linux-kernel - Re: [PATCH v3] x86/gart/kcore: Exclude GART aperture from kcore

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Date:   Wed, 6 Mar 2019 16:20:31 +0800
From:   Kairui Song <kasong@...hat.com>
To:     Jiri Bohac <jbohac@...e.cz>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Omar Sandoval <osandov@...com>, Baoquan He <bhe@...hat.com>,
        Dave Young <dyoung@...hat.com>
Subject: Re: [PATCH v3] x86/gart/kcore: Exclude GART aperture from kcore

On Fri, Mar 1, 2019 at 7:12 AM Jiri Bohac <jbohac@...e.cz> wrote:
>
> On Wed, Feb 13, 2019 at 04:28:00PM +0800, Kairui Song wrote:
> > @@ -465,6 +472,12 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
> >                               goto out;
> >                       }
> >                       m = NULL;       /* skip the list anchor */
> > +             } else if (m->type == KCORE_NORAM) {
> > +                     /* for NORAM area just fill zero */
> > +                     if (clear_user(buffer, tsz)) {
> > +                             ret = -EFAULT;
> > +                             goto out;
> > +                     }
>
> I don't think this works reliably. The loop filling the buffer
> has this logic at the top:
>
>         while (buflen) {
>                 /*
>                  * If this is the first iteration or the address is not within
>                  * the previous entry, search for a matching entry.
>                  */
>                 if (!m || start < m->addr || start >= m->addr + m->size) {
>                         list_for_each_entry(m, &kclist_head, list) {
>                                 if (start >= m->addr &&
>                                     start < m->addr + m->size)
>                                         break;
>                         }
>                 }
>
> This sets m to the kclist entry that contains the memory being
> read. But if we do a large read that starts in valid KCORE_RAM
> memory below the GART overlap and extends into the overlap, m
> will not be set to the KCORE_NORAM entry. It will keep pointing
> to the KCORE_RAM entry and the patch will have no effect.
>
> But this seems already broken in existing cases as well, various
> KCORE_* types overlap with KCORE_RAM, don't they?  So maybe
> bf991c2231117d50a7645792b514354fc8d19dae ("proc/kcore: optimize
> multiple page reads") broke this and once fixed, this KCORE_NORAM
> approach will work. Omar?
>

Thanks for the review! You are right, although I hid the NORAM region
from the elf header, but didn't notice this potential risk of having
overlapped region.
I didn't see other kcore regions overlap for now, if so the
optimization should be totally good.
Better to keep using a hook just like what we did in vmcore or we will
have a performance drop for "fixing" this.
Will send V4 using the previous approach if there are no further comments.

-- 
Best Regards,
Kairui Song