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

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Message-Id: <20240626135855.a4b64612a9104ff163e30bd7@linux-foundation.org>
Date: Wed, 26 Jun 2024 13:58:55 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
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

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?