linux-kernel - Re: [PATCH v2 00/15] Avoid MAP

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Date: Wed, 26 Jun 2024 21:15:18 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lstoakes@...il.com>,
        Matthew Wilcox <willy@...radead.org>, sidhartha.kumar@...cle.com,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Bert Karwatzki <spasswolf@....de>, Jiri Olsa <olsajiri@...il.com>,
        linux-kernel@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v2 00/15] Avoid MAP_FIXED gap exposure

* Andrew Morton <akpm@...ux-foundation.org> [240626 16:59]:
> On Tue, 25 Jun 2024 15:11:30 -0400 "Liam R. Howlett" <Liam.Howlett@...cle.com> wrote:
> 
> > It is now possible to walk the vma tree using the rcu read locks and is
> > beneficial to do so to reduce lock contention.  Doing so while a
> > MAP_FIXED mapping is executing means that a reader may see a gap in the
> > vma tree that should never logically exist - and does not when using the
> > mmap lock in read mode.  The temporal gap exists because mmap_region()
> > calls munmap() prior to installing the new mapping.
> 
> What are the consequences when this race hits?  IOW, why do we need to
> change anything?
> 

In the (near) future, we want to walk the vma tree to produce
/proc/<pid>/maps.  Without this change we will see the temporal gap and
expose it to the user.  This series was initially sent to Suren as part
of his patch set.

We also have the new interface for an ioctl request to a vma at or above
an address. I had highlighted that an rcu reader would be ideal, but
proved too difficult at this time. These patches by Andrii are currently
not using the rcu reading method as this and a per-vma locking
clarification are needed.

Since there were two users for this code, I decided to send it out
before the other patches.

Thanks,
Liam