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

There is a race situation when we are opening a file, if there is a
small fd was closed at that time, maybe it will be reused by safeExe.
Because of Go stdlib fds shuffling bug, if the fd of safeExe is too
small, go stdlib will dup3 it to another fd, or dup3 a other fd to this
fd, then it will cause the fd type cmd.Path refers to a random path,
and it can lead to an error "permission denied" when starting the process.
Please see #4294 and <golang/go#61751>.
So we should not use the original fd of safeExe, but use the fd after
shuffled by Go stdlib. Because Go stdlib will guarantee this fd refers to
the correct file.

Signed-off-by: lfbzhm <lifubang@acmcoder.com>

libcontainer/container_linux.go

@@ -560,7 +560,33 @@ func (c *Container) newParentProcess(p *Process) (parentProcess, error) {
 			if err != nil {
 				return nil, fmt.Errorf("unable to create safe /proc/self/exe clone for runc init: %w", err)
 			}
-			exePath = "/proc/self/fd/" + strconv.Itoa(int(safeExe.Fd()))
+
+			// TODO: After https://go-review.googlesource.com/c/go/+/515799 included
+			// in go versions supported by us, we can remove this logic and change
+			// it back to `exePath = "/proc/self/fd/" + strconv.Itoa(safeExe.Fd())`.
+
+			// 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
+			// 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>.
+			p.ExtraFiles = append(p.ExtraFiles, safeExe)
+
+			// There is a race situation when we are opening a file, if there is a
+			// small fd was closed at that time, maybe it will be reused by safeExe.
+			// Because of Go stdlib fds shuffling bug, if the fd of safeExe is too
+			// small, go stdlib will dup3 it to another fd, or dup3 a other fd to this
+			// fd, then it will cause the fd type cmd.Path refers to a random path,
+			// and it can lead to an error "permission denied" when starting the process.
+			// Please see #4294.
+			// So we should not use the original fd of safeExe, but use the fd after
+			// shuffled by Go stdlib. Because Go stdlib will guarantee this fd refers to
+			// the correct file.
+			exePath = "/proc/self/fd/" + strconv.Itoa(stdioFdCount+len(p.ExtraFiles)-1)
+
 			p.clonedExes = append(p.clonedExes, safeExe)
 			logrus.Debug("runc-dmz: using /proc/self/exe clone") // used for tests
 		}
@@ -618,18 +644,6 @@ 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
-		// 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)
-	}
-
 	// NOTE: when running a container with no PID namespace and the parent
 	//       process spawning the container is PID1 the pdeathsig is being
 	//       delivered to the container's init process by the kernel for some