arc4random: simplify design for better safety

Rather than buffering 16 MiB of entropy in userspace (by way of
chacha20), simply call getrandom() every time.

This approach is doubtlessly slower, for now, but trying to prematurely
optimize arc4random appears to be leading toward all sorts of nasty
properties and gotchas. Instead, this patch takes a much more
conservative approach. The interface is added as a basic loop wrapper
around getrandom(), and then later, the kernel and libc together can
work together on optimizing that.

This prevents numerous issues in which userspace is unaware of when it
really must throw away its buffer, since we avoid buffering all
together. Future improvements may include userspace learning more from
the kernel about when to do that, which might make these sorts of
chacha20-based optimizations more possible. The current heuristic of 16
MiB is meaningless garbage that doesn't correspond to anything the
kernel might know about. So for now, let's just do something
conservative that we know is correct and won't lead to cryptographic
issues for users of this function.

This patch might be considered along the lines of, "optimization is the
root of all evil," in that the much more complex implementation it
replaces moves too fast without considering security implications,
whereas the incremental approach done here is a much safer way of going
about things. Once this lands, we can take our time in optimizing this
properly using new interplay between the kernel and userspace.

getrandom(0) is used, since that's the one that ensures the bytes
returned are cryptographically secure. But on systems without it, we
fallback to using /dev/urandom. This is unfortunate because it means
opening a file descriptor, but there's not much of a choice. Secondly,
as part of the fallback, in order to get more or less the same
properties of getrandom(0), we poll on /dev/random, and if the poll
succeeds at least once, then we assume the RNG is initialized. This is a
rough approximation, as the ancient "non-blocking pool" initialized
after the "blocking pool", not before, and it may not port back to all
ancient kernels, though it does to all kernels supported by glibc
(≥3.2), so generally it's the best approximation we can do.

The motivation for including arc4random, in the first place, is to have
source-level compatibility with existing code. That means this patch
doesn't attempt to litigate the interface itself. It does, however,
choose a conservative approach for implementing it.

Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Cristian Rodríguez <crrodriguez@opensuse.org>
Cc: Paul Eggert <eggert@cs.ucla.edu>
Cc: Mark Harris <mark.hsj@gmail.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: linux-crypto@vger.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

6 files changed, 0 insertions, 345 deletions

diff --git a/sysdeps/powerpc/powerpc64/be/multiarch/Makefile b/sysdeps/powerpc/powerpc64/be/multiarch/Makefile
deleted file mode 100644
index 8c75165f7f..0000000000
--- a/sysdeps/powerpc/powerpc64/be/multiarch/Makefile
+++ /dev/null
@@ -1,4 +0,0 @@
-ifeq ($(subdir),stdlib)
-sysdep_routines += chacha20-ppc
-CFLAGS-chacha20-ppc.c += -mcpu=power8
-endif
diff --git a/sysdeps/powerpc/powerpc64/be/multiarch/chacha20-ppc.c b/sysdeps/powerpc/powerpc64/be/multiarch/chacha20-ppc.c
deleted file mode 100644
index cf9e735326..0000000000
--- a/sysdeps/powerpc/powerpc64/be/multiarch/chacha20-ppc.c
+++ /dev/null
@@ -1 +0,0 @@
-#include <sysdeps/powerpc/powerpc64/power8/chacha20-ppc.c>
diff --git a/sysdeps/powerpc/powerpc64/be/multiarch/chacha20_arch.h b/sysdeps/powerpc/powerpc64/be/multiarch/chacha20_arch.h
deleted file mode 100644
index 08494dc045..0000000000
--- a/sysdeps/powerpc/powerpc64/be/multiarch/chacha20_arch.h
+++ /dev/null
@@ -1,42 +0,0 @@
-/* PowerPC optimization for ChaCha20.
-   Copyright (C) 2022 Free Software Foundation, Inc.
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library; if not, see
-   <https://www.gnu.org/licenses/>.  */
-
-#include <stdbool.h>
-#include <ldsodefs.h>
-
-unsigned int __chacha20_power8_blocks4 (uint32_t *state, uint8_t *dst,
-					const uint8_t *src, size_t nblks)
-     attribute_hidden;
-
-static void
-chacha20_crypt (uint32_t *state, uint8_t *dst,
-		const uint8_t *src, size_t bytes)
-{
-  _Static_assert (CHACHA20_BUFSIZE % 4 == 0,
-		  "CHACHA20_BUFSIZE not multiple of 4");
-  _Static_assert (CHACHA20_BUFSIZE >= CHACHA20_BLOCK_SIZE * 4,
-		  "CHACHA20_BUFSIZE < CHACHA20_BLOCK_SIZE * 4");
-
-  unsigned long int hwcap = GLRO(dl_hwcap);
-  unsigned long int hwcap2 = GLRO(dl_hwcap2);
-  if (hwcap2 & PPC_FEATURE2_ARCH_2_07 && hwcap & PPC_FEATURE_HAS_ALTIVEC)
-    __chacha20_power8_blocks4 (state, dst, src,
-			       CHACHA20_BUFSIZE / CHACHA20_BLOCK_SIZE);
-  else
-    chacha20_crypt_generic (state, dst, src, bytes);
-}
diff --git a/sysdeps/powerpc/powerpc64/power8/Makefile b/sysdeps/powerpc/powerpc64/power8/Makefile
index abb0aa3f11..71a59529f3 100644
--- a/sysdeps/powerpc/powerpc64/power8/Makefile
+++ b/sysdeps/powerpc/powerpc64/power8/Makefile
@@ -1,8 +1,3 @@
 ifeq ($(subdir),string)
 sysdep_routines += strcasestr-ppc64
 endif
-
-ifeq ($(subdir),stdlib)
-sysdep_routines += chacha20-ppc
-CFLAGS-chacha20-ppc.c += -mcpu=power8
-endif
diff --git a/sysdeps/powerpc/powerpc64/power8/chacha20-ppc.c b/sysdeps/powerpc/powerpc64/power8/chacha20-ppc.c
deleted file mode 100644
index 0bbdcb9363..0000000000
--- a/sysdeps/powerpc/powerpc64/power8/chacha20-ppc.c
+++ /dev/null
@@ -1,256 +0,0 @@
-/* Optimized PowerPC implementation of ChaCha20 cipher.
-   Copyright (C) 2022 Free Software Foundation, Inc.
-
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library; if not, see
-   <https://www.gnu.org/licenses/>.  */
-
-/* chacha20-ppc.c - PowerPC vector implementation of ChaCha20
-   Copyright (C) 2019 Jussi Kivilinna <jussi.kivilinna@iki.fi>
-
-   This file is part of Libgcrypt.
-
-   Libgcrypt is free software; you can redistribute it and/or modify
-   it under the terms of the GNU Lesser General Public License as
-   published by the Free Software Foundation; either version 2.1 of
-   the License, or (at your option) any later version.
-
-   Libgcrypt is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-   GNU Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with this program; if not, see <https://www.gnu.org/licenses/>.
- */
-
-#include <altivec.h>
-#include <endian.h>
-#include <stddef.h>
-#include <stdint.h>
-#include <sys/cdefs.h>
-
-typedef vector unsigned char vector16x_u8;
-typedef vector unsigned int vector4x_u32;
-typedef vector unsigned long long vector2x_u64;
-
-#if __BYTE_ORDER == __BIG_ENDIAN
-static const vector16x_u8 le_bswap_const =
-  { 3, 2, 1, 0, 7, 6, 5, 4, 11, 10, 9, 8, 15, 14, 13, 12 };
-#endif
-
-static inline vector4x_u32
-vec_rol_elems (vector4x_u32 v, unsigned int idx)
-{
-#if __BYTE_ORDER != __BIG_ENDIAN
-  return vec_sld (v, v, (16 - (4 * idx)) & 15);
-#else
-  return vec_sld (v, v, (4 * idx) & 15);
-#endif
-}
-
-static inline vector4x_u32
-vec_load_le (unsigned long offset, const unsigned char *ptr)
-{
-  vector4x_u32 vec;
-  vec = vec_vsx_ld (offset, (const uint32_t *)ptr);
-#if __BYTE_ORDER == __BIG_ENDIAN
-  vec = (vector4x_u32) vec_perm ((vector16x_u8)vec, (vector16x_u8)vec,
-				 le_bswap_const);
-#endif
-  return vec;
-}
-
-static inline void
-vec_store_le (vector4x_u32 vec, unsigned long offset, unsigned char *ptr)
-{
-#if __BYTE_ORDER == __BIG_ENDIAN
-  vec = (vector4x_u32)vec_perm((vector16x_u8)vec, (vector16x_u8)vec,
-			       le_bswap_const);
-#endif
-  vec_vsx_st (vec, offset, (uint32_t *)ptr);
-}
-
-
-static inline vector4x_u32
-vec_add_ctr_u64 (vector4x_u32 v, vector4x_u32 a)
-{
-#if __BYTE_ORDER == __BIG_ENDIAN
-  static const vector16x_u8 swap32 =
-    { 4, 5, 6, 7, 0, 1, 2, 3, 12, 13, 14, 15, 8, 9, 10, 11 };
-  vector2x_u64 vec, add, sum;
-
-  vec = (vector2x_u64)vec_perm ((vector16x_u8)v, (vector16x_u8)v, swap32);
-  add = (vector2x_u64)vec_perm ((vector16x_u8)a, (vector16x_u8)a, swap32);
-  sum = vec + add;
-  return (vector4x_u32)vec_perm ((vector16x_u8)sum, (vector16x_u8)sum, swap32);
-#else
-  return (vector4x_u32)((vector2x_u64)(v) + (vector2x_u64)(a));
-#endif
-}
-
-/**********************************************************************
-  4-way chacha20
- **********************************************************************/
-
-#define ROTATE(v1,rolv)			\
-	__asm__ ("vrlw %0,%1,%2\n\t" : "=v" (v1) : "v" (v1), "v" (rolv))
-
-#define PLUS(ds,s) \
-	((ds) += (s))
-
-#define XOR(ds,s) \
-	((ds) ^= (s))
-
-#define ADD_U64(v,a) \
-	(v = vec_add_ctr_u64(v, a))
-
-/* 4x4 32-bit integer matrix transpose */
-#define transpose_4x4(x0, x1, x2, x3) ({ \
-	vector4x_u32 t1 = vec_mergeh(x0, x2); \
-	vector4x_u32 t2 = vec_mergel(x0, x2); \
-	vector4x_u32 t3 = vec_mergeh(x1, x3); \
-	x3 = vec_mergel(x1, x3); \
-	x0 = vec_mergeh(t1, t3); \
-	x1 = vec_mergel(t1, t3); \
-	x2 = vec_mergeh(t2, x3); \
-	x3 = vec_mergel(t2, x3); \
-      })
-
-#define QUARTERROUND2(a1,b1,c1,d1,a2,b2,c2,d2)			\
-	PLUS(a1,b1); PLUS(a2,b2); XOR(d1,a1); XOR(d2,a2);	\
-	    ROTATE(d1, rotate_16); ROTATE(d2, rotate_16);	\
-	PLUS(c1,d1); PLUS(c2,d2); XOR(b1,c1); XOR(b2,c2);	\
-	    ROTATE(b1, rotate_12); ROTATE(b2, rotate_12);	\
-	PLUS(a1,b1); PLUS(a2,b2); XOR(d1,a1); XOR(d2,a2);	\
-	    ROTATE(d1, rotate_8); ROTATE(d2, rotate_8);		\
-	PLUS(c1,d1); PLUS(c2,d2); XOR(b1,c1); XOR(b2,c2);	\
-	    ROTATE(b1, rotate_7); ROTATE(b2, rotate_7);
-
-unsigned int attribute_hidden
-__chacha20_power8_blocks4 (uint32_t *state, uint8_t *dst, const uint8_t *src,
-			   size_t nblks)
-{
-  vector4x_u32 counters_0123 = { 0, 1, 2, 3 };
-  vector4x_u32 counter_4 = { 4, 0, 0, 0 };
-  vector4x_u32 rotate_16 = { 16, 16, 16, 16 };
-  vector4x_u32 rotate_12 = { 12, 12, 12, 12 };
-  vector4x_u32 rotate_8 = { 8, 8, 8, 8 };
-  vector4x_u32 rotate_7 = { 7, 7, 7, 7 };
-  vector4x_u32 state0, state1, state2, state3;
-  vector4x_u32 v0, v1, v2, v3, v4, v5, v6, v7;
-  vector4x_u32 v8, v9, v10, v11, v12, v13, v14, v15;
-  vector4x_u32 tmp;
-  int i;
-
-  /* Force preload of constants to vector registers.  */
-  __asm__ ("": "+v" (counters_0123) :: "memory");
-  __asm__ ("": "+v" (counter_4) :: "memory");
-  __asm__ ("": "+v" (rotate_16) :: "memory");
-  __asm__ ("": "+v" (rotate_12) :: "memory");
-  __asm__ ("": "+v" (rotate_8) :: "memory");
-  __asm__ ("": "+v" (rotate_7) :: "memory");
-
-  state0 = vec_vsx_ld (0 * 16, state);
-  state1 = vec_vsx_ld (1 * 16, state);
-  state2 = vec_vsx_ld (2 * 16, state);
-  state3 = vec_vsx_ld (3 * 16, state);
-
-  do
-    {
-      v0 = vec_splat (state0, 0);
-      v1 = vec_splat (state0, 1);
-      v2 = vec_splat (state0, 2);
-      v3 = vec_splat (state0, 3);
-      v4 = vec_splat (state1, 0);
-      v5 = vec_splat (state1, 1);
-      v6 = vec_splat (state1, 2);
-      v7 = vec_splat (state1, 3);
-      v8 = vec_splat (state2, 0);
-      v9 = vec_splat (state2, 1);
-      v10 = vec_splat (state2, 2);
-      v11 = vec_splat (state2, 3);
-      v12 = vec_splat (state3, 0);
-      v13 = vec_splat (state3, 1);
-      v14 = vec_splat (state3, 2);
-      v15 = vec_splat (state3, 3);
-
-      v12 += counters_0123;
-      v13 -= vec_cmplt (v12, counters_0123);
-
-      for (i = 20; i > 0; i -= 2)
-	{
-	  QUARTERROUND2 (v0, v4,  v8, v12,   v1, v5,  v9, v13)
-	  QUARTERROUND2 (v2, v6, v10, v14,   v3, v7, v11, v15)
-	  QUARTERROUND2 (v0, v5, v10, v15,   v1, v6, v11, v12)
-	  QUARTERROUND2 (v2, v7,  v8, v13,   v3, v4,  v9, v14)
-	}
-
-      v0 += vec_splat (state0, 0);
-      v1 += vec_splat (state0, 1);
-      v2 += vec_splat (state0, 2);
-      v3 += vec_splat (state0, 3);
-      v4 += vec_splat (state1, 0);
-      v5 += vec_splat (state1, 1);
-      v6 += vec_splat (state1, 2);
-      v7 += vec_splat (state1, 3);
-      v8 += vec_splat (state2, 0);
-      v9 += vec_splat (state2, 1);
-      v10 += vec_splat (state2, 2);
-      v11 += vec_splat (state2, 3);
-      tmp = vec_splat( state3, 0);
-      tmp += counters_0123;
-      v12 += tmp;
-      v13 += vec_splat (state3, 1) - vec_cmplt (tmp, counters_0123);
-      v14 += vec_splat (state3, 2);
-      v15 += vec_splat (state3, 3);
-      ADD_U64 (state3, counter_4);
-
-      transpose_4x4 (v0, v1, v2, v3);
-      transpose_4x4 (v4, v5, v6, v7);
-      transpose_4x4 (v8, v9, v10, v11);
-      transpose_4x4 (v12, v13, v14, v15);
-
-      vec_store_le (v0, (64 * 0 + 16 * 0), dst);
-      vec_store_le (v1, (64 * 1 + 16 * 0), dst);
-      vec_store_le (v2, (64 * 2 + 16 * 0), dst);
-      vec_store_le (v3, (64 * 3 + 16 * 0), dst);
-
-      vec_store_le (v4, (64 * 0 + 16 * 1), dst);
-      vec_store_le (v5, (64 * 1 + 16 * 1), dst);
-      vec_store_le (v6, (64 * 2 + 16 * 1), dst);
-      vec_store_le (v7, (64 * 3 + 16 * 1), dst);
-
-      vec_store_le (v8, (64 * 0 + 16 * 2), dst);
-      vec_store_le (v9, (64 * 1 + 16 * 2), dst);
-      vec_store_le (v10, (64 * 2 + 16 * 2), dst);
-      vec_store_le (v11, (64 * 3 + 16 * 2), dst);
-
-      vec_store_le (v12, (64 * 0 + 16 * 3), dst);
-      vec_store_le (v13, (64 * 1 + 16 * 3), dst);
-      vec_store_le (v14, (64 * 2 + 16 * 3), dst);
-      vec_store_le (v15, (64 * 3 + 16 * 3), dst);
-
-      src += 4*64;
-      dst += 4*64;
-
-      nblks -= 4;
-    }
-  while (nblks);
-
-  vec_vsx_st (state3, 3 * 16, state);
-
-  return 0;
-}
diff --git a/sysdeps/powerpc/powerpc64/power8/chacha20_arch.h b/sysdeps/powerpc/powerpc64/power8/chacha20_arch.h
deleted file mode 100644
index ded06762b6..0000000000
--- a/sysdeps/powerpc/powerpc64/power8/chacha20_arch.h
+++ /dev/null
@@ -1,37 +0,0 @@
-/* PowerPC optimization for ChaCha20.
-   Copyright (C) 2022 Free Software Foundation, Inc.
-   This file is part of the GNU C Library.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library; if not, see
-   <https://www.gnu.org/licenses/>.  */
-
-#include <stdbool.h>
-#include <ldsodefs.h>
-
-unsigned int __chacha20_power8_blocks4 (uint32_t *state, uint8_t *dst,
-					const uint8_t *src, size_t nblks)
-     attribute_hidden;
-
-static void
-chacha20_crypt (uint32_t *state, uint8_t *dst,
-		const uint8_t *src, size_t bytes)
-{
-  _Static_assert (CHACHA20_BUFSIZE % 4 == 0,
-		  "CHACHA20_BUFSIZE not multiple of 4");
-  _Static_assert (CHACHA20_BUFSIZE >= CHACHA20_BLOCK_SIZE * 4,
-		  "CHACHA20_BUFSIZE < CHACHA20_BLOCK_SIZE * 4");
-
-  __chacha20_power8_blocks4 (state, dst, src,
-			     CHACHA20_BUFSIZE / CHACHA20_BLOCK_SIZE);
-}