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Date: Tue, 10 May 2022 09:12:02 -0500 From: "Gustavo A. R. Silva" <gustavoars@...nel.org> To: Kees Cook <keescook@...omium.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 01:54:32PM -0700, Kees Cook wrote: > 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? 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) 448 /* 449 * Make sure that pre-populating the pmds is atomic with 450 * respect to anything walking the pgd_list, so that they 451 * never see a partially populated pgd. 452 */ 453 spin_lock(&pgd_lock); 454 455 pgd_ctor(mm, pgd); 456 pgd_prepopulate_pmd(mm, pgd, pmds); 457 pgd_prepopulate_user_pmd(mm, pgd, u_pmds); 458 459 spin_unlock(&pgd_lock); 460 461 return pgd; So, my question here is why do you think the "all 0" case should only skip the preallocate_pmds() calls and not the pgd_prepopulate_pmd() calls too? > > Otherwise, they'll likely each need the same check that was added to > pgd_prepopulate_pmd() ages ago for a similar situation... uhm... that doesn't seem to have an impact nowadays, or at least now Wstringop-overflow sees the problem first, because now the issue is detected at the moment of passing the arguments to the the function and not when actually executing the function? otherwise, I think we wouldn't see this error: arch/x86/mm/pgtable.c:454:9: warning: 'pgd_prepopulate_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=] 454 | pgd_prepopulate_pmd(mm, pgd, pmds); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ arch/x86/mm/pgtable.c:454:9: 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[]) | ^~~~~~~~~~~~~~~~~~~ Thanks -- Gustavo
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