runc exec --cap: do not add capabilities to ambient

Commit 98fe566 removed setting inheritable capabilities from runc exec
--cap, but neglected to also remove ambient capabilities.

An ambient capability could only be set if the same inheritable
capability is set, so as a result of the above change ambient
capabilities were not set (but due to a bug in gocapability package,
those errors are never reported).

Once we start using a library with the fix [1], that bug will become
apparent. Alas, we do not have any tests for runc exec --cap, so add
one.

Yet, if some inheritable bits are already set from spec, let's set
ambient to avoid a possible regression. Add a test case for that, too.

[1]: kolyshkin/capability#3

Fixes: 98fe566 ("runc: do not set inheritable capabilities")
Co-authored-by: lifubang <lifubang@acmcoder.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

exec.go

@@ -234,7 +234,12 @@ func getProcess(context *cli.Context, bundle string) (*specs.Process, error) {
 			p.Capabilities.Bounding = append(p.Capabilities.Bounding, c)
 			p.Capabilities.Effective = append(p.Capabilities.Effective, c)
 			p.Capabilities.Permitted = append(p.Capabilities.Permitted, c)
-			p.Capabilities.Ambient = append(p.Capabilities.Ambient, c)
+			// Since ambient capabilities can't be set without inherritable,
+			// and runc exec --cap don't set inheritable, let's only set
+			// ambient if we already have some inheritable bits set from spec.
+			if p.Capabilities.Inheritable != nil {
+				p.Capabilities.Ambient = append(p.Capabilities.Ambient, c)
+			}
 		}
 	}
 	// append the passed env variables

tests/integration/capabilities.bats

@@ -53,3 +53,66 @@ function teardown() {
 	[[ "${output}" == *"CapPrm:	0000000000200000"* ]]
 	[[ "${output}" == *"NoNewPrivs:	1"* ]]
 }
+
+@test "runc exec --cap" {
+	update_config '	  .process.args = ["/bin/sh"]
+			| .process.capabilities = {}'
+	runc run -d --console-socket "$CONSOLE_SOCKET" test_exec_cap
+	[ "$status" -eq 0 ]
+
+	runc exec test_exec_cap cat /proc/self/status
+	[ "$status" -eq 0 ]
+	# Check no capabilities are set.
+	[[ "${output}" == *"CapInh:	0000000000000000"* ]]
+	[[ "${output}" == *"CapPrm:	0000000000000000"* ]]
+	[[ "${output}" == *"CapEff:	0000000000000000"* ]]
+	[[ "${output}" == *"CapBnd:	0000000000000000"* ]]
+	[[ "${output}" == *"CapAmb:	0000000000000000"* ]]
+
+	runc exec --cap CAP_KILL --cap CAP_AUDIT_WRITE test_exec_cap cat /proc/self/status
+	[ "$status" -eq 0 ]
+	# Check capabilities are added into bounding/effective/permitted only,
+	# but not to inheritable or ambient.
+	#
+	# CAP_KILL is 5, the bit mask is 0x20 (1 << 5).
+	# CAP_AUDIT_WRITE is 26, the bit mask is 0x20000000 (1 << 26).
+	[[ "${output}" == *"CapInh:	0000000000000000"* ]]
+	[[ "${output}" == *"CapPrm:	0000000020000020"* ]]
+	[[ "${output}" == *"CapEff:	0000000020000020"* ]]
+	[[ "${output}" == *"CapBnd:	0000000020000020"* ]]
+	[[ "${output}" == *"CapAmb:	0000000000000000"* ]]
+}
+
+@test "runc exec --cap [ambient is set from spec]" {
+	update_config '	  .process.args = ["/bin/sh"]
+			| .process.capabilities.inheritable = ["CAP_CHOWN", "CAP_SYSLOG"]
+			| .process.capabilities.permitted = ["CAP_KILL", "CAP_CHOWN"]
+			| .process.capabilities.effective = ["CAP_KILL"]
+			| .process.capabilities.bounding = ["CAP_KILL", "CAP_CHOWN", "CAP_SYSLOG"]
+			| .process.capabilities.ambient = ["CAP_CHOWN"]'
+	runc run -d --console-socket "$CONSOLE_SOCKET" test_some_caps
+	[ "$status" -eq 0 ]
+
+	runc exec test_some_caps cat /proc/self/status
+	[ "$status" -eq 0 ]
+	# Check that capabilities are as set in spec.
+	#
+	# CAP_CHOWN is 0, the bit mask is 0x1 (1 << 0)
+	# CAP_KILL is 5, the bit mask is 0x20 (1 << 5).
+	# CAP_SYSLOG is 34, the bit mask is 0x400000000 (1 << 34).
+	[[ "${output}" == *"CapInh:	0000000400000001"* ]]
+	[[ "${output}" == *"CapPrm:	0000000000000021"* ]]
+	[[ "${output}" == *"CapEff:	0000000000000021"* ]]
+	[[ "${output}" == *"CapBnd:	0000000400000021"* ]]
+	[[ "${output}" == *"CapAmb:	0000000000000001"* ]]
+
+	# Check that if config.json has an inheritable capability set,
+	# runc exec --cap adds ambient capabilities.
+	runc exec --cap CAP_SYSLOG test_some_caps cat /proc/self/status
+	[ "$status" -eq 0 ]
+	[[ "${output}" == *"CapInh:	0000000400000001"* ]]
+	[[ "${output}" == *"CapPrm:	0000000400000021"* ]]
+	[[ "${output}" == *"CapEff:	0000000400000021"* ]]
+	[[ "${output}" == *"CapBnd:	0000000400000021"* ]]
+	[[ "${output}" == *"CapAmb:	0000000400000001"* ]]
+}