linux-kernel - Re: [PATCH v8 2/3] mm: add a field to store names for private anonymous memory

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Message-ID: <CAJuCfpHWCtqCcuZdyfc4-virtynOMv2f_iU=OJUB_6b2Xz+k9g@mail.gmail.com>
Date:   Wed, 1 Sep 2021 08:42:29 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Colin Cross <ccross@...gle.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Al Viro <viro@...iv.linux.org.uk>,
        Randy Dunlap <rdunlap@...radead.org>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Peter Xu <peterx@...hat.com>, rppt@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
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        vincenzo.frascino@....com,
        Chinwen Chang (張錦文) 
        <chinwen.chang@...iatek.com>,
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        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH v8 2/3] mm: add a field to store names for private
 anonymous memory

On Wed, Sep 1, 2021 at 1:10 AM 'Michal Hocko' via kernel-team
<kernel-team@...roid.com> wrote:
>
> On Fri 27-08-21 12:18:57, Suren Baghdasaryan wrote:
> [...]
> > +static void replace_vma_anon_name(struct vm_area_struct *vma, const char *name)
> > +{
> > +     if (!name) {
> > +             free_vma_anon_name(vma);
> > +             return;
> > +     }
> > +
> > +     if (vma->anon_name) {
> > +             /* Should never happen, to dup use dup_vma_anon_name() */
> > +             WARN_ON(vma->anon_name == name);
>
> What is the point of this warning?

I wanted to make sure replace_vma_anon_name() is not used from inside
vm_area_dup() or some similar place (does not exist today but maybe in
the future) where "new" vma is a copy of "orig" vma and
new->anon_name==orig->anon_name. If someone by mistake calls
replace_vma_anon_name(new, orig->anon_name) and
new->anon_name==orig->anon_name then they will keep pointing to the
same name pointer, which breaks an assumption that ->anon_name
pointers are not shared among vmas even if the string is the same.
That would eventually lead to use-after-free error. After the next
patch implementing refcounting, the similar situation would lead to
both new and orig vma pointing to the same anon_vma_name structure
without raising the refcount, which would also lead to use-after-free
error. That's why the above comment asks to use dup_vma_anon_name() if
this warning ever happens.
I can remove the warning but I thought the problem is subtle enough to
put some safeguards.

>
> > +
> > +             /* Same name, nothing to do here */
> > +             if (!strcmp(name, vma->anon_name))
> > +                     return;
> > +
> > +             free_vma_anon_name(vma);
> > +     }
> > +     vma->anon_name = kstrdup(name, GFP_KERNEL);
> > +}
> --
> Michal Hocko
> SUSE Labs
>
> --
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>