crypto/rand: remove /dev/urandom fallback and improve getrandom batching

The fallback was reachable on

    - Linux, where starting in Go 1.24 we require a kernel with
      getrandom(2), see #67001.

    - FreeBSD, which added getrandom(2) in FreeBSD 12.0, which we
      require since Go 1.19.

    - OpenBSD, which added getentropy(2) in OpenBSD 5.6, and we only
      support the latest version.

    - DragonFly BSD, which has getrandom(2) and where we support only
      the latest version.

    - NetBSD, where we switched to kern.arandom in CL 511036, available
      since NetBSD 4.0.

    - illumos, which has getrandom(2). (Supported versions unclear.)

    - Solaris, which had getrandom(2) at least since Oracle
      Solaris 11.4.

    - AIX, which... ugh, fine, but that code is now in rand_aix.go.

At the end of the day the platform-specific code is just a global
func(b []byte) error, so simplified the package around that assumption.

This also includes the following change, which used to be a separate CL.

    crypto/rand: improve getrandom batching and retry logic

    The previous logic assumed getrandom never returned short, and then
    applied stricter-than-necessary batch size limits, presumably to
    avoid short returns.

    This was still not sufficient because above 256 bytes getrandom(2)
    can be interrupted by a signal and return short *or* it can simply
    return EINTR if the pool is not initialized (regardless of buffer
    size).

    https://man.archlinux.org/man/getrandom.2#Interruption_by_a_signal_handler

    Whether this ever failed in practice is unknown: it would have been
    masked by the /dev/urandom fallback before.

    Instead, we apply buffer size limits only where necessary (really,
    only Solaris in practice and FreeBSD in theory) and then handle
    gracefully short returns and EINTR.

    Change-Id: I8677b457aab68a8fb6137a3b43538efc62eb7c93

It turns out that we now know that large getrandom calls *did* fail in
practice, falling back on /dev/urandom, because when we removed the
fallback TestBidiStreamReverseProxy with its 4KiB read started failing.

https://cr-buildbucket.appspot.com/build/8740779846954406033

For #66821

Change-Id: Iaca62997604f326501a51401cdc2659c2790ff22
Reviewed-on: https://go-review.googlesource.com/c/go/+/602495
Reviewed-by: Roland Shoemaker <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Daniel McCarney <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: FiloSottile

src/crypto/rand/rand.go

@@ -6,21 +6,55 @@
 // random number generator.
 package rand
 
-import "io"
+import (
+	"crypto/internal/boring"
+	"io"
+	"sync/atomic"
+	"time"
+)
 
 // Reader is a global, shared instance of a cryptographically
-// secure random number generator.
+// secure random number generator. It is safe for concurrent use.
 //
-//   - On Linux, FreeBSD, Dragonfly, and Solaris, Reader uses getrandom(2)
-//     if available, and /dev/urandom otherwise.
+//   - On Linux, FreeBSD, Dragonfly, and Solaris, Reader uses getrandom(2).
 //   - On macOS and iOS, Reader uses arc4random_buf(3).
-//   - On OpenBSD and NetBSD, Reader uses getentropy(2).
-//   - On other Unix-like systems, Reader reads from /dev/urandom.
+//   - On OpenBSD, Reader uses getentropy(2).
+//   - On NetBSD, Reader uses the kern.arandom sysctl.
 //   - On Windows, Reader uses the ProcessPrng API.
 //   - On js/wasm, Reader uses the Web Crypto API.
-//   - On wasip1/wasm, Reader uses random_get from wasi_snapshot_preview1.
+//   - On wasip1/wasm, Reader uses random_get.
 var Reader io.Reader
 
+func init() {
+	if boring.Enabled {
+		Reader = boring.RandReader
+		return
+	}
+	Reader = &reader{}
+}
+
+var firstUse atomic.Bool
+
+func warnBlocked() {
+	println("crypto/rand: blocked for 60 seconds waiting to read random data from the kernel")
+}
+
+type reader struct{}
+
+func (r *reader) Read(b []byte) (n int, err error) {
+	boring.Unreachable()
+	if firstUse.CompareAndSwap(false, true) {
+		// First use of randomness. Start timer to warn about
+		// being blocked on entropy not being available.
+		t := time.AfterFunc(time.Minute, warnBlocked)
+		defer t.Stop()
+	}
+	if err := read(b); err != nil {
+		return 0, err
+	}
+	return len(b), nil
+}
+
 // Read is a helper function that calls Reader.Read using io.ReadFull.
 // On return, n == len(b) if and only if err == nil.
 func Read(b []byte) (n int, err error) {

src/crypto/rand/rand_aix.go

@@ -0,0 +1,56 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package rand
+
+import (
+	"errors"
+	"io"
+	"os"
+	"sync"
+	"sync/atomic"
+	"syscall"
+)
+
+const urandomDevice = "/dev/urandom"
+
+var (
+	f    io.Reader
+	mu   sync.Mutex
+	used atomic.Bool
+)
+
+func read(b []byte) error {
+	if !used.Load() {
+		mu.Lock()
+		if !used.Load() {
+			dev, err := os.Open(urandomDevice)
+			if err != nil {
+				mu.Unlock()
+				return err
+			}
+			f = hideAgainReader{dev}
+			used.Store(true)
+		}
+		mu.Unlock()
+	}
+	if _, err := io.ReadFull(f, b); err != nil {
+		return err
+	}
+	return nil
+}
+
+// hideAgainReader masks EAGAIN reads from /dev/urandom.
+// See golang.org/issue/9205.
+type hideAgainReader struct {
+	r io.Reader
+}
+
+func (hr hideAgainReader) Read(p []byte) (n int, err error) {
+	n, err = hr.r.Read(p)
+	if errors.Is(err, syscall.EAGAIN) {
+		err = nil
+	}
+	return
+}

src/crypto/rand/rand_darwin.go

@@ -6,14 +6,15 @@ package rand
 
 import "internal/syscall/unix"
 
-func init() {
-	// arc4random_buf is the recommended application CSPRNG, accepts buffers of
-	// any size, and never returns an error.
-	//
-	// "The subsystem is re-seeded from the kernel random number subsystem on a
-	// regular basis, and also upon fork(2)." - arc4random(3)
-	//
-	// Note that despite its legacy name, it uses a secure CSPRNG (not RC4) in
-	// all supported macOS versions.
-	altGetRandom = func(b []byte) error { unix.ARC4Random(b); return nil }
+// arc4random_buf is the recommended application CSPRNG, accepts buffers of
+// any size, and never returns an error.
+//
+// "The subsystem is re-seeded from the kernel random number subsystem on a
+// regular basis, and also upon fork(2)." - arc4random(3)
+//
+// Note that despite its legacy name, it uses a secure CSPRNG (not RC4) in
+// all supported macOS versions.
+func read(b []byte) error {
+	unix.ARC4Random(b)
+	return nil
 }

src/crypto/rand/rand_getentropy.go

@@ -8,7 +8,5 @@ package rand
 
 import "internal/syscall/unix"
 
-func init() {
-	// getentropy(2) returns a maximum of 256 bytes per call.
-	altGetRandom = batched(unix.GetEntropy, 256)
-}
+// getentropy(2) returns a maximum of 256 bytes per call.
+var read = batched(unix.GetEntropy, 256)

src/crypto/rand/rand_getrandom.go

@@ -7,42 +7,53 @@
 package rand
 
 import (
+	"errors"
 	"internal/syscall/unix"
+	"math"
 	"runtime"
 	"syscall"
 )
 
-func init() {
-	var maxGetRandomRead int
-	switch runtime.GOOS {
-	case "linux", "android":
-		// Per the manpage:
-		//     When reading from the urandom source, a maximum of 33554431 bytes
-		//     is returned by a single call to getrandom() on systems where int
-		//     has a size of 32 bits.
-		maxGetRandomRead = (1 << 25) - 1
-	case "dragonfly", "freebsd", "illumos", "solaris":
-		maxGetRandomRead = 1 << 8
-	default:
-		panic("no maximum specified for GetRandom")
-	}
-	altGetRandom = batched(getRandom, maxGetRandomRead)
-}
+func read(b []byte) error {
+	// Linux, DragonFly, and illumos don't have a limit on the buffer size.
+	// FreeBSD has a limit of IOSIZE_MAX, which seems to be either INT_MAX or
+	// SSIZE_MAX. 2^31-1 is a safe and high enough value to use for all of them.
+	//
+	// Note that Linux returns "a maximum of 32Mi-1 bytes", but that will only
+	// result in a short read, not an error. Short reads can also happen above
+	// 256 bytes due to signals. Reads up to 256 bytes are guaranteed not to
+	// return short (and not to return an error IF THE POOL IS INITIALIZED) on
+	// at least Linux, FreeBSD, DragonFly, and Oracle Solaris, but we don't make
+	// use of that.
+	maxSize := math.MaxInt32
 
-// If the kernel is too old to support the getrandom syscall(),
-// unix.GetRandom will immediately return ENOSYS and we will then fall back to
-// reading from /dev/urandom in rand_unix.go. unix.GetRandom caches the ENOSYS
-// result so we only suffer the syscall overhead once in this case.
-// If the kernel supports the getrandom() syscall, unix.GetRandom will block
-// until the kernel has sufficient randomness (as we don't use GRND_NONBLOCK).
-// In this case, unix.GetRandom will not return an error.
-func getRandom(p []byte) error {
-	n, err := unix.GetRandom(p, 0)
-	if err != nil {
-		return err
+	// Oracle Solaris has a limit of 133120 bytes. Very specific.
+	//
+	//    The getrandom() and getentropy() functions fail if: [...]
+	//
+	//    - bufsz is <= 0 or > 133120, when GRND_RANDOM is not set
+	//
+	// https://docs.oracle.com/cd/E88353_01/html/E37841/getrandom-2.html
+	if runtime.GOOS == "solaris" {
+		maxSize = 133120
 	}
-	if n != len(p) {
-		return syscall.EIO
+
+	for len(b) > 0 {
+		size := len(b)
+		if size > maxSize {
+			size = maxSize
+		}
+		n, err := unix.GetRandom(b[:size], 0)
+		if errors.Is(err, syscall.EINTR) {
+			// If getrandom(2) is blocking, either because it is waiting for the
+			// entropy pool to become initialized or because we requested more
+			// than 256 bytes, it might get interrupted by a signal.
+			continue
+		}
+		if err != nil {
+			return err
+		}
+		b = b[n:]
 	}
 	return nil
 }

src/crypto/rand/rand_js.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build js && wasm
-
 package rand
 
 import "syscall/js"
@@ -12,27 +10,13 @@ import "syscall/js"
 // https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues#exceptions
 const maxGetRandomRead = 64 << 10
 
-var batchedGetRandom func([]byte) error
-
-func init() {
-	Reader = &reader{}
-	batchedGetRandom = batched(getRandom, maxGetRandomRead)
-}
-
-var jsCrypto = js.Global().Get("crypto")
-var uint8Array = js.Global().Get("Uint8Array")
-
-// reader implements a pseudorandom generator
+// read implements a pseudorandom generator
 // using JavaScript crypto.getRandomValues method.
 // See https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues.
-type reader struct{}
+var read = batched(getRandom, maxGetRandomRead)
 
-func (r *reader) Read(b []byte) (int, error) {
-	if err := batchedGetRandom(b); err != nil {
-		return 0, err
-	}
-	return len(b), nil
-}
+var jsCrypto = js.Global().Get("crypto")
+var uint8Array = js.Global().Get("Uint8Array")
 
 func getRandom(b []byte) error {
 	a := uint8Array.New(len(b))

src/crypto/rand/rand_plan9.go

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Plan9 cryptographically secure pseudorandom number
-// generator.
-
 package rand
 
 import (
@@ -18,44 +15,40 @@ import (
 
 const randomDevice = "/dev/random"
 
-func init() {
-	Reader = &reader{}
-}
+// This is a pseudorandom generator that seeds itself by reading from
+// /dev/random. The read function always returns the full amount asked for, or
+// else it returns an error. The generator is a fast key erasure RNG.
 
-// reader is a new pseudorandom generator that seeds itself by
-// reading from /dev/random. The Read method on the returned
-// reader always returns the full amount asked for, or else it
-// returns an error. The generator is a fast key erasure RNG.
-type reader struct {
+var (
 	mu      sync.Mutex
 	seeded  sync.Once
 	seedErr error
 	key     [32]byte
-}
+)
 
-func (r *reader) Read(b []byte) (n int, err error) {
-	r.seeded.Do(func() {
+func read(b []byte) error {
+	seeded.Do(func() {
 		t := time.AfterFunc(time.Minute, func() {
 			println("crypto/rand: blocked for 60 seconds waiting to read random data from the kernel")
 		})
 		defer t.Stop()
 		entropy, err := os.Open(randomDevice)
 		if err != nil {
-			r.seedErr = err
+			seedErr = err
 			return
 		}
 		defer entropy.Close()
-		_, r.seedErr = io.ReadFull(entropy, r.key[:])
+		_, seedErr = io.ReadFull(entropy, key[:])
 	})
-	if r.seedErr != nil {
-		return 0, r.seedErr
+	if seedErr != nil {
+		return seedErr
 	}
 
-	r.mu.Lock()
-	blockCipher, err := aes.NewCipher(r.key[:])
+	mu.Lock()
+	blockCipher, err := aes.NewCipher(key[:])
 	if err != nil {
-		r.mu.Unlock()
-		return 0, err
+		mu.Unlock()
+		return err
 	}
 	var (
 		counter uint64
@@ -68,13 +61,12 @@ func (r *reader) Read(b []byte) (n int, err error) {
 		}
 		byteorder.LePutUint64(block[:], counter)
 	}
-	blockCipher.Encrypt(r.key[:aes.BlockSize], block[:])
+	blockCipher.Encrypt(key[:aes.BlockSize], block[:])
 	inc()
-	blockCipher.Encrypt(r.key[aes.BlockSize:], block[:])
+	blockCipher.Encrypt(key[aes.BlockSize:], block[:])
 	inc()
-	r.mu.Unlock()
+	mu.Unlock()
 
-	n = len(b)
 	for len(b) >= aes.BlockSize {
 		blockCipher.Encrypt(b[:aes.BlockSize], block[:])
 		inc()
@@ -84,5 +76,5 @@ func (r *reader) Read(b []byte) (n int, err error) {
 		blockCipher.Encrypt(block[:], block[:])
 		copy(b, block[:])
 	}
-	return n, nil
+	return nil
 }