fix an error caused by fd reuse race when starting runc init

Due to a Go stdlib bug, it is possible for the stdlib to clobber the fd
during forkAndExecInChild1 and replace it with some other file that
might be malicious. See <golang/go#61751>.
It will cause runc init process can't start. (opencontainers#4294)
It only occurs when we are using a fd type string, for example:
proc/self/fd/7, as a cmd path to start runc init, because there is a fd
reuse race, if some small fd closed, the kernel may reuse this fd to
refer to runc binary. If this fd num is small than the length of
`cmd.ExtraFiles`, it will hit this Go stdlib bug. If we found this
situation, we can dup it as a new bigger fd num to avoid.

Signed-off-by: lfbzhm <[email protected]>

libcontainer/container_linux.go

@@ -619,15 +619,29 @@ func (c *Container) newParentProcess(p *Process) (parentProcess, error) {
 	}
 
 	if safeExe != nil {
-		// Due to a Go stdlib bug, we need to add safeExe to the set of
-		// ExtraFiles otherwise it is possible for the stdlib to clobber the fd
+		// Due to a Go stdlib bug, it is possible for the stdlib to clobber the fd
 		// during forkAndExecInChild1 and replace it with some other file that
-		// might be malicious. This is less than ideal (because the descriptor
-		// will be non-O_CLOEXEC) however we have protections in "runc init" to
-		// stop us from leaking extra file descriptors.
-		//
-		// See <https://github.com/golang/go/issues/61751>.
-		cmd.ExtraFiles = append(cmd.ExtraFiles, safeExe)
+		// might be malicious. See <https://github.com/golang/go/issues/61751>.
+		// It will cause runc init process can't start. (#4294)
+		minFd := stdioFdCount + len(cmd.ExtraFiles)
+		if p.Init {
+			// If this is init process, we need to attach fifo fd.
+			minFd++
+		}
+		if int(safeExe.Fd()) <= minFd {
+			maxFd, err := utils.GetMaxFds()
+			if err != nil {
+				return nil, fmt.Errorf("unable to get the max opened fd of current process: %w", err)
+			}
+			maxFd++
+			if err := unix.Dup3(int(safeExe.Fd()), maxFd, unix.O_CLOEXEC); err != nil {
+				return nil, fmt.Errorf("unable to dup3 the fd from %d to %d: %w", safeExe.Fd(), maxFd, err)
+			}
+			cmd.Path = "/proc/self/fd/" + strconv.Itoa(maxFd)
+			if err := safeExe.Close(); err != nil {
+				return nil, fmt.Errorf("unable to close old safe exe(%d): %w", safeExe.Fd(), err)
+			}
+		}
 	}
 
 	// NOTE: when running a container with no PID namespace and the parent

libcontainer/utils/utils_unix.go

@@ -97,6 +97,17 @@ func fdRangeFrom(minFd int, fn fdFunc) error {
 	return nil
 }
 
+// GetMaxFds returns the max opened fd of current process.
+func GetMaxFds() (int, error) {
+	maxFd := -1
+	err := fdRangeFrom(-1, func(fd int) {
+		if fd > maxFd {
+			maxFd = fd
+		}
+	})
+	return maxFd, err
+}
+
 // CloseExecFrom sets the O_CLOEXEC flag on all file descriptors greater or
 // equal to minFd in the current process.
 func CloseExecFrom(minFd int) error {