Re: [PATCH v2 1/1] mm: fix use-after-free when anon vma name is used after vma is freed

[Suren Baghdasaryan]

On Thu, Feb 10, 2022 at 7:27 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Thu, Feb 10, 2022 at 07:18:24AM -0800, Suren Baghdasaryan wrote:
> > On Thu, Feb 10, 2022 at 4:40 AM 'Michal Hocko' via kernel-team
> > <kernel-team@xxxxxxxxxxx> wrote:
> > >
> > > On Wed 09-02-22 20:32:15, Suren Baghdasaryan wrote:
> > > > When adjacent vmas are being merged it can result in the vma that was
> > > > originally passed to madvise_update_vma being destroyed. In the current
> > > > implementation, the name parameter passed to madvise_update_vma points
> > > > directly to vma->anon_name->name and it is used after the call to
> > > > vma_merge. In the cases when vma_merge merges the original vma and
> > > > destroys it, this will result in use-after-free bug as shown below:
> > > >
> > > > madvise_vma_behavior << passes vma->anon_name->name as name param
> > > >   madvise_update_vma(name)
> > > >     vma_merge
> > > >       __vma_adjust
> > > >         vm_area_free <-- frees the vma
> > > >     replace_vma_anon_name(name) <-- UAF
> > > >
> > > > Fix this by raising the name refcount and stabilizing it. Introduce
> > > > vma_anon_name_{get/put} API for this purpose.
> > >
> > > What is the reason that madvise_update_vma uses the naked name rather
> > > than the encapsulated anon_vma_name? This really just begs for problems.
> >
> > The reason for that is the second place it's being used from the prctl syscall:
> >
> > prctl_set_vma
> >   madvise_set_anon_name
> >     madvise_vma_anon_name
> >       madvise_update_vma
> >
> > In that case the name parameter is not part of any anon_vma_name
> > struct and therefore is stable. I can add a comment to
> > madvise_update_vma indicating that the name parameter has to be stable
> > if that helps.
>
> Seems to me it'd simplify things if replace_vma_anon_name() and
> madvise_vma_anon_name() took a struct anon_vma_name instead of
> a bare char *.  You could construct it in madvise_set_anon_name().

Ok, this can be done. However I don't think changing
replace_vma_anon_name() to accept a struct anon_vma_name would be a
good idea. Reader might think that the object being passed will become
the vma->anon_name of the vma, while in reality that's not the case.
Keeping it a char* makes it obvious that the function will construct a
new anon_vma_name struct.
I'll post a v3 shortly implementing these suggestions.
Thanks for the review folks!