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: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

3 files changed, 66 insertions, 14 deletions

diff --git a/arch/arm/crypto/Kconfig b/arch/arm/crypto/Kconfig
index ef0c7feea6e2..0473a8f68389 100644
--- a/arch/arm/crypto/Kconfig
+++ b/arch/arm/crypto/Kconfig
@@ -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
diff --git a/arch/arm/crypto/aes-cipher-core.S b/arch/arm/crypto/aes-cipher-core.S
index 184d6c2d15d5..f2d67c095e59 100644
--- a/arch/arm/crypto/aes-cipher-core.S
+++ b/arch/arm/crypto/aes-cipher-core.S
@@ -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
diff --git a/crypto/aes_generic.c b/crypto/aes_generic.c
index ca554d57d01e..13df33aca463 100644
--- a/crypto/aes_generic.c
+++ b/crypto/aes_generic.c
@@ -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,