configs: make id mappings int64 to better handle 32-bit

(This is a cherry-pick of 482e563.)

Using ints for all of our mapping structures means that a 32-bit binary
errors out when trying to parse /proc/self/*id_map:

  failed to cache mappings for userns: failed to parse uid_map of userns /proc/1/ns/user:
  parsing id map failed: invalid format in line "         0          0 4294967295": integer overflow on token 4294967295

This issue was unearthed by commit 1912d59 ("*: actually support
joining a userns with a new container") but the underlying issue has
been present since the docker/libcontainer days.

In theory, switching to uint32 (to match the spec) instead of int64
would also work, but keeping everything signed seems much less
error-prone. It's also important to note that a mapping might be too
large for an int on 32-bit, so we detect this during the mapping.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

libcontainer/configs/config.go

@@ -21,9 +21,9 @@ type Rlimit struct {
 
 // IDMap represents UID/GID Mappings for User Namespaces.
 type IDMap struct {
-	ContainerID int `json:"container_id"`
-	HostID      int `json:"host_id"`
-	Size        int `json:"size"`
+	ContainerID int64 `json:"container_id"`
+	HostID      int64 `json:"host_id"`
+	Size        int64 `json:"size"`
 }
 
 // Seccomp represents syscall restrictions

libcontainer/configs/config_linux.go

@@ -1,6 +1,10 @@
 package configs
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+	"math"
+)
 
 var (
 	errNoUIDMap   = errors.New("User namespaces enabled, but no uid mappings found.")
@@ -16,11 +20,18 @@ func (c Config) HostUID(containerId int) (int, error) {
 		if c.UidMappings == nil {
 			return -1, errNoUIDMap
 		}
-		id, found := c.hostIDFromMapping(containerId, c.UidMappings)
+		id, found := c.hostIDFromMapping(int64(containerId), c.UidMappings)
 		if !found {
 			return -1, errNoUserMap
 		}
-		return id, nil
+		// If we are a 32-bit binary running on a 64-bit system, it's possible
+		// the mapped user is too large to store in an int, which means we
+		// cannot do the mapping. We can't just return an int64, because
+		// os.Setuid() takes an int.
+		if id > math.MaxInt {
+			return -1, fmt.Errorf("mapping for uid %d (host id %d) is larger than native integer size (%d)", containerId, id, math.MaxInt)
+		}
+		return int(id), nil
 	}
 	// Return unchanged id.
 	return containerId, nil
@@ -39,11 +50,18 @@ func (c Config) HostGID(containerId int) (int, error) {
 		if c.GidMappings == nil {
 			return -1, errNoGIDMap
 		}
-		id, found := c.hostIDFromMapping(containerId, c.GidMappings)
+		id, found := c.hostIDFromMapping(int64(containerId), c.GidMappings)
 		if !found {
 			return -1, errNoGroupMap
 		}
-		return id, nil
+		// If we are a 32-bit binary running on a 64-bit system, it's possible
+		// the mapped user is too large to store in an int, which means we
+		// cannot do the mapping. We can't just return an int64, because
+		// os.Setgid() takes an int.
+		if id > math.MaxInt {
+			return -1, fmt.Errorf("mapping for gid %d (host id %d) is larger than native integer size (%d)", containerId, id, math.MaxInt)
+		}
+		return int(id), nil
 	}
 	// Return unchanged id.
 	return containerId, nil
@@ -57,7 +75,7 @@ func (c Config) HostRootGID() (int, error) {
 
 // Utility function that gets a host ID for a container ID from user namespace map
 // if that ID is present in the map.
-func (c Config) hostIDFromMapping(containerID int, uMap []IDMap) (int, bool) {
+func (c Config) hostIDFromMapping(containerID int64, uMap []IDMap) (int64, bool) {
 	for _, m := range uMap {
 		if (containerID >= m.ContainerID) && (containerID <= (m.ContainerID + m.Size - 1)) {
 			hostID := m.HostID + (containerID - m.ContainerID)

libcontainer/container_linux.go

@@ -2268,7 +2268,7 @@ func ignoreTerminateErrors(err error) error {
 
 func requiresRootOrMappingTool(c *configs.Config) bool {
 	gidMap := []configs.IDMap{
-		{ContainerID: 0, HostID: os.Getegid(), Size: 1},
+		{ContainerID: 0, HostID: int64(os.Getegid()), Size: 1},
 	}
 	return !reflect.DeepEqual(c.GidMappings, gidMap)
 }

libcontainer/specconv/spec_linux.go

@@ -927,9 +927,9 @@ next:
 func setupUserNamespace(spec *specs.Spec, config *configs.Config) error {
 	create := func(m specs.LinuxIDMapping) configs.IDMap {
 		return configs.IDMap{
-			HostID:      int(m.HostID),
-			ContainerID: int(m.ContainerID),
-			Size:        int(m.Size),
+			HostID:      int64(m.HostID),
+			ContainerID: int64(m.ContainerID),
+			Size:        int64(m.Size),
 		}
 	}
 	if spec.Linux != nil {