linux-hardening - Re: inconsistence in mprotect_fixup mlock_fixup madvise_update

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Date:   Tue, 13 Jun 2023 14:29:14 -0700
From:   Jeff Xu <jeffxu@...omium.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-hardening@...r.kernel.org,
        Liam.Howlett@...cle.com, zhangpeng.00@...edance.com,
        akpm@...ux-foundation.org, koct9i@...il.com, david@...hat.com,
        ak@...ux.intel.com, hughd@...gle.com, emunson@...mai.com,
        rppt@...ux.ibm.com, aarcange@...hat.com,
        linux-kernel@...r.kernel.org, Lorenzo Stoakes <lstoakes@...il.com>
Subject: Re: inconsistence in mprotect_fixup mlock_fixup madvise_update_vma

Hello Peter,

Thanks for responding.

On Tue, Jun 13, 2023 at 1:16 PM Peter Xu <peterx@...hat.com> wrote:
>
> Hi, Jeff,
>
> On Tue, Jun 13, 2023 at 08:26:26AM -0700, Jeff Xu wrote:
> > + more ppl to the list.
> >
> > On Mon, Jun 12, 2023 at 6:04 PM Jeff Xu <jeffxu@...omium.org> wrote:
> > >
> > > Hello,
> > >
> > > There seems to be inconsistency in different VMA fixup
> > > implementations, for example:
> > > mlock_fixup will skip VMA that is hugettlb, etc, but those checks do
> > > not exist in mprotect_fixup and madvise_update_vma. Wouldn't this be a
> > > problem? the merge/split skipped by mlock_fixup, might get acted on in
> > > the madvice/mprotect case.
> > >
> > > mlock_fixup currently check for
> > > if (newflags == oldflags || (oldflags & VM_SPECIAL) ||
> > > is_vm_hugetlb_page(vma) || vma == get_gate_vma(current->mm) ||
> > > vma_is_dax(vma) || vma_is_secretmem(vma))
>
> The special handling you mentioned in mlock_fixup mostly makes sense to me.
>
> E.g., I think we can just ignore mlock a hugetlb page if it won't be
> swapped anyway.
>
> Do you encounter any issue with above?
>
> > > Should there be a common function to handle VMA merge/split ?
>
> IMHO vma_merge() and split_vma() are the "common functions".  Copy Lorenzo
> as I think he has plan to look into the interface to make it even easier to
> use.
>
The mprotect_fixup doesn't have the same check as mlock_fixup. When
userspace calls mlock(), two VMAs might not merge or split because of
vma_is_secretmem check, However, when user space calls mprotect() with
the same address range, it will merge/split.  If mlock() is doing the
right thing to merge/split the VMAs, then mprotect() is not ?

Also skipping merge of VMA might be OK, but skipping split doesn't,
wouldn't that cause inconsistent between vma->vm_flags and what is
provisioned in the page ?

Thanks
-Jeff Xu


> --
> Peter Xu
>