On Wed, Nov 25, 2020 at 08:22:16AM +0100, Ard Biesheuvel wrote:
> ARM Cortex-A72 cores running in 32-bit mode are affected by a silicon
> erratum (1655431: ELR recorded incorrectly on interrupt taken between
> cryptographic instructions in a sequence [0]) where the second instruction
> of a AES instruction pair may execute twice if an interrupt is taken right
> after the first instruction consumes an input register of which a single
> 32-bit lane has been updated the last time it was modified.
>
> This is not such a rare occurrence as it may seem: in counter mode, only
> the least significant 32-bit word is incremented in the absence of a
> carry, which makes our counter mode implementation susceptible to the
> erratum.
>
> So let's shuffle the counter assignments around a bit so that the most
> recent updates when the AES instruction pair executes are 128-bit wide.
>
> [0] ARM-EPM-012079 v11.0 Cortex-A72 MPCore Software Developers Errata Notice
>
> Cc: <sta...@vger.kernel.org> # v5.4+
> Signed-off-by: Ard Biesheuvel <a...@kernel.org>
> ---
> arch/arm/crypto/aes-ce-core.S | 20 ++++++++++----------
> 1 file changed, 10 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm/crypto/aes-ce-core.S b/arch/arm/crypto/aes-ce-core.S
> index 4d1707388d94..c0ef9680d90b 100644
> --- a/arch/arm/crypto/aes-ce-core.S
> +++ b/arch/arm/crypto/aes-ce-core.S
> @@ -386,20 +386,20 @@ ENTRY(ce_aes_ctr_encrypt)
> .Lctrloop4x:
> subs r4, r4, #4
> bmi .Lctr1x
> - add r6, r6, #1
> + add ip, r6, #1
> vmov q0, q7
> + rev ip, ip
> + add lr, r6, #2
> + vmov s31, ip
> + add ip, r6, #3
> + rev lr, lr
> vmov q1, q7
> - rev ip, r6
> - add r6, r6, #1
> + vmov s31, lr
> + rev ip, ip
> vmov q2, q7
> - vmov s7, ip
> - rev ip, r6
> - add r6, r6, #1
> + vmov s31, ip
> + add r6, r6, #4
> vmov q3, q7
> - vmov s11, ip
> - rev ip, r6
> - add r6, r6, #1
> - vmov s15, ip
> vld1.8 {q4-q5}, [r1]!
> vld1.8 {q6}, [r1]!
> vld1.8 {q15}, [r1]!
Seems like this could use a comment that explains that things need to be done in
a certain way to avoid an erratum.
- Eric
