linux-kernel - Re: [tip:x86/urgent] x86/mm: Avoid VLA in pgd

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Date:   Tue, 9 Oct 2018 09:33:56 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Borislav Petkov <bp@...en8.de>, linux-tip-commits@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Joerg Roedel <jroedel@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Toshimitsu Kani <toshi.kani@....com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [tip:x86/urgent] x86/mm: Avoid VLA in pgd_alloc()

On Tue, Oct 9, 2018 at 1:28 AM Kees Cook <keescook@...omium.org> wrote:
> On Mon, Oct 8, 2018 at 1:22 PM, Borislav Petkov <bp@...en8.de> wrote:
> > On Fri, Oct 05, 2018 at 09:24:53AM -0700, tip-bot for Arnd Bergmann wrote:

> >
> > For whatever reason - probably because it forced
> > MAX_PREALLOCATED_USER_PMDS be KERNEL_PGD_PTRS and not 0 (and I don't
> > have CONFIG_PAGE_TABLE_ISOLATION so it was 0 here with my .config
> > before) but this patch causes the fun below.
> >
> > If I revert it, no splat.
> >
> > Also, config has CONFIG_X86_PAE=y. And CONFIG_STACKPROTECTOR_STRONG=y. If I
> > disable _STRONG, it boots too. Attached.
>
> This really should mean that the stack canary changed. Either the
> stack canary wasn't prepared yet (but this is from run_init_process(),
> which is WELL after boot_init_stack_canary()), or the canary was
> actually stomped on, which would certainly be a bug in the existing
> code.
>
> Ah! I see it now. "pmds" shouldn't have changed, it's not .._USER_PMDS...
>
> -       pmd_t *u_pmds[PREALLOCATED_USER_PMDS];
> -       pmd_t *pmds[PREALLOCATED_PMDS];
> +       pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];
> +       pmd_t *pmds[MAX_PREALLOCATED_USER_PMDS];

Ah, cool, thanks for the analysis. Is the patch already reverted?
I.e. should I send a replacement patch, or a relative fix, or is
someone else already on it?

      Arnd