ssh/tailssh: fix privilege dropping on FreeBSD; add tests

On FreeBSD and Darwin, changing a process's supplementary groups with
setgroups(2) will also change the egid of the process, setting it to the
first entry in the provided list. This is distinct from the behaviour on
other platforms (and possibly a violation of the POSIX standard).

Because of this, on FreeBSD with no TTY, our incubator code would
previously not change the process's gid, because it would read the
newly-changed egid, compare it against the expected egid, and since they
matched, not change the gid. Because we didn't use the 'login' program
on FreeBSD without a TTY, this would propagate to a child process.

This could be observed by running "id -p" in two contexts. The expected
output, and the output returned when running from a SSH shell, is:

    andrew@freebsd:~ $ id -p
    uid         andrew
    groups      andrew

However, when run via "ssh andrew@freebsd id -p", the output would be:

    $ ssh andrew@freebsd id -p
    login       root
    uid         andrew
    rgid        wheel
    groups      andrew

(this could also be observed via "id -g -r" to print just the gid)

We fix this by pulling the details of privilege dropping out into their
own function and prepending the expected gid to the start of the list on
Darwin and FreeBSD.

Finally, we add some tests that run a child process, drop privileges,
and assert that the final UID/GID/additional groups are what we expect.

More information can be found in the following article:
    https://www.usenix.org/system/files/login/articles/325-tsafrir.pdf

Updates #7616
Alternative to #7609

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I0e6513c31b121108b50fe561c89e5816d84a45b9

ssh/tailssh/incubator.go

@@ -235,6 +235,7 @@ func beIncubator(args []string) error {
 	if err == nil && sessionCloser != nil {
 		defer sessionCloser()
 	}
+
 	var groupIDs []int
 	for _, g := range strings.Split(ia.groups, ",") {
 		gid, err := strconv.ParseInt(g, 10, 32)
@@ -244,22 +245,10 @@ func beIncubator(args []string) error {
 		groupIDs = append(groupIDs, int(gid))
 	}
 
-	if err := setGroups(groupIDs); err != nil {
+	if err := dropPrivileges(logf, int(ia.uid), ia.gid, groupIDs); err != nil {
 		return err
 	}
-	if egid := os.Getegid(); egid != ia.gid {
-		if err := syscall.Setgid(int(ia.gid)); err != nil {
-			logf(err.Error())
-			os.Exit(1)
-		}
-	}
-	if euid != ia.uid {
-		// Switch users if required before starting the desired process.
-		if err := syscall.Setuid(int(ia.uid)); err != nil {
-			logf(err.Error())
-			os.Exit(1)
-		}
-	}
+
 	if ia.isSFTP {
 		logf("handling sftp")
 
@@ -304,6 +293,108 @@ func beIncubator(args []string) error {
 	return err
 }
 
+// TODO(andrew-d): verify that this works in more configurations before
+// enabling by default.
+const assertDropPrivileges = false
+
+// dropPrivileges contains all the logic for dropping privileges to a different
+// UID, GID, and set of supplementary groups. This function is
+// security-sensitive and ordering-dependent; please be very cautious if/when
+// refactoring.
+//
+// WARNING: if you change this function, you *MUST* run the TestDropPrivileges
+// test in this package as root on at least Linux, FreeBSD and Darwin. This can
+// be done by running:
+//
+//	go test -c ./ssh/tailssh/ && sudo ./tailssh.test -test.v -test.run TestDropPrivileges
+func dropPrivileges(logf logger.Logf, wantUid, wantGid int, supplementaryGroups []int) error {
+	fatalf := func(format string, args ...any) {
+		logf(format, args...)
+		os.Exit(1)
+	}
+
+	euid := os.Geteuid()
+	egid := os.Getegid()
+
+	if runtime.GOOS == "darwin" || runtime.GOOS == "freebsd" {
+		// On FreeBSD and Darwin, the first entry returned from the
+		// getgroups(2) syscall is the egid, and changing it with
+		// setgroups(2) changes the egid of the process. This is
+		// technically a violation of the POSIX standard; see the
+		// following article for more detail:
+		//    https://www.usenix.org/system/files/login/articles/325-tsafrir.pdf
+		//
+		// In this case, we add an entry at the beginning of the
+		// groupIDs list containing the expected gid if it's not
+		// already there, which modifies the egid and additional groups
+		// as one unit.
+		if len(supplementaryGroups) == 0 || supplementaryGroups[0] != wantGid {
+			supplementaryGroups = append([]int{wantGid}, supplementaryGroups...)
+		}
+	}
+
+	if err := setGroups(supplementaryGroups); err != nil {
+		return err
+	}
+	if egid != wantGid {
+		// On FreeBSD and Darwin, we may have already called the
+		// equivalent of setegid(wantGid) via the call to setGroups,
+		// above. However, per the manpage, setgid(getegid()) is an
+		// allowed operation regardless of privilege level.
+		//
+		// FreeBSD:
+		//	The setgid() system call is permitted if the specified ID
+		//	is equal to the real group ID or the effective group ID
+		//	of the process, or if the effective user ID is that of
+		//	the super user.
+		//
+		// Darwin:
+		//	The setgid() function is permitted if the effective
+		//	user ID is that of the super user, or if the specified
+		//	group ID is the same as the effective group ID.  If
+		//	not, but the specified group ID is the same as the real
+		//	group ID, setgid() will set the effective group ID to
+		//	the real group ID.
+		if err := syscall.Setgid(wantGid); err != nil {
+			fatalf("Setgid(%d): %v", wantGid, err)
+		}
+	}
+	if euid != wantUid {
+		// Switch users if required before starting the desired process.
+		if err := syscall.Setuid(wantUid); err != nil {
+			fatalf("Setuid(%d): %v", wantUid, err)
+		}
+	}
+
+	// If we changed either the UID or GID, defensively assert that we
+	// cannot reset the it back to our original values, and that the
+	// current egid/euid are the expected values after we change
+	// everything; if not, we exit the process.
+	if assertDropPrivileges {
+		if egid != wantGid {
+			if err := syscall.Setegid(egid); err == nil {
+				fatalf("unexpectedly able to set egid back to %d", egid)
+			}
+		}
+		if euid != wantUid {
+			if err := syscall.Seteuid(euid); err == nil {
+				fatalf("unexpectedly able to set euid back to %d", euid)
+			}
+		}
+
+		if got := os.Getegid(); got != wantGid {
+			fatalf("got egid=%d, want %d", got, wantGid)
+		}
+		if got := os.Geteuid(); got != wantUid {
+			fatalf("got euid=%d, want %d", got, wantUid)
+		}
+
+		// TODO(andrew-d): assert that our supplementary groups are correct
+	}
+
+	return nil
+}
+
 // launchProcess launches an incubator process for the provided session.
 // It is responsible for configuring the process execution environment.
 // The caller can wait for the process to exit by calling cmd.Wait().