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[23.128.96.18]) by mx.google.com with ESMTP id k18si7445959edf.89.2022.01.31.01.41.34; Mon, 31 Jan 2022 01:41:59 -0800 (PST) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20210112 header.b=fEJYvnHu; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348211AbiA1Lp6 (ORCPT + 99 others); Fri, 28 Jan 2022 06:45:58 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60780 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1348196AbiA1Lpz (ORCPT ); Fri, 28 Jan 2022 06:45:55 -0500 Received: from mail-wm1-x34a.google.com (mail-wm1-x34a.google.com [IPv6:2a00:1450:4864:20::34a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C52D5C061714 for ; Fri, 28 Jan 2022 03:45:54 -0800 (PST) Received: by mail-wm1-x34a.google.com with SMTP id v185-20020a1cacc2000000b0034906580813so5782830wme.1 for ; Fri, 28 Jan 2022 03:45:54 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:in-reply-to:message-id:mime-version:references:subject:from:to :cc; bh=wFWaeqF62cKOElQQL7pp3r58nzR0TbsroVad70HicBs=; b=fEJYvnHuyG464XI/DpthcQvizr2SjMlBfEuvtx0cNBlSwi5tg6ICnFeqdOsPNED6Zf GURh9bBjH6WhPfTpSMj10rpf+j3KbZ29fEi3pNHCYpTiGF6NicIK1sHx9zvobpNuTZKr RMsEu4Vsw02ns5ISGzKxbqUr3vT01QKV9KfWiWQh1gkXADLxOSboWbzG3t6he2cyHQWd m4KT2O6UVO+Svo1P3pFm5Gq4v1YYmasEn6PZdQuD7VF3TETsfNoEcYcJM+If7j5ZoHbf MsyfoGf2XG4pobrNWRnB2JgKpHfRgXIzp9hKRor6fqv0nUNWl1+qo6QlR6zpygSogVM7 SjpA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=wFWaeqF62cKOElQQL7pp3r58nzR0TbsroVad70HicBs=; b=wAGCmLUZc7eCjuQUhExWGRvSO6s4xQXPp/rFcKhZI3CRF2utSfOO4nqHp5h7cB/mgn TEKY4K4t5WnLy75/Y+MVu7LqsI65TUFWH0O0e8SzYKCjhH9g3PzArJWs64EjxyCHKiN/ dblrNEDQfo8S+BpPsy/umqVv6QGcU4QubmI9HGCYcjzuyzt6dK+PUTqPsc/BOYlqSvyO CI4qCCIygAnQSWg3kg9scKnY65eLKYrsn053mlV8Mb5TwiLahv2C0p/JktDUzQEJDGjL feDS/AmgYhbzxL7UOMmo3zhVG6bYbI6dPTsoaiuHfYxDBJ87XUu77NXpkQWD1yD6J/WR fQIQ== X-Gm-Message-State: AOAM531igkBsqNmusowRSfszEiIAR63eH36g+QT8IUC+9874jbfJIL9Y FkO5kwLCCiJjTifFj535kzV+WATYIA== X-Received: from elver.muc.corp.google.com ([2a00:79e0:15:13:f088:5245:7f91:d730]) (user=elver job=sendgmr) by 2002:a05:600c:4e0d:: with SMTP id b13mr7191313wmq.188.1643370353320; Fri, 28 Jan 2022 03:45:53 -0800 (PST) Date: Fri, 28 Jan 2022 12:44:46 +0100 In-Reply-To: <20220128114446.740575-1-elver@google.com> Message-Id: <20220128114446.740575-2-elver@google.com> Mime-Version: 1.0 References: <20220128114446.740575-1-elver@google.com> X-Mailer: git-send-email 2.35.0.rc0.227.g00780c9af4-goog Subject: [PATCH 2/2] stack: Constrain stack offset randomization with Clang builds From: Marco Elver To: elver@google.com, Thomas Gleixner , Kees Cook Cc: Peter Zijlstra , Ingo Molnar , Nathan Chancellor , Nick Desaulniers , Elena Reshetova , Alexander Potapenko , llvm@lists.linux.dev, kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org All supported versions of Clang perform auto-init of __builtin_alloca() when stack auto-init is on (CONFIG_INIT_STACK_ALL_{ZERO,PATTERN}). add_random_kstack_offset() uses __builtin_alloca() to add a stack offset. This means, when CONFIG_INIT_STACK_ALL_{ZERO,PATTERN} is enabled, add_random_kstack_offset() will auto-init that unused portion of the stack used to add an offset. There are several problems with this: 1. These offsets can be as large as 1023 bytes. Performing memset() on them isn't exactly cheap, and this is done on every syscall entry. 2. Architectures adding add_random_kstack_offset() to syscall entry implemented in C require them to be 'noinstr' (e.g. see x86 and s390). The potential problem here is that a call to memset may occur, which is not noinstr. A x86_64 defconfig kernel with Clang 11 and CONFIG_VMLINUX_VALIDATION shows: | vmlinux.o: warning: objtool: do_syscall_64()+0x9d: call to memset() leaves .noinstr.text section | vmlinux.o: warning: objtool: do_int80_syscall_32()+0xab: call to memset() leaves .noinstr.text section | vmlinux.o: warning: objtool: __do_fast_syscall_32()+0xe2: call to memset() leaves .noinstr.text section | vmlinux.o: warning: objtool: fixup_bad_iret()+0x2f: call to memset() leaves .noinstr.text section Clang 14 (unreleased) will introduce a way to skip alloca initialization via __builtin_alloca_uninitialized() (https://reviews.llvm.org/D115440). Constrain RANDOMIZE_KSTACK_OFFSET to only be enabled if no stack auto-init is enabled, the compiler is GCC, or Clang is version 14+. Use __builtin_alloca_uninitialized() if the compiler provides it, as is done by Clang 14. Link: https://lkml.kernel.org/r/YbHTKUjEejZCLyhX@elver.google.com Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall") Signed-off-by: Marco Elver --- arch/Kconfig | 1 + include/linux/randomize_kstack.h | 14 ++++++++++++-- 2 files changed, 13 insertions(+), 2 deletions(-) diff --git a/arch/Kconfig b/arch/Kconfig index 2cde48d9b77c..c5b50bfe31c1 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -1163,6 +1163,7 @@ config RANDOMIZE_KSTACK_OFFSET bool "Support for randomizing kernel stack offset on syscall entry" if EXPERT default y depends on HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET + depends on INIT_STACK_NONE || !CC_IS_CLANG || CLANG_VERSION >= 140000 help The kernel stack offset can be randomized (after pt_regs) by roughly 5 bits of entropy, frustrating memory corruption diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h index 91f1b990a3c3..5c711d73ed10 100644 --- a/include/linux/randomize_kstack.h +++ b/include/linux/randomize_kstack.h @@ -17,8 +17,18 @@ DECLARE_PER_CPU(u32, kstack_offset); * alignment. Also, since this use is being explicitly masked to a max of * 10 bits, stack-clash style attacks are unlikely. For more details see * "VLAs" in Documentation/process/deprecated.rst + * + * The normal alloca() can be initialized with INIT_STACK_ALL. Initializing the + * unused area on each syscall entry is expensive, and generating an implicit + * call to memset() may also be problematic (such as in noinstr functions). + * Therefore, if the compiler provides it, use the "uninitialized" variant. */ -void *__builtin_alloca(size_t size); +#if __has_builtin(__builtin_alloca_uninitialized) +#define __kstack_alloca __builtin_alloca_uninitialized +#else +#define __kstack_alloca __builtin_alloca +#endif + /* * Use, at most, 10 bits of entropy. We explicitly cap this to keep the * "VLA" from being unbounded (see above). 10 bits leaves enough room for @@ -37,7 +47,7 @@ void *__builtin_alloca(size_t size); if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ &randomize_kstack_offset)) { \ u32 offset = raw_cpu_read(kstack_offset); \ - u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset)); \ + u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \ /* Keep allocation even after "ptr" loses scope. */ \ asm volatile("" :: "r"(ptr) : "memory"); \ } \ -- 2.35.0.rc0.227.g00780c9af4-goog