linux-kernel - Re: [PATCH] mm: make 'highmem' symbol ro_after

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 for Android: free password hash cracker in your pocket

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

Message-ID: <CAMj1kXHrS-=5Lc7rFz9Jaq8Nshss+VDgoP6n95D+w+1WB3OZEQ@mail.gmail.com>
Date:   Fri, 28 Jan 2022 10:07:54 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: make 'highmem' symbol ro_after_init

On Wed, 26 Jan 2022 at 22:38, Mike Rapoport <rppt@...nel.org> wrote:
>
> On Mon, Jan 24, 2022 at 06:05:55PM +0100, Ard Biesheuvel wrote:
> > The 'highmem' variable is only set at boot, so we can make it
> > ro_after_init and prevent it from being corrupted inadvertently, or from
> > ending up in a contended cacheline.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
>
> Acked-by: Mike Rapoport <rppt@...ux.ibm.com>
>
> > ---
> >  mm/memory.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index c125c4969913..50e82cb94ccc 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -106,7 +106,7 @@ EXPORT_SYMBOL(mem_map);
> >   * highstart_pfn must be the same; there must be no gap between ZONE_NORMAL
> >   * and ZONE_HIGHMEM.
> >   */
> > -void *high_memory;
> > +void *high_memory __ro_after_init;
> >  EXPORT_SYMBOL(high_memory);
> >
> >  /*
> > --
> > 2.30.2
> >
> >
>


This turns out to break CONFIG_MEMORY_HOTPLUG on x86, as it updates
max_pfn, max_low_pfn and high_memory after init.

So probably best to make all of those __read_mostly instead.