd as read from the container filesystem.
func getExecUserHome(ctx context.Context, rootMns *fs.MountNamespace, uid uint32) (string, error) {
// The default user home directory to return if no user matching the user
// if found in the /etc/passwd found in the image.
const defaultHome = "/"
// Open the /etc/passwd file from the dirent via the root mount namespace.
mnsRoot := rootMns.Root()
maxTraversals := uint(linux.MaxSymlinkTraversals)
dirent, err := rootMns.FindInode(ctx, mnsRoot, nil, "/etc/passwd", &maxTraversals)
if err != nil {
// NOTE: Ignore errors opening the passwd file. If the passwd file
// doesn't exist we will return the default home directory.
return defaultHome, nil
}
defer dirent.DecRef()
// Check read permissions on the file.
if err := dirent.Inode.CheckPermission(ctx, fs.PermMask{Read: true}); err != nil {
// NOTE: Ignore permissions errors here and return default root dir.
return defaultHome, nil
}
// Only open regular files. We don't open other files like named pipes as
// they may block and might present some attack surface to the container.
// Note that runc does not seem to do this kind of checking.
if !fs.IsRegular(dirent.Inode.StableAttr) {
return defaultHome, nil
}
f, err := dirent.Inode.GetFile(ctx, dirent, fs.FileFlags{Read: true, Directory: false})
if err != nil {
return "", err
}
defer f.DecRef()
r := &fileReader{
Ctx: ctx,
File: f,
}
homeDir, err := findHomeInPasswd(uid, r, defaultHome)
if err != nil {
return "", err
}
return homeDir, nil
}
// findHomeInPasswd parses a passwd file and returns the given user's home
// directory. This function does it's best to replicate the runc's behavior.
func findHomeInPasswd(uid uint32, passwd io.Reader, defaultHome string) (string, error) {
s := bufio.NewScanner(passwd)
for s.Scan() {
if err := s.Err(); err != nil {
return "", err
}
line := strings.TrimSpace(s.Text())
if line == "" {
continue
}
// Pull out part of passwd entry. Loosely parse the passwd entry as some
// passwd files could be poorly written and for compatibility with runc.
//
// Per 'man 5 passwd'
// /etc/passwd contains one line for each user account, with seven
// fields delimited by colons (“:”). These fields are:
//
// - login name
// - optional encrypted password
// - numerical user ID
// - numerical group ID
// - user name or comment field
// - user home directory
// - optional user command interpreter
parts := strings.Split(line, ":")
found := false
homeDir := ""
for i, p := range parts {
switch i {
case 2:
parsedUID, err := strconv.ParseUint(p, 10, 32)
if err == nil && parsedUID == uint64(uid) {
found = true
}
case 5:
homeDir = p
}
}
if found {
// NOTE: If the uid is present but the home directory is not
// present in the /etc/passwd entry we return an empty string. This
// is, for better or worse, what runc does.
return homeDir, nil
}
}
return defaultHome, nil
}
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