linux-kernel - Re: [PATCH 4/7] mm/mremap: Replace can_modify_mm with can_modify

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Message-ID: <CALmYWFu_cnS1nHcQxVmjp=a+SdsACi+4YWWvfDGTB=pj=fmS3A@mail.gmail.com>
Date: Tue, 6 Aug 2024 16:09:11 -0700
From: Jeff Xu <jeffxu@...gle.com>
To: Pedro Falcato <pedro.falcato@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, oliver.sang@...el.com, 
	torvalds@...ux-foundation.org, Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 4/7] mm/mremap: Replace can_modify_mm with can_modify_vma

On Tue, Aug 6, 2024 at 2:28 PM Pedro Falcato <pedro.falcato@...il.com> wrote:
>
> Delegate all can_modify checks to the proper places. Unmap checks are
> done in do_unmap (et al).
>
> This patch allows for mremap partial failure in certain cases (for
> instance, when destination VMAs aren't sealed, but the source VMA is).
> It shouldn't be too troublesome, as you'd need to go out of your way to
> do illegal operations on a VMA.
>
> Signed-off-by: Pedro Falcato <pedro.falcato@...il.com>
> ---
>  mm/mremap.c | 33 +++++++--------------------------
>  1 file changed, 7 insertions(+), 26 deletions(-)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index e7ae140fc64..8af877d7bb0 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -676,6 +676,9 @@ static unsigned long move_vma(struct vm_area_struct *vma,
>         if (unlikely(flags & MREMAP_DONTUNMAP))
>                 to_account = new_len;
>
> +       if (!can_modify_vma(vma))
> +               return -EPERM;
> +
I m not 100% sure, but I suspect you don't need this check? Is
vma_to_resize already checking the src address ?

PS. Is it possible to consolidate all the related changes (except the
fix for madvise) to a single commit ?
 It would be easier to look for dependency, e.g. the remap depends on munmap().

Also selftest is helpful to prove the correctness of the change. (And
I can also test it)

>         if (vma->vm_ops && vma->vm_ops->may_split) {
>                 if (vma->vm_start != old_addr)
>                         err = vma->vm_ops->may_split(vma, old_addr);
> @@ -821,6 +824,10 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
>         if (!vma)
>                 return ERR_PTR(-EFAULT);
>
> +       /* Don't allow vma expansion when it has already been sealed */
> +       if (!can_modify_vma(vma))
> +               return ERR_PTR(-EPERM);
> +
>         /*
>          * !old_len is a special case where an attempt is made to 'duplicate'
>          * a mapping.  This makes no sense for private mappings as it will
> @@ -902,19 +909,6 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
>         if ((mm->map_count + 2) >= sysctl_max_map_count - 3)
>                 return -ENOMEM;
>
> -       /*
> -        * In mremap_to().
> -        * Move a VMA to another location, check if src addr is sealed.
> -        *
> -        * Place can_modify_mm here because mremap_to()
> -        * does its own checking for address range, and we only
> -        * check the sealing after passing those checks.
> -        *
> -        * can_modify_mm assumes we have acquired the lock on MM.
> -        */
> -       if (unlikely(!can_modify_mm(mm, addr, addr + old_len)))
> -               return -EPERM;
> -
>         if (flags & MREMAP_FIXED) {
>                 /*
>                  * In mremap_to().
> @@ -1079,19 +1073,6 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
>                 goto out;
>         }
>
> -       /*
> -        * Below is shrink/expand case (not mremap_to())
> -        * Check if src address is sealed, if so, reject.
> -        * In other words, prevent shrinking or expanding a sealed VMA.
> -        *
> -        * Place can_modify_mm here so we can keep the logic related to
> -        * shrink/expand together.
> -        */
> -       if (unlikely(!can_modify_mm(mm, addr, addr + old_len))) {
> -               ret = -EPERM;
> -               goto out;
> -       }
> -
>         /*
>          * Always allow a shrinking remap: that just unmaps
>          * the unnecessary pages..
> --
> 2.46.0
>