linux-kernel - Re: [PATCH v2 4/5] mm: make vma cache SLAB_TYPESAFE_BY

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <sdfh56itaffzhpk4rft2tsjm7r44auhjomfthzgxzrmj5632eq@noi2uhgp3a3h>
Date: Wed, 13 Nov 2024 07:38:02 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        willy@...radead.org, lorenzo.stoakes@...cle.com, mhocko@...e.com,
        hannes@...xchg.org, mjguzik@...il.com, oliver.sang@...el.com,
        mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com,
        oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org,
        brauner@...nel.org, dhowells@...hat.com, hdanton@...a.com,
        hughd@...gle.com, minchan@...gle.com, jannh@...gle.com,
        shakeel.butt@...ux.dev, souravpanda@...gle.com,
        pasha.tatashin@...een.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 4/5] mm: make vma cache SLAB_TYPESAFE_BY_RCU

* Vlastimil Babka <vbabka@...e.cz> [241113 03:58]:
> On 11/12/24 20:46, Suren Baghdasaryan wrote:
> > To enable SLAB_TYPESAFE_BY_RCU for vma cache we need to ensure that
> > object reuse before RCU grace period is over will be detected inside
> > lock_vma_under_rcu().
> > lock_vma_under_rcu() enters RCU read section, finds the vma at the
> > given address, locks the vma and checks if it got detached or remapped
> > to cover a different address range. These last checks are there
> > to ensure that the vma was not modified after we found it but before
> > locking it. Vma reuse introduces a possibility that in between those
> > events of finding and locking the vma, it can get detached, reused,
> > added into a tree and be marked as attached. Current checks will help
> > detecting cases when:
> > - vma was reused but not yet added into the tree (detached check)
> > - vma was reused at a different address range (address check)
> > If vma is covering a new address range which still includes the address
> > we were looking for, it's not a problem unless the reused vma was added
> > into a different address space. Therefore checking that vma->vm_mm is
> > still the same is the the only missing check to detect vma reuse.
> 
> Hi, I was wondering if we actually need the detached flag. Couldn't
> "detached" simply mean vma->vm_mm == NULL and we save 4 bytes? Do we ever
> need a vma that's detached but still has a mm pointer? I'd hope the places
> that set detached to false have the mm pointer around so it's not inconvenient.

I think the gate vmas ruin this plan.