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 11FDCC433FE for ; Mon, 9 May 2022 20:42:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229787AbiEIUqB (ORCPT ); Mon, 9 May 2022 16:46:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52362 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229760AbiEIUpl (ORCPT ); Mon, 9 May 2022 16:45:41 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E55B3248D1; Mon, 9 May 2022 13:41:45 -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 72320B81980; Mon, 9 May 2022 20:41:44 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 02D56C385BA; Mon, 9 May 2022 20:41:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1652128903; bh=J8j9VzcB8Y4gabiwK3Jn4PyF1nARtTRu3BMbfR7/Pu4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=W1rGwqCqVBponkVtMWTvVdEuLOszyeOvBvx2bPU/eDR5SQTj1L0gmKM3gf+/laYR8 4VVpewzhS/3j9ng5/FE2+uOQV8nuan7ZGFSLvF09D8CB8y02sNe4F5fkcyaz5rldyg BrWWMGDW2tMk53H0eMlUAA2KGVzN7d/tojdLbv7pMV/+o+V+Q8gTRQiwOnL/iwLOj8 D9TUcJRX1Ry1wMG0Fg4mw9RZEtsfwijkTLPqTzD5P7ajl18+RuiIOX3/pf5qEkCzXu 8+awz7QDbUAQ0lLipKDFuSqDifz9iiLdqZuDONRffma3WxVFIeEPGr+oFbF8tr4TXz TyH7KrPc9tOrg== Date: Mon, 9 May 2022 15:50:56 -0500 From: "Gustavo A. R. Silva" To: Kees Cook Cc: Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , x86@kernel.org, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: Re: [PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings Message-ID: <20220509205056.GA109715@embeddedor> References: <20220509194541.GA91598@embeddedor> <202205091251.5703DE2@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <202205091251.5703DE2@keescook> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 09, 2022 at 12:59:15PM -0700, Kees Cook wrote: > On Mon, May 09, 2022 at 02:45:41PM -0500, Gustavo A. R. Silva wrote: > > Fix the following -Wstringop-overflow warnings when building with GCC-12.1: > > > > arch/x86/mm/pgtable.c:437:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=] > > arch/x86/mm/pgtable.c:440:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=] > > arch/x86/mm/pgtable.c:462:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=] > > arch/x86/mm/pgtable.c:454:9: warning: 'pgd_prepopulate_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=] > > arch/x86/mm/pgtable.c:455:9: warning: 'pgd_prepopulate_user_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=] > > arch/x86/mm/pgtable.c:464:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=] > > > > There is a case in which PREALLOCATED_PMDS, MAX_PREALLOCATED_PMDS, > > PREALLOCATED_USER_PMDS and MAX_PREALLOCATED_USER_PMDS are defined as > > zero: > > > > 204 #else /* !CONFIG_X86_PAE */ > > 205 > > 206 /* No need to prepopulate any pagetable entries in non-PAE modes. */ > > 207 #define PREALLOCATED_PMDS 0 > > 208 #define MAX_PREALLOCATED_PMDS 0 > > 209 #define PREALLOCATED_USER_PMDS 0 > > 210 #define MAX_PREALLOCATED_USER_PMDS 0 > > 211 #endif /* CONFIG_X86_PAE */ > > > > It seems that GCC is legitimately complaining about the fact that, under > > certain circumstances, u_pmds and pmds are declared as zero-length arrays > > in the stack and, of course, they are not flexible arrays. > > Ah yeah, I've run into this a few times. Since the relationship between > the macro pairs can't be seen by GCC, it gets upset (i.e. sizeof(u_pmds) > has no relationship wtih PREALLOCATED_USER_PMDS and the calls weren't > inlined, so it can't see that it'll always be 0 and 0). > > > 424 pgd_t *pgd_alloc(struct mm_struct *mm) > > 425 { > > 426 pgd_t *pgd; > > 427 pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS]; > > 428 pmd_t *pmds[MAX_PREALLOCATED_PMDS]; > > 429 > > > > Notice that "Accessing elements of zero-length arrays declared in such > > contexts is undefined and may be diagnosed."[1] > > > > We can fix this by checking that MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS > > are different than zero, prior to passing u_pmds amd pmds as arguments to any > > function, in this case to functions preallocate_pmds(), pgd_prepopulate_pmd() > > and free_pmds(). > > > > This helps with the ongoing efforts to globally enable > > -Wstringop-overflow. > > > > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html > > > > Link: https://github.com/KSPP/linux/issues/181 > > Signed-off-by: Gustavo A. R. Silva > > --- > > Changes in v2: > > - Check MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS > > instead of using pointer notation. > > Link: https://lore.kernel.org/linux-hardening/20220401005834.GA182932@embeddedor/ > > - Update changelog text. > > > > arch/x86/mm/pgtable.c | 16 ++++++++++------ > > 1 file changed, 10 insertions(+), 6 deletions(-) > > > > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c > > index f16059e9a85e..96c3f402a1da 100644 > > --- a/arch/x86/mm/pgtable.c > > +++ b/arch/x86/mm/pgtable.c > > @@ -434,14 +434,18 @@ pgd_t *pgd_alloc(struct mm_struct *mm) > > > > mm->pgd = pgd; > > > > - if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0) > > - goto out_free_pgd; > > + if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) { > > + if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0) > > + goto out_free_pgd; > > > > - if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0) > > - goto out_free_pmds; > > + if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0) > > + goto out_free_pmds; > > > > - if (paravirt_pgd_alloc(mm) != 0) > > - goto out_free_user_pmds; > > + if (paravirt_pgd_alloc(mm) != 0) > > + goto out_free_user_pmds; > > + } else { > > + goto out_free_pgd; > > The "all 0" case shouldn't be a failure mode; it should just skip the > preallocate_pmds() calls. Do you mean something like this: diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c index f16059e9a85e..4dae168408f1 100644 --- a/arch/x86/mm/pgtable.c +++ b/arch/x86/mm/pgtable.c @@ -434,11 +434,13 @@ pgd_t *pgd_alloc(struct mm_struct *mm) mm->pgd = pgd; - if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0) - goto out_free_pgd; + if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) { + if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0) + goto out_free_pgd; - if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0) - goto out_free_pmds; + if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0) + goto out_free_pmds; + } if (paravirt_pgd_alloc(mm) != 0) goto out_free_user_pmds; It seems that the above is not enough, because we have the same issue when calling pgd_prepopulate_pmd(), pgd_prepopulate_user_pmd() and free_pmds(): CC arch/x86/mm/pgtable.o arch/x86/mm/pgtable.c: In function 'pgd_alloc': arch/x86/mm/pgtable.c:464:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=] 464 | free_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ arch/x86/mm/pgtable.c:464:9: note: referencing argument 2 of type 'pmd_t *[0]' arch/x86/mm/pgtable.c:213:13: note: in a call to function 'free_pmds' 213 | static void free_pmds(struct mm_struct *mm, pmd_t *pmds[], int count) | ^~~~~~~~~ arch/x86/mm/pgtable.c:466:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=] 466 | free_pmds(mm, pmds, PREALLOCATED_PMDS); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ arch/x86/mm/pgtable.c:466:9: note: referencing argument 2 of type 'pmd_t *[0]' arch/x86/mm/pgtable.c:213:13: note: in a call to function 'free_pmds' 213 | static void free_pmds(struct mm_struct *mm, pmd_t *pmds[], int count) | ^~~~~~~~~ arch/x86/mm/pgtable.c:456:9: warning: 'pgd_prepopulate_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=] 456 | pgd_prepopulate_pmd(mm, pgd, pmds); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ arch/x86/mm/pgtable.c:456:9: note: referencing argument 3 of type 'pmd_t *[0]' arch/x86/mm/pgtable.c:296:13: note: in a call to function 'pgd_prepopulate_pmd' 296 | static void pgd_prepopulate_pmd(struct mm_struct *mm, pgd_t *pgd, pmd_t *pmds[]) | ^~~~~~~~~~~~~~~~~~~ arch/x86/mm/pgtable.c:457:9: warning: 'pgd_prepopulate_user_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=] 457 | pgd_prepopulate_user_pmd(mm, pgd, u_pmds); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ arch/x86/mm/pgtable.c:457:9: note: referencing argument 3 of type 'pmd_t *[0]' arch/x86/mm/pgtable.c:320:13: note: in a call to function 'pgd_prepopulate_user_pmd' 320 | static void pgd_prepopulate_user_pmd(struct mm_struct *mm, | ^~~~~~~~~~~~~~~~~~~~~~~~ -- Gustavo > > > + } > > > > /* > > * Make sure that pre-populating the pmds is atomic with > > -- > > 2.27.0 > > > > -- > Kees Cook