From 36a964f86a1f30a7c3d8900312d8f5f2beff9831 Mon Sep 17 00:00:00 2001
From: lfbzhm <lifubang@acmcoder.com>
Date: Thu, 17 Oct 2024 17:48:23 +0000
Subject: [PATCH] fix a fd resue race when starting runc init

In #3987(0e9a335), we may use a memfd to copy run to start runc init,
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>.

But because of we have added safeExe to the set of ExtraFiles, if the
fd of safeExe is too small, go stdlib will dup3 it to another fd, then
it will cause the original fd closed. (#4294)

Signed-off-by: lfbzhm <lifubang@acmcoder.com>
---
 libcontainer/container_linux.go  | 15 +++++++++++++++
 libcontainer/utils/utils_unix.go | 11 +++++++++++
 2 files changed, 26 insertions(+)

diff --git a/libcontainer/container_linux.go b/libcontainer/container_linux.go
index 0b17168c2bb..6d0ecdb2177 100644
--- a/libcontainer/container_linux.go
+++ b/libcontainer/container_linux.go
@@ -628,6 +628,21 @@ func (c *Container) newParentProcess(p *Process) (parentProcess, error) {
 		//
 		// See <https://github.com/golang/go/issues/61751>.
 		cmd.ExtraFiles = append(cmd.ExtraFiles, safeExe)
+
+		// But because of we have added safeExe to the set of ExtraFiles, if the
+		// fd of safeExe is too small, go stdlib will dup3 it to another fd, then
+		// it will cause the original fd closed. (#4294)
+		if int(safeExe.Fd()) <= stdioFdCount+len(cmd.ExtraFiles) {
+			maxFd, err := utils.GetMaxFds()
+			if err != nil {
+				return nil, fmt.Errorf("unable to get the max opened fd of current process: %w", err)
+			}
+			maxFd = maxFd + 1
+			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)
+		}
 	}
 
 	// NOTE: when running a container with no PID namespace and the parent
diff --git a/libcontainer/utils/utils_unix.go b/libcontainer/utils/utils_unix.go
index cc84597a7ce..e76b5f17e5a 100644
--- a/libcontainer/utils/utils_unix.go
+++ b/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 {