From patchwork Mon Jan 17 21:56:17 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "brian m. carlson" X-Patchwork-Id: 12715688 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 974C8C433EF for ; Mon, 17 Jan 2022 21:56:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243419AbiAQV4Y (ORCPT ); Mon, 17 Jan 2022 16:56:24 -0500 Received: from ring.crustytoothpaste.net ([172.105.110.227]:35368 "EHLO ring.crustytoothpaste.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231401AbiAQV4X (ORCPT ); Mon, 17 Jan 2022 16:56:23 -0500 Received: from camp.crustytoothpaste.net (unknown [IPv6:2001:470:b056:101:a6ae:7d13:8741:9028]) (using TLSv1.2 with cipher ECDHE-RSA-CHACHA20-POLY1305 (256/256 bits)) (No client certificate requested) by ring.crustytoothpaste.net (Postfix) with ESMTPSA id D51BB5B27E; Mon, 17 Jan 2022 21:56:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=crustytoothpaste.net; s=default; t=1642456582; bh=5tcSeRws+FPWHA9PPK/oyQ3KxCkCZT2X+03wvATI5Gw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From:Reply-To: Subject:Date:To:CC:Resent-Date:Resent-From:Resent-To:Resent-Cc: In-Reply-To:References:Content-Type:Content-Disposition; b=DI6H3+rjY8qT9StQrZ71JXrMuxML3hNjAUKqAM34fzdQjCvuU+iB0un287hAQoMbl Sm/P1qEROF+Mj992Pdc05ATdUUlYxypSWz6gePc5cWCBp0RuiCV1q/6lHuOI6WVGRY GGa8RY5dWD1uudI6OXjJcnvKWxPNzNVZjOZztDFa7CW8Yqu1/DCSfOYFaTIVMgOW1L L8tqmaUFxfFB6S3A663bA1vFwXwqSpk0rP+K0jU6ro8x7STn+YsSWwWJO3YbaqKtuF qN/8lxEyBlW03B/pNnoSG7116qY9Ufs+Ry49fWUmHbbhTdoY3U0L8kc+5e0Fy+Stie 9D6igd4Sbh8u1gdGfx2XnI6Ew+ofgRnqOShB3g4TCjHo1oHQKnOa62EKUZmlQn2q65 vwTJCAiJEeQ4Dh59Jf4/W4NVFAAi8hDvgK16lCnzrNqP15tJ9PsyFLHd0j0o5aksWj gKwd6kCsI/pqfMxSYnwuNewEMLG8oo2k9W3k/G9dbJHBRDcIO3y From: "brian m. carlson" To: Cc: Junio C Hamano , rsbecker@nexbridge.com, Taylor Blau , =?utf-8?q?Carlo_Marcelo_Arenas_Bel=C3=B3n?= , Johannes Schindelin Subject: [PATCH v3 2/2] wrapper: use a CSPRNG to generate random file names Date: Mon, 17 Jan 2022 21:56:17 +0000 Message-Id: <20220117215617.843190-3-sandals@crustytoothpaste.net> X-Mailer: git-send-email 2.34.1.575.g55b058a8bb In-Reply-To: <20220117215617.843190-1-sandals@crustytoothpaste.net> References: <20220104015555.3387101-1-sandals@crustytoothpaste.net> <20220117215617.843190-1-sandals@crustytoothpaste.net> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org The current way we generate random file names is by taking the seconds and microseconds, plus the PID, and mixing them together, then encoding them. If this fails, we increment the value by 7777, and try again up to TMP_MAX times. Unfortunately, this is not the best idea from a security perspective. If we're writing into TMPDIR, an attacker can guess these values easily and prevent us from creating any temporary files at all by creating them all first. Even though we set TMP_MAX to 16384, this may be achievable in some contexts, even if unlikely to occur in practice. Fortunately, we can simply solve this by using the system cryptographically secure pseudorandom number generator (CSPRNG) to generate a random 64-bit value, and use that as before. Note that there is still a small bias here, but because a six-character sequence chosen out of 62 characters provides about 36 bits of entropy, the bias here is less than 2^-28, which is acceptable, especially considering we'll retry several times. Note that the use of a CSPRNG in generating temporary file names is also used in many libcs. glibc recently changed from an approach similar to ours to using a CSPRNG, and FreeBSD and OpenBSD also use a CSPRNG in this case. Even if the likelihood of an attack is low, we should still be at least as responsible in creating temporary files as libc is. Signed-off-by: brian m. carlson --- wrapper.c | 15 ++++----------- 1 file changed, 4 insertions(+), 11 deletions(-) diff --git a/wrapper.c b/wrapper.c index 1052356703..3258cdb171 100644 --- a/wrapper.c +++ b/wrapper.c @@ -463,8 +463,6 @@ int git_mkstemps_mode(char *pattern, int suffix_len, int mode) static const int num_letters = ARRAY_SIZE(letters) - 1; static const char x_pattern[] = "XXXXXX"; static const int num_x = ARRAY_SIZE(x_pattern) - 1; - uint64_t value; - struct timeval tv; char *filename_template; size_t len; int fd, count; @@ -485,12 +483,13 @@ int git_mkstemps_mode(char *pattern, int suffix_len, int mode) * Replace pattern's XXXXXX characters with randomness. * Try TMP_MAX different filenames. */ - gettimeofday(&tv, NULL); - value = ((uint64_t)tv.tv_usec << 16) ^ tv.tv_sec ^ getpid(); filename_template = &pattern[len - num_x - suffix_len]; for (count = 0; count < TMP_MAX; ++count) { - uint64_t v = value; int i; + uint64_t v; + if (csprng_bytes(&v, sizeof(v)) < 0) + return error_errno("unable to get random bytes for temporary file"); + /* Fill in the random bits. */ for (i = 0; i < num_x; i++) { filename_template[i] = letters[v % num_letters]; @@ -506,12 +505,6 @@ int git_mkstemps_mode(char *pattern, int suffix_len, int mode) */ if (errno != EEXIST) break; - /* - * This is a random value. It is only necessary that - * the next TMP_MAX values generated by adding 7777 to - * VALUE are different with (module 2^32). - */ - value += 7777; } /* We return the null string if we can't find a unique file name. */ pattern[0] = '\0';