[PATCH URGENT crypto] crypto: arm64/chacha - correctly walk through blocks

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Prior, passing in chunks of 2, 3, or 4, followed by any additional
chunks would result in the chacha state counter getting out of sync,
resulting in incorrect encryption/decryption, which is a pretty nasty
crypto vuln, dating back to 2018. WireGuard users never experienced this
prior, because we have always, out of tree, used a different crypto
library, until the recent Frankenzinc addition. This commit fixes the
issue by advancing the pointers and state counter by the actual size
processed.

Fixes: f2ca1cbd0fb5 ("crypto: arm64/chacha - optimize for arbitrary length inputs")
Reported-and-tested-by: Emil Renner Berthing <kernel@esmil.dk>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: stable@vger.kernel.org
---
 arch/arm64/crypto/chacha-neon-glue.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/crypto/chacha-neon-glue.c b/arch/arm64/crypto/chacha-neon-glue.c
index c1f9660d104c..debb1de0d3dd 100644
--- a/arch/arm64/crypto/chacha-neon-glue.c
+++ b/arch/arm64/crypto/chacha-neon-glue.c
@@ -55,10 +55,10 @@ static void chacha_doneon(u32 *state, u8 *dst, const u8 *src,
 			break;
 		}
 		chacha_4block_xor_neon(state, dst, src, nrounds, l);
-		bytes -= CHACHA_BLOCK_SIZE * 5;
-		src += CHACHA_BLOCK_SIZE * 5;
-		dst += CHACHA_BLOCK_SIZE * 5;
-		state[12] += 5;
+		bytes -= l;
+		src += l;
+		dst += l;
+		state[12] += round_up(l, CHACHA_BLOCK_SIZE) / CHACHA_BLOCK_SIZE;
 	}
 }
 
-- 
2.25.1