From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1B88CC433EF for ; Fri, 27 May 2022 11:49:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1351900AbiE0Ls4 (ORCPT ); Fri, 27 May 2022 07:48:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57450 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1351935AbiE0LpJ (ORCPT ); Fri, 27 May 2022 07:45:09 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2CA2413C351; Fri, 27 May 2022 04:41:31 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id BD52461C3F; Fri, 27 May 2022 11:41:30 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C766FC385A9; Fri, 27 May 2022 11:41:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1653651690; bh=5Qvl9MRNPbfZ5zmGTUMKK2Aok2MPBZGiLGgl4WAat1s=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=lYwx4WEvQ1SNgCUPKmPlqp283w+Z+TdbnJXAsOeZfxvvRtoncvKizDdFf9ENfhsCr HUsiNZovbwAGvHpv7EGiKgl/PgZWjTEnFfobjhc0z78T37aT4x3hruDaGkEmC0TeHB zoD+Gt0K7LtLxmsdp0HtQlfKnnLEXu1J6xHLuc9o= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Jann Horn , Theodore Tso , "Jason A. Donenfeld" Subject: [PATCH 5.17 063/111] random: check for signals every PAGE_SIZE chunk of /dev/[u]random Date: Fri, 27 May 2022 10:49:35 +0200 Message-Id: <20220527084828.462781628@linuxfoundation.org> X-Mailer: git-send-email 2.36.1 In-Reply-To: <20220527084819.133490171@linuxfoundation.org> References: <20220527084819.133490171@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Jason A. Donenfeld" commit e3c1c4fd9e6d14059ed93ebfe15e1c57793b1a05 upstream. In 1448769c9cdb ("random: check for signal_pending() outside of need_resched() check"), Jann pointed out that we previously were only checking the TIF_NOTIFY_SIGNAL and TIF_SIGPENDING flags if the process had TIF_NEED_RESCHED set, which meant in practice, super long reads to /dev/[u]random would delay signal handling by a long time. I tried this using the below program, and indeed I wasn't able to interrupt a /dev/urandom read until after several megabytes had been read. The bug he fixed has always been there, and so code that reads from /dev/urandom without checking the return value of read() has mostly worked for a long time, for most sizes, not just for <= 256. Maybe it makes sense to keep that code working. The reason it was so small prior, ignoring the fact that it didn't work anyway, was likely because /dev/random used to block, and that could happen for pretty large lengths of time while entropy was gathered. But now, it's just a chacha20 call, which is extremely fast and is just operating on pure data, without having to wait for some external event. In that sense, /dev/[u]random is a lot more like /dev/zero. Taking a page out of /dev/zero's read_zero() function, it always returns at least one chunk, and then checks for signals after each chunk. Chunk sizes there are of length PAGE_SIZE. Let's just copy the same thing for /dev/[u]random, and check for signals and cond_resched() for every PAGE_SIZE amount of data. This makes the behavior more consistent with expectations, and should mitigate the impact of Jann's fix for the age-old signal check bug. ---- test program ---- #include #include #include #include static unsigned char x[~0U]; static void handle(int) { } int main(int argc, char *argv[]) { pid_t pid = getpid(), child; signal(SIGUSR1, handle); if (!(child = fork())) { for (;;) kill(pid, SIGUSR1); } pause(); printf("interrupted after reading %zd bytes\n", getrandom(x, sizeof(x), 0)); kill(child, SIGTERM); return 0; } Cc: Jann Horn Cc: Theodore Ts'o Signed-off-by: Jason A. Donenfeld Signed-off-by: Greg Kroah-Hartman --- drivers/char/random.c | 17 +++++++---------- 1 file changed, 7 insertions(+), 10 deletions(-) --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -523,7 +523,6 @@ EXPORT_SYMBOL(get_random_bytes); static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes) { - bool large_request = nbytes > 256; ssize_t ret = 0; size_t len; u32 chacha_state[CHACHA_STATE_WORDS]; @@ -549,15 +548,6 @@ static ssize_t get_random_bytes_user(voi } do { - if (large_request) { - if (signal_pending(current)) { - if (!ret) - ret = -ERESTARTSYS; - break; - } - cond_resched(); - } - chacha20_block(chacha_state, output); if (unlikely(chacha_state[12] == 0)) ++chacha_state[13]; @@ -571,6 +561,13 @@ static ssize_t get_random_bytes_user(voi nbytes -= len; buf += len; ret += len; + + BUILD_BUG_ON(PAGE_SIZE % CHACHA_BLOCK_SIZE != 0); + if (!(ret % PAGE_SIZE) && nbytes) { + if (signal_pending(current)) + break; + cond_resched(); + } } while (nbytes); memzero_explicit(output, sizeof(output));