Commit 913a3aa0 authored by Eric Biggers's avatar Eric Biggers Committed by Herbert Xu

crypto: arm/aes - add some hardening against cache-timing attacks

Make the ARM scalar AES implementation closer to constant-time by
disabling interrupts and prefetching the tables into L1 cache.  This is
feasible because due to ARM's "free" rotations, the main tables are only
1024 bytes instead of the usual 4096 used by most AES implementations.

On ARM Cortex-A7, the speed loss is only about 5%.  The resulting code
is still over twice as fast as aes_ti.c.  Responsiveness is potentially
a concern, but interrupts are only disabled for a single AES block.

Note that even after these changes, the implementation still isn't
necessarily guaranteed to be constant-time; see
https://cr.yp.to/antiforgery/cachetiming-20050414.pdf for a discussion
of the many difficulties involved in writing truly constant-time AES
software.  But it's valuable to make such attacks more difficult.

Much of this patch is based on patches suggested by Ard Biesheuvel.
Suggested-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
Reviewed-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
parent 0a6a40c2
......@@ -69,6 +69,15 @@ config CRYPTO_AES_ARM
help
Use optimized AES assembler routines for ARM platforms.
On ARM processors without the Crypto Extensions, this is the
fastest AES implementation for single blocks. For multiple
blocks, the NEON bit-sliced implementation is usually faster.
This implementation may be vulnerable to cache timing attacks,
since it uses lookup tables. However, as countermeasures it
disables IRQs and preloads the tables; it is hoped this makes
such attacks very difficult.
config CRYPTO_AES_ARM_BS
tristate "Bit sliced AES using NEON instructions"
depends on KERNEL_MODE_NEON
......
......@@ -10,6 +10,7 @@
*/
#include <linux/linkage.h>
#include <asm/assembler.h>
#include <asm/cache.h>
.text
......@@ -41,7 +42,7 @@
.endif
.endm
.macro __hround, out0, out1, in0, in1, in2, in3, t3, t4, enc, sz, op
.macro __hround, out0, out1, in0, in1, in2, in3, t3, t4, enc, sz, op, oldcpsr
__select \out0, \in0, 0
__select t0, \in1, 1
__load \out0, \out0, 0, \sz, \op
......@@ -73,6 +74,14 @@
__load t0, t0, 3, \sz, \op
__load \t4, \t4, 3, \sz, \op
.ifnb \oldcpsr
/*
* This is the final round and we're done with all data-dependent table
* lookups, so we can safely re-enable interrupts.
*/
restore_irqs \oldcpsr
.endif
eor \out1, \out1, t1, ror #24
eor \out0, \out0, t2, ror #16
ldm rk!, {t1, t2}
......@@ -83,14 +92,14 @@
eor \out1, \out1, t2
.endm
.macro fround, out0, out1, out2, out3, in0, in1, in2, in3, sz=2, op
.macro fround, out0, out1, out2, out3, in0, in1, in2, in3, sz=2, op, oldcpsr
__hround \out0, \out1, \in0, \in1, \in2, \in3, \out2, \out3, 1, \sz, \op
__hround \out2, \out3, \in2, \in3, \in0, \in1, \in1, \in2, 1, \sz, \op
__hround \out2, \out3, \in2, \in3, \in0, \in1, \in1, \in2, 1, \sz, \op, \oldcpsr
.endm
.macro iround, out0, out1, out2, out3, in0, in1, in2, in3, sz=2, op
.macro iround, out0, out1, out2, out3, in0, in1, in2, in3, sz=2, op, oldcpsr
__hround \out0, \out1, \in0, \in3, \in2, \in1, \out2, \out3, 0, \sz, \op
__hround \out2, \out3, \in2, \in1, \in0, \in3, \in1, \in0, 0, \sz, \op
__hround \out2, \out3, \in2, \in1, \in0, \in3, \in1, \in0, 0, \sz, \op, \oldcpsr
.endm
.macro __rev, out, in
......@@ -118,13 +127,14 @@
.macro do_crypt, round, ttab, ltab, bsz
push {r3-r11, lr}
// Load keys first, to reduce latency in case they're not cached yet.
ldm rk!, {r8-r11}
ldr r4, [in]
ldr r5, [in, #4]
ldr r6, [in, #8]
ldr r7, [in, #12]
ldm rk!, {r8-r11}
#ifdef CONFIG_CPU_BIG_ENDIAN
__rev r4, r4
__rev r5, r5
......@@ -138,6 +148,25 @@
eor r7, r7, r11
__adrl ttab, \ttab
/*
* Disable interrupts and prefetch the 1024-byte 'ft' or 'it' table into
* L1 cache, assuming cacheline size >= 32. This is a hardening measure
* intended to make cache-timing attacks more difficult. They may not
* be fully prevented, however; see the paper
* https://cr.yp.to/antiforgery/cachetiming-20050414.pdf
* ("Cache-timing attacks on AES") for a discussion of the many
* difficulties involved in writing truly constant-time AES software.
*/
save_and_disable_irqs t0
.set i, 0
.rept 1024 / 128
ldr r8, [ttab, #i + 0]
ldr r9, [ttab, #i + 32]
ldr r10, [ttab, #i + 64]
ldr r11, [ttab, #i + 96]
.set i, i + 128
.endr
push {t0} // oldcpsr
tst rounds, #2
bne 1f
......@@ -151,8 +180,21 @@
\round r4, r5, r6, r7, r8, r9, r10, r11
b 0b
2: __adrl ttab, \ltab
\round r4, r5, r6, r7, r8, r9, r10, r11, \bsz, b
2: .ifb \ltab
add ttab, ttab, #1
.else
__adrl ttab, \ltab
// Prefetch inverse S-box for final round; see explanation above
.set i, 0
.rept 256 / 64
ldr t0, [ttab, #i + 0]
ldr t1, [ttab, #i + 32]
.set i, i + 64
.endr
.endif
pop {rounds} // oldcpsr
\round r4, r5, r6, r7, r8, r9, r10, r11, \bsz, b, rounds
#ifdef CONFIG_CPU_BIG_ENDIAN
__rev r4, r4
......@@ -175,7 +217,7 @@
.endm
ENTRY(__aes_arm_encrypt)
do_crypt fround, crypto_ft_tab, crypto_ft_tab + 1, 2
do_crypt fround, crypto_ft_tab,, 2
ENDPROC(__aes_arm_encrypt)
.align 5
......
......@@ -63,7 +63,8 @@ static inline u8 byte(const u32 x, const unsigned n)
static const u32 rco_tab[10] = { 1, 2, 4, 8, 16, 32, 64, 128, 27, 54 };
__visible const u32 crypto_ft_tab[4][256] = {
/* cacheline-aligned to facilitate prefetching into cache */
__visible const u32 crypto_ft_tab[4][256] __cacheline_aligned = {
{
0xa56363c6, 0x847c7cf8, 0x997777ee, 0x8d7b7bf6,
0x0df2f2ff, 0xbd6b6bd6, 0xb16f6fde, 0x54c5c591,
......@@ -327,7 +328,7 @@ __visible const u32 crypto_ft_tab[4][256] = {
}
};
__visible const u32 crypto_fl_tab[4][256] = {
__visible const u32 crypto_fl_tab[4][256] __cacheline_aligned = {
{
0x00000063, 0x0000007c, 0x00000077, 0x0000007b,
0x000000f2, 0x0000006b, 0x0000006f, 0x000000c5,
......@@ -591,7 +592,7 @@ __visible const u32 crypto_fl_tab[4][256] = {
}
};
__visible const u32 crypto_it_tab[4][256] = {
__visible const u32 crypto_it_tab[4][256] __cacheline_aligned = {
{
0x50a7f451, 0x5365417e, 0xc3a4171a, 0x965e273a,
0xcb6bab3b, 0xf1459d1f, 0xab58faac, 0x9303e34b,
......@@ -855,7 +856,7 @@ __visible const u32 crypto_it_tab[4][256] = {
}
};
__visible const u32 crypto_il_tab[4][256] = {
__visible const u32 crypto_il_tab[4][256] __cacheline_aligned = {
{
0x00000052, 0x00000009, 0x0000006a, 0x000000d5,
0x00000030, 0x00000036, 0x000000a5, 0x00000038,
......
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