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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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