linux-kernel - Re: [PATCH] mm: swap: check for xa_zero

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: <fec5ad61-7caf-4076-ba81-7b454ec7018d@lucifer.local>
Date: Mon, 11 Aug 2025 13:14:31 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Charan Teja Kalla <quic_charante@...cinc.com>
Cc: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org,
        shikemeng@...weicloud.com, kasong@...cent.com, nphamcs@...il.com,
        bhe@...hat.com, baohua@...nel.org, chrisl@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm: swap: check for xa_zero_entry() on vma in swapoff
 path

On Mon, Aug 11, 2025 at 03:13:14PM +0530, Charan Teja Kalla wrote:
> Thanks David, for the reply!!
> On 8/8/2025 5:34 PM, David Hildenbrand wrote:
> >>         if (mpnt) {
> >>             mas_set_range(&vmi.mas, mpnt->vm_start, mpnt->vm_end - 1);
> >>             mas_store(&vmi.mas, XA_ZERO_ENTRY);
> >>             /* Avoid OOM iterating a broken tree */
> >>             set_bit(MMF_OOM_SKIP, &mm->flags);
> >>         }
> >>         /*
> >>          * The mm_struct is going to exit, but the locks will be dropped
> >>          * first.  Set the mm_struct as unstable is advisable as it is
> >>          * not fully initialised.
> >>          */
> >>         set_bit(MMF_UNSTABLE, &mm->flags);
> >>     }
> >>
> >> Shouldn't we just remove anything from the tree here that was not copied
> >> immediately?
> >
> > Another fix would be to just check MMF_UNSTABLE in unuse_mm(). But
> > having these MMF_UNSTABLE checks all over the place feels a bit like
> > whack-a-mole.
> >
> Seems MMF_UNSTABLE is the expectation per the commit,
> 64c37e134b12("kernel: be more careful about dup_mmap() failures and
> uprobe registering"). Excerpt(s) from the commit message:

This really is whack-a-mole yeah.

>
> This patch sets the MMF_OOM_SKIP to avoid the iteration of the vmas on
> the oom side (even though this is extremely unlikely to be selected as
> an oom victim in the race window), and __sets MMF_UNSTABLE to avoid
> other potential users from using a partially initialised mm_struct.
>

But... maybe this is better for the _hotfix_ version as a nicer way of
doing this.

> When registering vmas for uprobe, skip the vmas in an mm that is marked
> unstable.  Modifying a vma in an unstable mm may cause issues if the mm
> isn't fully initialised.__
>
> > Is there anything preventing us from just leaving a proper tree that
> > reflects reality in place before we drop the write lock?
>
> When you mean proper tree, is this about the your previous question? --
> Shouldn't we just remove anything from the tree here that was not copied
> immediately?

Commit d24062914837 (" fork: use __mt_dup() to duplicate maple tree in
dup_mmap()") did this for efficiency, so it'd be a regression to do this.

See
https://lore.kernel.org/linux-mm/20231016032226.59199-1-zhangpeng.00@bytedance.com/

>
> Anyway, would request Liam/Lorenzo to comment on this.

Hi! :)

Really want Liam's input on this too, as he looked at this previously and
was, I believe, at least potentially going to take a look around this whole
logic.

Fork is sadly an area in which things become horrible quick...

Cheers, Lorenzo