linux-kernel - Re: [PATCHv3 12/27] x86/vdso: Restrict splitting VVAR VMA

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Message-ID: <CAG48ez1NSNWx1vixsHrxkqyafEu1i=qC4dQwBLXc+gLnBM2Y0Q@mail.gmail.com>
Date:   Thu, 25 Apr 2019 20:41:19 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        Adrian Reber <adrian@...as.de>,
        Andrei Vagin <avagin@...nvz.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jeff Dike <jdike@...toit.com>, Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        containers@...ts.linux-foundation.org, criu@...nvz.org,
        Linux API <linux-api@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCHv3 12/27] x86/vdso: Restrict splitting VVAR VMA

On Thu, Apr 25, 2019 at 6:17 PM Dmitry Safonov <dima@...sta.com> wrote:
> Although, time namespace can work with VVAR VMA split, it seems worth
> to forbid splitting VVAR resulting in stricter ABI and reducing amount
> of corner-cases to consider while working further on VDSO.
[...]
> diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
> index babc4e7a519c..ff9875a4d53b 100644
> --- a/arch/x86/entry/vdso/vma.c
> +++ b/arch/x86/entry/vdso/vma.c
[...]
> +static int vvar_mremap(const struct vm_special_mapping *sm,
> +               struct vm_area_struct *new_vma)
> +{
> +       unsigned long new_size = new_vma->vm_end - new_vma->vm_start;
> +       const struct vdso_image *image = current->mm->context.vdso_image;

Using current->mm in here is a bit awkward. So far the ->mremap
handler can only be invoked for `current`, but still, maybe
new_vma->vm_mm would be better here?

> +       if (new_size != -image->sym_vvar_start)
> +               return -EINVAL;
> +
> +       return 0;
> +}
[...]