Add support for ambient capabilities (#11)

Ambient capabilities were added in Linux 4.3 and provide a way
to pass on capabilities to unprivileged processes easily.

Signed-off-by: Justin Cormack <[email protected]>

capability/capability.go

@@ -10,42 +10,42 @@ package capability
 type Capabilities interface {
 	// Get check whether a capability present in the given
 	// capabilities set. The 'which' value should be one of EFFECTIVE,
-	// PERMITTED, INHERITABLE or BOUNDING.
+	// PERMITTED, INHERITABLE, BOUNDING or AMBIENT.
 	Get(which CapType, what Cap) bool
 
 	// Empty check whether all capability bits of the given capabilities
 	// set are zero. The 'which' value should be one of EFFECTIVE,
-	// PERMITTED, INHERITABLE or BOUNDING.
+	// PERMITTED, INHERITABLE, BOUNDING or AMBIENT.
 	Empty(which CapType) bool
 
 	// Full check whether all capability bits of the given capabilities
 	// set are one. The 'which' value should be one of EFFECTIVE,
-	// PERMITTED, INHERITABLE or BOUNDING.
+	// PERMITTED, INHERITABLE, BOUNDING or AMBIENT.
 	Full(which CapType) bool
 
 	// Set sets capabilities of the given capabilities sets. The
 	// 'which' value should be one or combination (OR'ed) of EFFECTIVE,
-	// PERMITTED, INHERITABLE or BOUNDING.
+	// PERMITTED, INHERITABLE, BOUNDING or AMBIENT.
 	Set(which CapType, caps ...Cap)
 
 	// Unset unsets capabilities of the given capabilities sets. The
 	// 'which' value should be one or combination (OR'ed) of EFFECTIVE,
-	// PERMITTED, INHERITABLE or BOUNDING.
+	// PERMITTED, INHERITABLE, BOUNDING or AMBIENT.
 	Unset(which CapType, caps ...Cap)
 
 	// Fill sets all bits of the given capabilities kind to one. The
-	// 'kind' value should be one or combination (OR'ed) of CAPS or
-	// BOUNDS.
+	// 'kind' value should be one or combination (OR'ed) of CAPS,
+	// BOUNDS or AMBS.
 	Fill(kind CapType)
 
 	// Clear sets all bits of the given capabilities kind to zero. The
-	// 'kind' value should be one or combination (OR'ed) of CAPS or
-	// BOUNDS.
+	// 'kind' value should be one or combination (OR'ed) of CAPS,
+	// BOUNDS or AMBS.
 	Clear(kind CapType)
 
 	// String return current capabilities state of the given capabilities
 	// set as string. The 'which' value should be one of EFFECTIVE,
-	// PERMITTED, INHERITABLE or BOUNDING.
+	// PERMITTED, INHERITABLE BOUNDING or AMBIENT
 	StringCap(which CapType) string
 
 	// String return current capabilities state as string.

capability/capability_linux.go

@@ -235,9 +235,10 @@ func (c *capsV1) Apply(kind CapType) error {
 }
 
 type capsV3 struct {
-	hdr    capHeader
-	data   [2]capData
-	bounds [2]uint32
+	hdr     capHeader
+	data    [2]capData
+	bounds  [2]uint32
+	ambient [2]uint32
 }
 
 func (c *capsV3) Get(which CapType, what Cap) bool {
@@ -256,6 +257,8 @@ func (c *capsV3) Get(which CapType, what Cap) bool {
 		return (1<<uint(what))&c.data[i].inheritable != 0
 	case BOUNDING:
 		return (1<<uint(what))&c.bounds[i] != 0
+	case AMBIENT:
+		return (1<<uint(what))&c.ambient[i] != 0
 	}
 
 	return false
@@ -275,6 +278,9 @@ func (c *capsV3) getData(which CapType, dest []uint32) {
 	case BOUNDING:
 		dest[0] = c.bounds[0]
 		dest[1] = c.bounds[1]
+	case AMBIENT:
+		dest[0] = c.ambient[0]
+		dest[1] = c.ambient[1]
 	}
 }
 
@@ -313,6 +319,9 @@ func (c *capsV3) Set(which CapType, caps ...Cap) {
 		if which&BOUNDING != 0 {
 			c.bounds[i] |= 1 << uint(what)
 		}
+		if which&AMBIENT != 0 {
+			c.ambient[i] |= 1 << uint(what)
+		}
 	}
 }
 
@@ -336,6 +345,9 @@ func (c *capsV3) Unset(which CapType, caps ...Cap) {
 		if which&BOUNDING != 0 {
 			c.bounds[i] &= ^(1 << uint(what))
 		}
+		if which&AMBIENT != 0 {
+			c.ambient[i] &= ^(1 << uint(what))
+		}
 	}
 }
 
@@ -353,6 +365,10 @@ func (c *capsV3) Fill(kind CapType) {
 		c.bounds[0] = 0xffffffff
 		c.bounds[1] = 0xffffffff
 	}
+	if kind&AMBS == AMBS {
+		c.ambient[0] = 0xffffffff
+		c.ambient[1] = 0xffffffff
+	}
 }
 
 func (c *capsV3) Clear(kind CapType) {
@@ -369,6 +385,10 @@ func (c *capsV3) Clear(kind CapType) {
 		c.bounds[0] = 0
 		c.bounds[1] = 0
 	}
+	if kind&AMBS == AMBS {
+		c.ambient[0] = 0
+		c.ambient[1] = 0
+	}
 }
 
 func (c *capsV3) StringCap(which CapType) (ret string) {
@@ -410,6 +430,10 @@ func (c *capsV3) Load() (err error) {
 			fmt.Sscanf(line[4:], "nd:  %08x%08x", &c.bounds[1], &c.bounds[0])
 			break
 		}
+		if strings.HasPrefix(line, "CapA") {
+			fmt.Sscanf(line[4:], "mb:  %08x%08x", &c.ambient[1], &c.ambient[0])
+			break
+		}
 	}
 	f.Close()
 
@@ -442,7 +466,25 @@ func (c *capsV3) Apply(kind CapType) (err error) {
 	}
 
 	if kind&CAPS == CAPS {
-		return capset(&c.hdr, &c.data[0])
+		err = capset(&c.hdr, &c.data[0])
+		if err != nil {
+			return
+		}
+	}
+
+	if kind&AMBS == AMBS {
+		for i := Cap(0); i <= CAP_LAST_CAP; i++ {
+			action := pr_CAP_AMBIENT_LOWER
+			if c.Get(AMBIENT, i) {
+				action = pr_CAP_AMBIENT_RAISE
+			}
+			err := prctl(pr_CAP_AMBIENT, action, uintptr(i), 0, 0)
+			// Ignore EINVAL as not supported on kernels before 4.3
+			if errno, ok := err.(syscall.Errno); ok && errno == syscall.EINVAL {
+				err = nil
+				continue
+			}
+		}
 	}
 
 	return

capability/enum.go

@@ -20,6 +20,8 @@ func (c CapType) String() string {
 		return "bounding"
 	case CAPS:
 		return "caps"
+	case AMBIENT:
+		return "ambient"
 	}
 	return "unknown"
 }
@@ -29,9 +31,11 @@ const (
 	PERMITTED
 	INHERITABLE
 	BOUNDING
+	AMBIENT
 
 	CAPS   = EFFECTIVE | PERMITTED | INHERITABLE
 	BOUNDS = BOUNDING
+	AMBS   = AMBIENT
 )
 
 //go:generate go run enumgen/gen.go

capability/syscall_linux.go

@@ -38,6 +38,15 @@ func capset(hdr *capHeader, data *capData) (err error) {
 	return
 }
 
+// not yet in syscall
+const (
+	pr_CAP_AMBIENT           = 47
+	pr_CAP_AMBIENT_IS_SET    = uintptr(1)
+	pr_CAP_AMBIENT_RAISE     = uintptr(2)
+	pr_CAP_AMBIENT_LOWER     = uintptr(3)
+	pr_CAP_AMBIENT_CLEAR_ALL = uintptr(4)
+)
+
 func prctl(option int, arg2, arg3, arg4, arg5 uintptr) (err error) {
 	_, _, e1 := syscall.Syscall6(syscall.SYS_PRCTL, uintptr(option), arg2, arg3, arg4, arg5, 0)
 	if e1 != 0 {