linux-kernel - Re: [PATCH v9 2/8] mm: rust: add vm_area_struct methods that require read access

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Message-ID: <f0da7967-2d0b-4890-a4a7-bd5e36538eea@lucifer.local>
Date: Fri, 29 Nov 2024 11:58:35 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Jann Horn <jannh@...gle.com>, Miguel Ojeda <ojeda@...nel.org>,
        Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, Christian Brauner <brauner@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, rust-for-linux@...r.kernel.org,
        Andreas Hindborg <a.hindborg@...nel.org>
Subject: Re: [PATCH v9 2/8] mm: rust: add vm_area_struct methods that require
 read access

On Fri, Nov 29, 2024 at 12:44:10PM +0100, Alice Ryhl wrote:
> On Wed, Nov 27, 2024 at 4:41 PM Jann Horn <jannh@...gle.com> wrote:
> >
> > On Wed, Nov 27, 2024 at 1:01 PM Alice Ryhl <aliceryhl@...gle.com> wrote:
> > > On Tue, Nov 26, 2024 at 11:10 PM Jann Horn <jannh@...gle.com> wrote:
> > > >
> > > > On Fri, Nov 22, 2024 at 4:41 PM Alice Ryhl <aliceryhl@...gle.com> wrote:
> > > > > This adds a type called VmAreaRef which is used when referencing a vma
> > > > > that you have read access to. Here, read access means that you hold
> > > > > either the mmap read lock or the vma read lock (or stronger).
> > > > >
> > > > > Additionally, a vma_lookup method is added to the mmap read guard, which
> > > > > enables you to obtain a &VmAreaRef in safe Rust code.
> > > > >
> > > > > This patch only provides a way to lock the mmap read lock, but a
> > > > > follow-up patch also provides a way to just lock the vma read lock.
> > > > >
> > > > > Acked-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com> (for mm bits)
> > > > > Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> > > >
> > > > Reviewed-by: Jann Horn <jannh@...gle.com>
> > >
> > > Thanks!
> > >
> > > > with one comment:
> > > >
> > > > > +    /// Zap pages in the given page range.
> > > > > +    ///
> > > > > +    /// This clears page table mappings for the range at the leaf level, leaving all other page
> > > > > +    /// tables intact, and freeing any memory referenced by the VMA in this range. That is,
> > > > > +    /// anonymous memory is completely freed, file-backed memory has its reference count on page
> > > > > +    /// cache folio's dropped, any dirty data will still be written back to disk as usual.
> > > > > +    #[inline]
> > > > > +    pub fn zap_page_range_single(&self, address: usize, size: usize) {
> > > > > +        // SAFETY: By the type invariants, the caller has read access to this VMA, which is
> > > > > +        // sufficient for this method call. This method has no requirements on the vma flags. Any
> > > > > +        // value of `address` and `size` is allowed.
> > > >
> > > > If we really want to allow any address and size, we might want to add
> > > > an early bailout in zap_page_range_single(). The comment on top of
> > > > zap_page_range_single() currently says "The range must fit into one
> > > > VMA", and it looks like by the point we reach a bailout, we could have
> > > > gone through an interval tree walk via
> > > > mmu_notifier_invalidate_range_start()->__mmu_notifier_invalidate_range_start()->mn_itree_invalidate()
> > > > for a range that ends before it starts; I don't know how safe that is.
> > >
> > > I could change the comment on zap_page_range_single() to say:
> > >
> > > "The range should be contained within a single VMA. Otherwise an error
> > > is returned."
> > >
> > > And then I can add an overflow check at the top of
> > > zap_page_range_single(). Sounds ok?
> >
> > Yes, I think changing the comment like that and adding a check for
> > whether address+size wraps around there addresses this.
>
> Hmm. Looking at this again now ...
>
> For one, the function returns void so we can at best handle overflow
> by performing a no-op.
>
> Another question is, are we actually okay to call it with ranges
> outside the vma? Does that just no-op or what? Maybe the Rust side
> should just bail out early if the address range is not a subset of the
> vma?

In unmap_single_vma() invoked by zap_page_range_single() we check this:

	unsigned long start = max(vma->vm_start, start_addr);

	...

	if (start >= vma->vm_end)
		return;
	end = min(vma->vm_end, end_addr);
	if (end <= vma->vm_start)
		return;

So you could specify start <= vma->vm_end and end > vma->vm_start and it'll get
clamped like:

0                                        MAX
        start  <-clamp->
<---------------------->xxxxxxxxxxxxxxxxxxxx
               |--------| VMA
xxxxxxxxxxxxxxxx<-------------------------->
	        <-clamp->

However note that we will tell mmum_notifier_range_init() and
hugetlb_zap_begin() incorrect values in this case... not sure.

I'd prefer if rust would strictly only allowed ranges within the VMA.

>
> Alice