linux-kernel - Re: [PATCH v3 4/5] mm/mseal: Simplify and rename VMA gap check

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Message-ID: <00283a52-4aa8-4e0e-8ec1-62fbbca90479@lucifer.local>
Date: Fri, 25 Jul 2025 06:33:15 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jeff Xu <jeffxu@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        David Hildenbrand <david@...hat.com>, Vlastimil Babka <vbabka@...e.cz>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Kees Cook <kees@...nel.org>, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3 4/5] mm/mseal: Simplify and rename VMA gap check

On Thu, Jul 24, 2025 at 11:40:59AM -0700, Jeff Xu wrote:
> Hi Lorenzo,
>
> On Wed, Jul 16, 2025 at 10:38 AM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
> >
> > The check_mm_seal() function is doing something general - checking whether
> > a range contains only VMAs (or rather that it does NOT contain any
> > unmapped regions).
> >
> > So rename this function to range_contains_unmapped().
> >
> > Additionally simplify the logic, we are simply checking whether the last
> > vma->vm_end has either a VMA starting after it or ends before the end
> > parameter.
> >
> > This check is rather dubious, so it is sensible to keep it local to
> > mm/mseal.c as at a later stage it may be removed, and we don't want any
> > other mm code to perform such a check.
> >
> > No functional change intended.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > Acked-by: David Hildenbrand <david@...hat.com>
> > ---
> >  mm/mseal.c | 36 +++++++++++-------------------------
> >  1 file changed, 11 insertions(+), 25 deletions(-)
> >
> > diff --git a/mm/mseal.c b/mm/mseal.c
> > index adbcc65e9660..61c07b1369cb 100644
> > --- a/mm/mseal.c
> > +++ b/mm/mseal.c
> > @@ -37,32 +37,22 @@ static int mseal_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >         return ret;
> >  }
> >
> > -/*
> > - * Check for do_mseal:
> > - * 1> start is part of a valid vma.
> > - * 2> end is part of a valid vma.
> > - * 3> No gap (unallocated address) between start and end.
> > - * 4> map is sealable.
> > - */
> > -static int check_mm_seal(unsigned long start, unsigned long end)
> Is it possible to leave the check_mm_seal() function together with its
> header comments? My original reason was to have a contract that
> documents the exact entry check for mseal(). That way, no matter how
> the code is refactored in the future, as long as the contract remains
> true, I won't need to worry about behavior changes for mseal(). This
> could be helpful if you move range_contains_unmapped into vma.c in the
> future.
>
> Note: "4> map is sealable." can be removed,  which is obsolete, we no
> longer use sealable flags.

Sure, I will add in a comment to make this abundantly clear.

>
> Thanks and regards,
> -Jeff
> > +/* Does the [start, end) range contain any unmapped memory? */
> > +static bool range_contains_unmapped(struct mm_struct *mm,
> > +               unsigned long start, unsigned long end)
> >  {
> >         struct vm_area_struct *vma;
> > -       unsigned long nstart = start;
> > +       unsigned long prev_end = start;
> >         VMA_ITERATOR(vmi, current->mm, start);
> >
> > -       /* going through each vma to check. */
> >         for_each_vma_range(vmi, vma, end) {
> > -               if (vma->vm_start > nstart)
> > -                       /* unallocated memory found. */
> > -                       return -ENOMEM;
> > -
> > -               if (vma->vm_end >= end)
> > -                       return 0;
> > +               if (vma->vm_start > prev_end)
> > +                       return true;
> >
> > -               nstart = vma->vm_end;
> > +               prev_end = vma->vm_end;
> >         }
> >
> > -       return -ENOMEM;
> > +       return prev_end < end;
> >  }
> >
> >  /*
> > @@ -184,14 +174,10 @@ int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> >         if (mmap_write_lock_killable(mm))
> >                 return -EINTR;
> >
> > -       /*
> > -        * First pass, this helps to avoid
> > -        * partial sealing in case of error in input address range,
> > -        * e.g. ENOMEM error.
> > -        */
> > -       ret = check_mm_seal(start, end);
> > -       if (ret)
> > +       if (range_contains_unmapped(mm, start, end)) {
> > +               ret = -ENOMEM;
> >                 goto out;
> > +       }
> >
> >         /*
> >          * Second pass, this should success, unless there are errors
> > --
> > 2.50.1
> >