On Wed, Nov 22, 2017 at 08:51:57PM +0000, Ard Biesheuvel wrote: > On 22 November 2017 at 19:51, Eric Biggers <ebiggers3@xxxxxxxxx> wrote: > > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > > > When chacha20_block() outputs the keystream block, it uses 'u32' stores > > directly. However, the callers (crypto/chacha20_generic.c and > > drivers/char/random.c) declare the keystream buffer as a 'u8' array, > > which is not guaranteed to have the needed alignment. > > > > Fix it by having both callers declare the keystream as a 'u32' array. > > For now this is preferable to switching over to the unaligned access > > macros because chacha20_block() is only being used in cases where we can > > easily control the alignment (stack buffers). > > > > Given this paragraph, I think we agree the correct way to fix this > would be to make chacha20_block() adhere to its prototype, so if we > deviate from that, there should be a good reason. On which > architecture that cares about alignment is this expected to result in > a measurable performance benefit? > Well, variables on the stack tend to be 4 or even 8-byte aligned anyway, so this change probably doesn't make a difference in practice currently. But it still should be fixed, in case it does become a problem. We could certainly leave the type as u8 array and use put_unaligned_le32() instead; that would be a simpler change. But that would be slower on architectures where a potentially-unaligned access requires multiple instructions. Eric
