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Date: Mon, 9 May 2022 13:54:32 -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 Mon, May 09, 2022 at 03:50:56PM -0500, Gustavo A. R. Silva wrote: > 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 <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; > > > > 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); > | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Ugh. Perhaps just marking both preallocate_pmds() and free_pmds() as inline is enough to let the compiler "see" everything correctly? Otherwise, they'll likely each need the same check that was added to pgd_prepopulate_pmd() ages ago for a similar situation... -- Kees Cook
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