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>

1 files changed, 5 insertions, 4 deletions

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,