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

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Message-ID: <CABi2SkWgp1M5FWbo8p6_byF+5FzQtcSg3ui0rQ9Mpy6z2_BB5Q@mail.gmail.com>
Date: Thu, 24 Jul 2025 11:40:59 -0700
From: Jeff Xu <jeffxu@...omium.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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

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.

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
>