From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by sourceware.org (Postfix) with ESMTPS id EC47D3858CDA for ; Tue, 26 Jul 2022 10:41:18 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org EC47D3858CDA 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 81CFA61194; Tue, 26 Jul 2022 10:41:18 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 631C9C341C0; Tue, 26 Jul 2022 10:41:17 +0000 (UTC) Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 5d45f5c3 (TLSv1.3:AEAD-AES256-GCM-SHA384:256:NO); Tue, 26 Jul 2022 10:41:15 +0000 (UTC) Date: Tue, 26 Jul 2022 12:41:13 +0200 From: "Jason A. Donenfeld" To: Mark Harris Cc: libc-alpha@sourceware.org, Florian Weimer , linux-crypto@vger.kernel.org Subject: Re: [PATCH v2] arc4random: simplify design for better safety Message-ID: References: <20220725225728.824128-1-Jason@zx2c4.com> <20220725232810.843433-1-Jason@zx2c4.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: X-Spam-Status: No, score=-2.6 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, HEADER_FROM_DIFFERENT_DOMAINS, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=no autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 26 Jul 2022 10:41:20 -0000 Hi Mark, On Mon, Jul 25, 2022 at 06:10:06PM -0700, Mark Harris wrote: > Jason A. Donenfeld wrote: > > + l = __getrandom_nocancel (p, n, 0); > > + if (l > 0) > > + { > > + if ((size_t) l == n) > > + return; /* Done reading, success. */ > > + p = (uint8_t *) p + l; > > + n -= l; > > + continue; /* Interrupted by a signal; keep going. */ > > + } > > + else if (l == 0) > > + arc4random_getrandom_failure (); /* Weird, should never happen. */ > > + else if (errno == ENOSYS) > > + { > > + have_getrandom = false; > > + break; /* No syscall, so fallback to /dev/urandom. */ > > + } > > + arc4random_getrandom_failure (); /* Unknown error, should never happen. */ > > Isn't EINTR also possible? Aborting in that case does not seem reasonable. Not in current kernels, where it always returns at least PAGE_SIZE bytes before checking for pending signals. In older kernels, if there was a signal pending at the top, it would do no work and return -ERESTARTSYS, which I believe should then get restarted by glibc's syscaller? I might be wrong about how restarts work though, so if you know better, please let me know. TEMP_FAILURE_RETRY relies on errno, so that's not what we want. I guess I can just add a case for it. > Also the __getrandom_nocancel function does not set errno on Linux; it > just returns INTERNAL_SYSCALL_CALL (getrandom, buf, buflen, flags). > So unless that is changed, it doesn't look like this ENOSYS check will > detect old Linux kernels. Thanks. It looks like INTERNAL_SYSCALL_CALL just returns the errno as-is as a return value, right? I'll adjust the code to account for that. > > + struct pollfd pfd = { .events = POLLIN }; > > + pfd.fd = TEMP_FAILURE_RETRY ( > > + __open64_nocancel ("/dev/random", O_RDONLY | O_CLOEXEC | O_NOCTTY)); > > + if (pfd.fd < 0) > > + arc4random_getrandom_failure (); > > + if (__poll (&pfd, 1, -1) < 0) > > + arc4random_getrandom_failure (); > > + if (__close_nocancel (pfd.fd) < 0) > > + arc4random_getrandom_failure (); > > The TEMP_FAILURE_RETRY handles EINTR on open, but __poll can also > result in EINTR. Thanks. I'll surround the __poll in TEMP_FAILURE_RETRY. Thank you for the review! v3 will have the above changes. Jason