From mboxrd@z Thu Jan 1 00:00:00 1970 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 1EC9FC433FE for ; Tue, 5 Apr 2022 10:50:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348860AbiDEKt1 (ORCPT ); Tue, 5 Apr 2022 06:49:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46028 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1344105AbiDEJmA (ORCPT ); Tue, 5 Apr 2022 05:42:00 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8AF66BD2FC; Tue, 5 Apr 2022 02:27:35 -0700 (PDT) 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 0C8D4B81CAF; Tue, 5 Apr 2022 09:27:34 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 57B9BC385A2; Tue, 5 Apr 2022 09:27:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1649150852; bh=HA4HSF/J+nRdjbcp1S15eTKIy9yKc9WCMNOb4KCX3L0=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=pLI/yZ/nTbSQGixRnXzUnAbTl21oAEhRGPz/4jNQgytBrdXvYb7CJ176Bea0HcaJK eWKTsFuMMwdmqDCxvzi+I2R+T761homJPAjZVIn7rnzZE9aPl2XO6MscaKw2ogR/sq nGdlononPDAe/w2H04fwtHUk/Uvg14hl78x+uDm8= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Marco Elver , Nathan Chancellor , Kees Cook , Sasha Levin Subject: [PATCH 5.15 203/913] stack: Constrain and fix stack offset randomization with Clang builds Date: Tue, 5 Apr 2022 09:21:05 +0200 Message-Id: <20220405070345.944099127@linuxfoundation.org> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20220405070339.801210740@linuxfoundation.org> References: <20220405070339.801210740@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Marco Elver [ Upstream commit efa90c11f62e6b7252fb75efe2787056872a627c ] 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 Reviewed-by: Nathan Chancellor Acked-by: Kees Cook Signed-off-by: Kees Cook Link: https://lore.kernel.org/r/20220131090521.1947110-2-elver@google.com Signed-off-by: Sasha Levin --- arch/Kconfig | 1 + include/linux/randomize_kstack.h | 16 ++++++++++++++-- 2 files changed, 15 insertions(+), 2 deletions(-) diff --git a/arch/Kconfig b/arch/Kconfig index d1e69d6e8498..191589f26b1a 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -1141,6 +1141,7 @@ config HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET config RANDOMIZE_KSTACK_OFFSET_DEFAULT bool "Randomize kernel stack offset on syscall entry" 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 bebc911161b6..d373f1bcbf7c 100644 --- a/include/linux/randomize_kstack.h +++ b/include/linux/randomize_kstack.h @@ -16,8 +16,20 @@ 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 __builtin_alloca() is initialized with INIT_STACK_ALL (currently + * only with Clang and not GCC). 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 + * supports it (which it should if it initializes allocas), always use the + * "uninitialized" variant of the builtin. */ -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 @@ -36,7 +48,7 @@ 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"); \ } \ -- 2.34.1