linux-hardening - Re: [PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings

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Date:   Wed, 11 May 2022 11:41:25 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings

On Tue, May 10, 2022 at 09:12:02AM -0500, Gustavo A. R. Silva wrote:
> It doesn't seem to work... however, the following piece of code implies
> that pmds and u_pmds should be first preallocated through preallocate_pmds(),
> which cannot happen if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0)

This works, weirdly:

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 3481b35cb4ec..937a87b404c3 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -424,8 +424,8 @@ static inline void _pgd_free(pgd_t *pgd)
 pgd_t *pgd_alloc(struct mm_struct *mm)
 {
 	pgd_t *pgd;
-	pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];
-	pmd_t *pmds[MAX_PREALLOCATED_PMDS];
+	pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS + 1];
+	pmd_t *pmds[MAX_PREALLOCATED_PMDS + 1];
 
 	pgd = _pgd_alloc();
 

-- 
Kees Cook