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

[Kees Cook <keescook@chromium.org>]

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@kernel.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.

> +	}
>  
>  	/*
>  	 * Make sure that pre-populating the pmds is atomic with
> -- 
> 2.27.0
> 

-- 
Kees Cook