From patchwork Fri Feb 3 00:27:10 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Eric Biggers X-Patchwork-Id: 13126844 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 C4394C63797 for ; Fri, 3 Feb 2023 00:28:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233238AbjBCA2V (ORCPT ); Thu, 2 Feb 2023 19:28:21 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54568 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233077AbjBCA2P (ORCPT ); Thu, 2 Feb 2023 19:28:15 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3D52B30187; Thu, 2 Feb 2023 16:28:09 -0800 (PST) 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 ams.source.kernel.org (Postfix) with ESMTPS id A5DB1B828DA; Fri, 3 Feb 2023 00:28:08 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0A745C433D2; Fri, 3 Feb 2023 00:28:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1675384087; bh=lwTsMszefw5rHZqf0ZxqDJa/9KBH+8q5OL/uceUU0fo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=rhd6VBWliuTiYnmfw71LvW/Shl1jqUGI0neLZFFSIH6Bb36u4D0Ud9p50cihtQ3+X EdzgeG9MSTbJPGhHgKnSZ5vTnp4yZfIwn1Wsl8TSJCxMTFlOHaQ/QXm1QCtDfYcbIL Gk+qbtS6leRYCs1y86to5/FfdLpsQf8VL+flmiWixMf6Zr1Xxtq2w2NUBTSWhMoGDJ m8ZfYN/FyYDJPbXL3QtQ6auJeKfLRMhnCzzloi9VhniEjm0fhot3zn/dwQTlB8UNM8 ymEKV1qWj3VX3QbFeAOXvUsxMhJc2r8OFAkny6rzhY/8xs4MAQ8Ux4Dk+ae/VExTPs EQtKpqRvypEyA== From: Eric Biggers To: stable@vger.kernel.org Cc: Harshit Mogalapalli , Kees Cook , SeongJae Park , Seth Jenkins , Jann Horn , "Eric W . Biederman" , linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org, Luis Chamberlain Subject: [PATCH 4.19 v2 08/15] exit: Put an upper limit on how often we can oops Date: Thu, 2 Feb 2023 16:27:10 -0800 Message-Id: <20230203002717.49198-9-ebiggers@kernel.org> X-Mailer: git-send-email 2.39.1 In-Reply-To: <20230203002717.49198-1-ebiggers@kernel.org> References: <20230203002717.49198-1-ebiggers@kernel.org> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org From: Jann Horn commit d4ccd54d28d3c8598e2354acc13e28c060961dbb upstream. Many Linux systems are configured to not panic on oops; but allowing an attacker to oops the system **really** often can make even bugs that look completely unexploitable exploitable (like NULL dereferences and such) if each crash elevates a refcount by one or a lock is taken in read mode, and this causes a counter to eventually overflow. The most interesting counters for this are 32 bits wide (like open-coded refcounts that don't use refcount_t). (The ldsem reader count on 32-bit platforms is just 16 bits, but probably nobody cares about 32-bit platforms that much nowadays.) So let's panic the system if the kernel is constantly oopsing. The speed of oopsing 2^32 times probably depends on several factors, like how long the stack trace is and which unwinder you're using; an empirically important one is whether your console is showing a graphical environment or a text console that oopses will be printed to. In a quick single-threaded benchmark, it looks like oopsing in a vfork() child with a very short stack trace only takes ~510 microseconds per run when a graphical console is active; but switching to a text console that oopses are printed to slows it down around 87x, to ~45 milliseconds per run. (Adding more threads makes this faster, but the actual oops printing happens under &die_lock on x86, so you can maybe speed this up by a factor of around 2 and then any further improvement gets eaten up by lock contention.) It looks like it would take around 8-12 days to overflow a 32-bit counter with repeated oopsing on a multi-core X86 system running a graphical environment; both me (in an X86 VM) and Seth (with a distro kernel on normal hardware in a standard configuration) got numbers in that ballpark. 12 days aren't *that* short on a desktop system, and you'd likely need much longer on a typical server system (assuming that people don't run graphical desktop environments on their servers), and this is a *very* noisy and violent approach to exploiting the kernel; and it also seems to take orders of magnitude longer on some machines, probably because stuff like EFI pstore will slow it down a ton if that's active. Signed-off-by: Jann Horn Link: https://lore.kernel.org/r/20221107201317.324457-1-jannh@google.com Reviewed-by: Luis Chamberlain Signed-off-by: Kees Cook Link: https://lore.kernel.org/r/20221117234328.594699-2-keescook@chromium.org Signed-off-by: Eric Biggers --- Documentation/sysctl/kernel.txt | 9 +++++++ kernel/exit.c | 43 +++++++++++++++++++++++++++++++++ 2 files changed, 52 insertions(+) diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt index db1676525ca35..fd65f4e651d55 100644 --- a/Documentation/sysctl/kernel.txt +++ b/Documentation/sysctl/kernel.txt @@ -51,6 +51,7 @@ show up in /proc/sys/kernel: - msgmnb - msgmni - nmi_watchdog +- oops_limit - osrelease - ostype - overflowgid @@ -555,6 +556,14 @@ scanned for a given scan. ============================================================== +oops_limit: + +Number of kernel oopses after which the kernel should panic when +``panic_on_oops`` is not set. Setting this to 0 or 1 has the same effect +as setting ``panic_on_oops=1``. + +============================================================== + osrelease, ostype & version: # cat osrelease diff --git a/kernel/exit.c b/kernel/exit.c index 2147bbc2c7c32..fa3004ff33362 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -68,6 +68,33 @@ #include #include +/* + * The default value should be high enough to not crash a system that randomly + * crashes its kernel from time to time, but low enough to at least not permit + * overflowing 32-bit refcounts or the ldsem writer count. + */ +static unsigned int oops_limit = 10000; + +#ifdef CONFIG_SYSCTL +static struct ctl_table kern_exit_table[] = { + { + .procname = "oops_limit", + .data = &oops_limit, + .maxlen = sizeof(oops_limit), + .mode = 0644, + .proc_handler = proc_douintvec, + }, + { } +}; + +static __init int kernel_exit_sysctls_init(void) +{ + register_sysctl_init("kernel", kern_exit_table); + return 0; +} +late_initcall(kernel_exit_sysctls_init); +#endif + static void __unhash_process(struct task_struct *p, bool group_dead) { nr_threads--; @@ -924,10 +951,26 @@ EXPORT_SYMBOL_GPL(do_exit); void __noreturn make_task_dead(int signr) { + static atomic_t oops_count = ATOMIC_INIT(0); + /* * Take the task off the cpu after something catastrophic has * happened. */ + + /* + * Every time the system oopses, if the oops happens while a reference + * to an object was held, the reference leaks. + * If the oops doesn't also leak memory, repeated oopsing can cause + * reference counters to wrap around (if they're not using refcount_t). + * This means that repeated oopsing can make unexploitable-looking bugs + * exploitable through repeated oopsing. + * To make sure this can't happen, place an upper bound on how often the + * kernel may oops without panic(). + */ + if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit)) + panic("Oopsed too often (kernel.oops_limit is %d)", oops_limit); + do_exit(signr); }