| 1 | //go:build linux |
| 2 | |
| 3 | package privatefile |
| 4 | |
| 5 | import ( |
| 6 | "io" |
| 7 | "os" |
| 8 | "path/filepath" |
| 9 | "strings" |
| 10 | |
| 11 | "golang.org/x/sys/unix" |
| 12 | ) |
| 13 | |
| 14 | // Walk using retained directory descriptors so a renamed ancestor cannot turn |
| 15 | // a checked path into a symlink traversal. |
| 16 | func parent(path string) (*os.File, string, error) { |
| 17 | if !filepath.IsAbs(path) || filepath.Clean(path) != path || path == "/" { |
| 18 | return nil, "", ErrUnavailable |
| 19 | } |
| 20 | parts := strings.Split(strings.TrimPrefix(path, "/"), "/") |
| 21 | fd, err := unix.Open("/", unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC, 0) |
| 22 | if err != nil { |
| 23 | return nil, "", ErrUnavailable |
| 24 | } |
| 25 | for _, part := range parts[:len(parts)-1] { |
| 26 | next, e := unix.Openat(fd, part, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0) |
| 27 | unix.Close(fd) |
| 28 | if e != nil { |
| 29 | return nil, "", ErrUnavailable |
| 30 | } |
| 31 | fd = next |
| 32 | } |
| 33 | return os.NewFile(uintptr(fd), "private directory"), parts[len(parts)-1], nil |
| 34 | } |
| 35 | func valid(f *os.File, max int) bool { |
| 36 | var st unix.Stat_t |
| 37 | return unix.Fstat(int(f.Fd()), &st) == nil && st.Mode&unix.S_IFMT == unix.S_IFREG && st.Mode&07777 == 0600 && st.Uid == uint32(os.Geteuid()) && st.Nlink == 1 && st.Size >= 0 && st.Size <= int64(max) |
| 38 | } |
| 39 | func read(path string, max int) ([]byte, error) { |
| 40 | if max <= 0 || max == int(^uint(0)>>1) { |
| 41 | return nil, ErrUnavailable |
| 42 | } |
| 43 | p, name, e := parent(path) |
| 44 | if e != nil { |
| 45 | return nil, ErrUnavailable |
| 46 | } |
| 47 | defer p.Close() |
| 48 | fd, e := unix.Openat(int(p.Fd()), name, unix.O_RDONLY|unix.O_NOFOLLOW|unix.O_NONBLOCK|unix.O_CLOEXEC, 0) |
| 49 | if e != nil { |
| 50 | return nil, ErrUnavailable |
| 51 | } |
| 52 | f := os.NewFile(uintptr(fd), "private input") |
| 53 | if !valid(f, max) { |
| 54 | f.Close() |
| 55 | return nil, ErrUnavailable |
| 56 | } |
| 57 | b, e := io.ReadAll(io.LimitReader(f, int64(max)+1)) |
| 58 | ok := valid(f, max) |
| 59 | ce := f.Close() |
| 60 | if e != nil || ce != nil || !ok || len(b) > max { |
| 61 | clear(b) |
| 62 | return nil, ErrUnavailable |
| 63 | } |
| 64 | return b, nil |
| 65 | } |
| 66 | |
| 67 | type durableFile interface { |
| 68 | Write([]byte) (int, error) |
| 69 | Sync() error |
| 70 | Close() error |
| 71 | } |
| 72 | |
| 73 | func finish(f durableFile, p interface{ Sync() error }, b []byte) error { |
| 74 | n, e := f.Write(b) |
| 75 | if e != nil || n != len(b) { |
| 76 | f.Close() |
| 77 | return ErrUnavailable |
| 78 | } |
| 79 | if f.Sync() != nil { |
| 80 | f.Close() |
| 81 | return ErrUnavailable |
| 82 | } |
| 83 | if f.Close() != nil { |
| 84 | return ErrUnavailable |
| 85 | } |
| 86 | if p.Sync() != nil { |
| 87 | return ErrUnavailable |
| 88 | } |
| 89 | return nil |
| 90 | } |
| 91 | func create(path string, b []byte, max int) error { |
| 92 | if max <= 0 || len(b) > max { |
| 93 | return ErrUnavailable |
| 94 | } |
| 95 | p, name, e := parent(path) |
| 96 | if e != nil { |
| 97 | return ErrUnavailable |
| 98 | } |
| 99 | defer p.Close() |
| 100 | var st unix.Stat_t |
| 101 | if unix.Fstat(int(p.Fd()), &st) != nil || st.Uid != uint32(os.Geteuid()) || st.Mode&0077 != 0 { |
| 102 | return ErrUnavailable |
| 103 | } |
| 104 | fd, e := unix.Openat(int(p.Fd()), name, unix.O_WRONLY|unix.O_CREAT|unix.O_EXCL|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0600) |
| 105 | if e != nil { |
| 106 | return ErrUnavailable |
| 107 | } |
| 108 | f := os.NewFile(uintptr(fd), "private output") |
| 109 | if !valid(f, max) { |
| 110 | f.Close() |
| 111 | return ErrUnavailable |
| 112 | } |
| 113 | return finish(f, p, b) |
| 114 | } |
| 115 | |
amadan