From patchwork Wed May 4 14:09:54 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Jon Medhurst (Tixy)" X-Patchwork-Id: 9014291 Return-Path: X-Original-To: patchwork-linux-arm@patchwork.kernel.org Delivered-To: patchwork-parsemail@patchwork2.web.kernel.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.136]) by patchwork2.web.kernel.org (Postfix) with ESMTP id 76F7EBF29F for ; Wed, 4 May 2016 14:12:12 +0000 (UTC) Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 83974203AA for ; Wed, 4 May 2016 14:12:11 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.9]) (using TLSv1.2 with cipher AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 9AA3F20373 for ; Wed, 4 May 2016 14:12:09 +0000 (UTC) Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.80.1 #2 (Red Hat Linux)) id 1axxVQ-0005TJ-2I; Wed, 04 May 2016 14:10:24 +0000 Received: from smarthost03d.mail.zen.net.uk ([212.23.1.23]) by bombadil.infradead.org with esmtp (Exim 4.80.1 #2 (Red Hat Linux)) id 1axxVL-0004EI-Of for linux-arm-kernel@lists.infradead.org; Wed, 04 May 2016 14:10:22 +0000 Received: from [82.69.122.217] (helo=computer5) by smarthost03d.mail.zen.net.uk with esmtpsa (TLS1.2:RSA_AES_128_CBC_SHA1:128) (Exim 4.80) (envelope-from ) id 1axxUx-0003QR-FI; Wed, 04 May 2016 14:09:55 +0000 Message-ID: <1462370994.2895.11.camel@linaro.org> Subject: [PATCH] arm64: Make arch_randomize_brk avoid stack area From: "Jon Medhurst (Tixy)" To: Catalin Marinas , Will Deacon Date: Wed, 04 May 2016 15:09:54 +0100 In-Reply-To: <1461848638.2848.19.camel@linaro.org> References: <1461848638.2848.19.camel@linaro.org> X-Mailer: Evolution 3.12.9-1+b1 Mime-Version: 1.0 X-Originating-smarthost03d-IP: [82.69.122.217] X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20160504_071019_984128_C9009EEB X-CRM114-Status: GOOD ( 22.05 ) X-Spam-Score: -1.9 (-) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Nicolas Pitre , Hector Marco , Kees Cook , linux-arm-kernel@lists.infradead.org Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org X-Spam-Status: No, score=-6.3 required=5.0 tests=BAYES_00, RCVD_IN_DNSWL_MED, RP_MATCHES_RCVD, UNPARSEABLE_RELAY autolearn=unavailable version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP When a process is created, various address randomisations could end up colluding to place the address of brk in the stack memory. This would mean processes making small heap based memory allocations are in danger of having them overwriting, or overwritten by, the stack. Another consequence, is that even for processes that make no use of brk, the output of /proc/*/maps may show the stack area listed as '[heap]' rather than '[stack]'. Apart from being misleading this causes fatal errors with the Android run-time like: "No [stack] line found in /proc/self/task/*/maps" To prevent this problem pick a limit for brk that allows for the stack's memory. At the same time we remove randomize_base() as that was only used by arch_randomize_brk(). Note, in practice, since commit d1fd836dcf00 ("mm: split ET_DYN ASLR from mmap ASLR") this problem shouldn't occur because the address chosen for loading binaries is well clear of the stack, however, prior to that the problem does occur because of the following... The memory layout of a task is determined by arch_pick_mmap_layout. If address randomisation is enabled (task has flag PF_RANDOMIZE) this sets mmap_base to a random address at the top of task memory just below a region calculated to allow for a stack which itself may have a random base address. Any mmap operations that then happen which require an address allocating will use the topdown allocation method, i.e. the first allocated memory will be at the top of memory, just below the area set aside for the stack. When a relocatable binary is loaded into a new process by load_elf_binary and randomised address are enabled, it uses a 'load_bias' of zero, so that when mmap is called to create a memory region for it, a new address is picked (address zero not being available). As this is the first memory region in the task, it gets the region just below the stack, as described previously. The loader then set's brk to the end of the elf data section, which will be near the end of the loaded binary and then it calls arch_randomize_brk. As this currently stands, this adds a random amount to brk, which unfortunately may take it into the address range where the stack lies. Testing: These changes have been tested on Linux v4.6-rc4 using 100000 invocations of a program [1] that can display the offset of a process's brk... $for i in $(seq 100000); do ./aslr --report brk ; done This shows values of brk are evenly distributed over a 1GB range, both before and after this change. With Linux version 3.18 (where the collision of brk and stack can happen and this change limits brk to avoid that) the distribution of brk values after the change shows a slope where lower values for brk are more common and upper values have about half the frequency of those. [1] http://bazaar.launchpad.net/~ubuntu-bugcontrol/qa-regression-testing/master/files/2499/scripts/kernel-security/aslr/ Signed-off-by: Jon Medhurst Cc: # 4.0 and earlier Acked-by: Kees Cook Acked-by: Catalin Marinas --- Changes since RFC. - Fixed compilation errors (finger trouble preparing original email) - Updated commit message to included notes on testing - Added CC stable for Linux '4.0 and earlier' arch/arm64/kernel/process.c | 24 ++++++++++++++++++------ 1 file changed, 18 insertions(+), 6 deletions(-) diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c index 8062482..26429a0 100644 --- a/arch/arm64/kernel/process.c +++ b/arch/arm64/kernel/process.c @@ -382,13 +382,25 @@ unsigned long arch_align_stack(unsigned long sp) return sp & ~0xf; } -static unsigned long randomize_base(unsigned long base) +unsigned long arch_randomize_brk(struct mm_struct *mm) { + unsigned long base = mm->brk; unsigned long range_end = base + (STACK_RND_MASK << PAGE_SHIFT) + 1; - return randomize_range(base, range_end, 0) ? : base; -} + unsigned long max_stack, range_limit; -unsigned long arch_randomize_brk(struct mm_struct *mm) -{ - return randomize_base(mm->brk); + /* + * Determine how much room we need to leave available for the stack. + * We limit this to a reasonable value, because extremely large or + * unlimited stacks are always going to bump up against brk at some + * point and we don't want to fail to randomise brk in those cases. + */ + max_stack = rlimit(RLIMIT_STACK); + if (max_stack > SZ_128M) + max_stack = SZ_128M; + + range_limit = mm->start_stack - max_stack - 1; + if (range_end > range_limit) + range_end = range_limit; + + return randomize_range(base, range_end, 0) ? : base; }