rootfs: switch to fd-based handling of mountpoint targets

An attacker could race with us during mount configuration in order to
trick us into mounting over an unexpected path. This would bypass
checkProcMount() and would allow for security profiles to be left
unapplied by mounting over /proc/self/attr/... (or even more serious
outcomes such as killing the entire system by tricking runc into writing
strings to /proc/sysrq-trigger).

This is a larger issue with our current mount infrastructure, and the
ideal solution would be to rewrite it all to be fd-based (which would
also allow us to support the "new" mount API, which also avoids a bunch
of other issues with mount(8)). However, such a rewrite is not really
workable as a security fix, so this patch is a bit of a compromise
approach to fix the issue while also moving us a bit towards that
eventual end-goal.

The core issue in CVE-2025-52881 is that we currently use the (insecure)
SecureJoin to re-resolve mountpoint target paths multiple times during
mounting. Rather than generating a string from createMountpoint(), we
instead open an *os.File handle to the target mountpoint directly and
then operate on that handle. This will make it easier to remove
utils.WithProcfd() and rework mountViaFds() in the future.

The only real issue we need to work around is that we need to re-open
the mount target after doing the mount in order to get a handle to the
mountpoint -- pathrs.Reopen() doesn't work in this case (it just
re-opens the inode under the mountpoint) so we need to do a naive
re-open using the full path. Note that if we used move_mount(2) this
wouldn't be a problem because we would have a handle to the mountpoint
itself.

Note that this is still somewhat of a temporary solution -- ideally
mountViaFds would use *os.File directly to let us avoid some other
issues with using bare /proc/... paths, as well as also letting us more
easily use the new mount API on modern kernels.

Fixes: GHSA-cgrx-mc8f-2prm CVE-2025-52881
Co-developed-by: lifubang <lifubang@acmcoder.com>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

libcontainer/utils/utils_unix.go

@@ -153,8 +153,8 @@ func NewSockPair(name string) (parent, child *os.File, err error) {
 // the passed closure (the file handle will be freed once the closure returns).
 func WithProcfd(root, unsafePath string, fn func(procfd string) error) error {
 	// Remove the root then forcefully resolve inside the root.
-	unsafePath = stripRoot(root, unsafePath)
-	path, err := securejoin.SecureJoin(root, unsafePath)
+	unsafePath = StripRoot(root, unsafePath)
+	fullPath, err := securejoin.SecureJoin(root, unsafePath)
 	if err != nil {
 		return fmt.Errorf("resolving path inside rootfs failed: %w", err)
 	}
@@ -163,7 +163,7 @@ func WithProcfd(root, unsafePath string, fn func(procfd string) error) error {
 	defer closer()
 
 	// Open the target path.
-	fh, err := os.OpenFile(path, unix.O_PATH|unix.O_CLOEXEC, 0)
+	fh, err := os.OpenFile(fullPath, unix.O_PATH|unix.O_CLOEXEC, 0)
 	if err != nil {
 		return fmt.Errorf("open o_path procfd: %w", err)
 	}
@@ -173,13 +173,24 @@ func WithProcfd(root, unsafePath string, fn func(procfd string) error) error {
 	// Double-check the path is the one we expected.
 	if realpath, err := os.Readlink(procfd); err != nil {
 		return fmt.Errorf("procfd verification failed: %w", err)
-	} else if realpath != path {
+	} else if realpath != fullPath {
 		return fmt.Errorf("possibly malicious path detected -- refusing to operate on %s", realpath)
 	}
 
 	return fn(procfd)
 }
 
+// WithProcfdFile is a very minimal wrapper around [ProcThreadSelfFd], intended
+// to make migrating from [WithProcfd] and [WithProcfdPath] usage easier. The
+// caller is responsible for making sure that the provided file handle is
+// actually safe to operate on.
+func WithProcfdFile(file *os.File, fn func(procfd string) error) error {
+	fdpath, closer := ProcThreadSelfFd(file.Fd())
+	defer closer()
+
+	return fn(fdpath)
+}
+
 type ProcThreadSelfCloser func()
 
 var (