runtime: overwrite startupRand instead of clearing it

AT_RANDOM is unfortunately used by libc before we run (so make sure it's
not cleared) but also is available to cgo programs after we did. It
would be unfortunate if a cgo program assumed it could use AT_RANDOM but
instead found all zeroes there.

Change-Id: I82eff34d8cf5a499b439052b7827b8ef7cabc21d
Reviewed-on: https://go-review.googlesource.com/c/go/+/608437
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

3 files changed, 46 insertions, 6 deletions

diff --git a/src/runtime/os_linux.go b/src/runtime/os_linux.go
index 979761cc6a..8f5cf6db8a 100644
--- a/src/runtime/os_linux.go
+++ b/src/runtime/os_linux.go
@@ -296,13 +296,19 @@ func sysargs(argc int32, argv **byte) {
 var secureMode bool
 
 func sysauxv(auxv []uintptr) (pairs int) {
+	// Process the auxiliary vector entries provided by the kernel when the
+	// program is executed. See getauxval(3).
 	var i int
 	for ; auxv[i] != _AT_NULL; i += 2 {
 		tag, val := auxv[i], auxv[i+1]
 		switch tag {
 		case _AT_RANDOM:
-			// The kernel provides a pointer to 16-bytes
-			// worth of random data.
+			// The kernel provides a pointer to 16 bytes of cryptographically
+			// random data. Note that in cgo programs this value may have
+			// already been used by libc at this point, and in particular glibc
+			// and musl use the value as-is for stack and pointer protector
+			// cookies from libc_start_main and/or dl_start. Also, cgo programs
+			// may use the value after we do.
 			startupRand = (*[16]byte)(unsafe.Pointer(val))[:]
 
 		case _AT_PAGESZ:
@@ -354,6 +360,8 @@ func osinit() {
 var urandom_dev = []byte("/dev/urandom\x00")
 
 func readRandom(r []byte) int {
+	// Note that all supported Linux kernels should provide AT_RANDOM which
+	// populates startupRand, so this fallback should be unreachable.
 	fd := open(&urandom_dev[0], 0 /* O_RDONLY */, 0)
 	n := read(fd, unsafe.Pointer(&r[0]), int32(len(r)))
 	closefd(fd)
diff --git a/src/runtime/os_openbsd.go b/src/runtime/os_openbsd.go
index 9a21d6a8d0..574bfa8b17 100644
--- a/src/runtime/os_openbsd.go
+++ b/src/runtime/os_openbsd.go
@@ -139,6 +139,9 @@ func osinit() {
 	physPageSize = getPageSize()
 }
 
+// TODO(#69781): set startupRand using the .openbsd.randomdata ELF section.
+// See SPECS.randomdata.
+
 var urandom_dev = []byte("/dev/urandom\x00")
 
 //go:nosplit
diff --git a/src/runtime/rand.go b/src/runtime/rand.go
index 11be6552aa..2e44858ee2 100644
--- a/src/runtime/rand.go
+++ b/src/runtime/rand.go
@@ -7,6 +7,7 @@
 package runtime
 
 import (
+	"internal/byteorder"
 	"internal/chacha8rand"
 	"internal/goarch"
 	"internal/runtime/math"
@@ -41,14 +42,15 @@ func randinit() {
 	}
 
 	seed := &globalRand.seed
-	if startupRand != nil {
+	if len(startupRand) >= 16 &&
+		// Check that at least the first two words of startupRand weren't
+		// cleared by any libc initialization.
+		!allZero(startupRand[:8]) && !allZero(startupRand[8:16]) {
 		for i, c := range startupRand {
 			seed[i%len(seed)] ^= c
 		}
-		clear(startupRand)
-		startupRand = nil
 	} else {
-		if readRandom(seed[:]) != len(seed) {
+		if readRandom(seed[:]) != len(seed) || allZero(seed[:]) {
 			// readRandom should never fail, but if it does we'd rather
 			// not make Go binaries completely unusable, so make up
 			// some random data based on the current time.
@@ -58,6 +60,25 @@ func randinit() {
 	}
 	globalRand.state.Init(*seed)
 	clear(seed[:])
+
+	if startupRand != nil {
+		// Overwrite startupRand instead of clearing it, in case cgo programs
+		// access it after we used it.
+		for len(startupRand) > 0 {
+			buf := make([]byte, 8)
+			for {
+				if x, ok := globalRand.state.Next(); ok {
+					byteorder.BePutUint64(buf, x)
+					break
+				}
+				globalRand.state.Refill()
+			}
+			n := copy(startupRand, buf)
+			startupRand = startupRand[n:]
+		}
+		startupRand = nil
+	}
+
 	globalRand.init = true
 	unlock(&globalRand.lock)
 }
@@ -88,6 +109,14 @@ func readTimeRandom(r []byte) {
 	}
 }
 
+func allZero(b []byte) bool {
+	var acc byte
+	for _, x := range b {
+		acc |= x
+	}
+	return acc == 0
+}
+
 // bootstrapRand returns a random uint64 from the global random generator.
 func bootstrapRand() uint64 {
 	lock(&globalRand.lock)