init: don't special-case logrus fds

We close the logfd before execve so there's no need to special case it.
In addition, it turns out that (*os.File).Fd() doesn't handle the case
where the file was closed and so it seems suspect to use that kind of
check.

Signed-off-by: Aleksa Sarai <[email protected]>

Author: cyphar

libcontainer/logs/logs.go

@@ -4,19 +4,10 @@ import (
 	"bufio"
 	"encoding/json"
 	"io"
-	"os"
 
 	"github.com/sirupsen/logrus"
 )
 
-// IsLogrusFd returns whether the provided fd matches the one that logrus is
-// currently outputting to. This should only ever be called by UnsafeCloseFrom
-// from `runc init`.
-func IsLogrusFd(fd uintptr) bool {
-	file, ok := logrus.StandardLogger().Out.(*os.File)
-	return ok && file.Fd() == fd
-}
-
 func ForwardLogs(logPipe io.ReadCloser) chan error {
 	done := make(chan error, 1)
 	s := bufio.NewScanner(logPipe)

libcontainer/utils/utils_unix.go

@@ -16,8 +16,6 @@ import (
 	securejoin "github.com/cyphar/filepath-securejoin"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
-
-	"github.com/opencontainers/runc/libcontainer/logs"
 )
 
 // EnsureProcHandle returns whether or not the given file handle is on procfs.
@@ -142,12 +140,6 @@ func UnsafeCloseFrom(minFd int) error {
 			// don't have any choice.
 			return
 		}
-		if logs.IsLogrusFd(uintptr(fd)) {
-			// Do not close the logrus output fd. We cannot exec a pipe, and
-			// the contents are quite limited (very little attacker control,
-			// JSON-encoded) making shellcode attacks unlikely.
-			return
-		}
 		// There's nothing we can do about errors from close(2), and the
 		// only likely error to be seen is EBADF which indicates the fd was
 		// already closed (in which case, we got what we wanted).