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Date: Mon, 9 May 2022 14:45:41 -0500 From: "Gustavo A. R. Silva" <gustavoars@...nel.org> To: 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> Cc: x86@...nel.org, linux-kernel@...r.kernel.org, "Gustavo A. R. Silva" <gustavoars@...nel.org>, linux-hardening@...r.kernel.org Subject: [PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings 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. 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 <gustavoars@...nel.org> --- 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; + } /* * Make sure that pre-populating the pmds is atomic with -- 2.27.0
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