ICP: Fix out of bounds write

If gcm_mode_encrypt_contiguous_blocks() is called more than once
in succession, with the accumulated lengths being less than
blocksize, ctx->copy_to will be incorrectly advanced. Later, if
out is NULL, the bcopy at line 114 will overflow
ctx->gcm_copy_to since ctx->gcm_remainder_len is larger than the
ctx->gcm_copy_to buffer can hold.

The fix is to set ctx->copy_to only if it's not already set.

For ZoL the issue may be academic, since in all my testing I wasn't
able to hit neither of both conditions needed to trigger it, but
other consumers can easily do so.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tom Caputi <[email protected]>
Signed-off-by: Attila Fülöp <[email protected]>
Closes #9660

Author: AttilaFueloep

module/icp/algs/modes/gcm.c

@@ -67,7 +67,9 @@ gcm_mode_encrypt_contiguous_blocks(gcm_ctx_t *ctx, char *data, size_t length,
 		    (uint8_t *)ctx->gcm_remainder + ctx->gcm_remainder_len,
 		    length);
 		ctx->gcm_remainder_len += length;
-		ctx->gcm_copy_to = datap;
+		if (ctx->gcm_copy_to == NULL) {
+			ctx->gcm_copy_to = datap;
+		}
 		return (CRYPTO_SUCCESS);
 	}