linux-kernel - Re: [PATCH mm-unstable 0/2] mseal system mappings fix + s390 enablement

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Message-ID: <6f106b28-7d5e-468f-8a94-e94357a950ee@lucifer.local>
Date: Tue, 11 Mar 2025 14:09:30 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Jeff Xu <jeffxu@...omium.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Kees Cook <kees@...nel.org>,
        Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH mm-unstable 0/2] mseal system mappings fix + s390
 enablement

On Tue, Mar 11, 2025 at 03:06:30PM +0100, Heiko Carstens wrote:
> On Tue, Mar 11, 2025 at 01:42:01PM +0000, Lorenzo Stoakes wrote:
> > > Just like for arm64, and x86_64 the s390 part is just adding the new
> > > vm flag to the _install_mappings() call in vdso code. Otherwise there
> > > is nothing to be considered.
> >
> > No, they are not just adding a flag, they are enabling the sealing of
> > system mappings, if a user chooses to make use of it, which already breaks
> > a number of useland applications that rely on remapping these.
> >
> > if the architecture code ever needs to unmap/remap these, then this breaks
> > your architecture.
> >
> > I think it would be sensible to clearly indicate that enabling this feature
> > does not break the s390 architecture and you've confirmed that by checking
> > the code and ensuring that nowhere does it rely upon doing this.
> >
> > Likely that's the case, but I'd suggest you ought to make sure...
> >
> > x86-64 and arm64 were checked for this and confirmed to not ever need this.
> >
> > This is why we're restricting by architecture.
>
> Ok, I was assuming more that whoever enables that config option knows
> what he or she is doing. However as far as I know there is no s390
> user space which relies on remapping vdso mappings.

Yeah, we allow for the 'user knows what they're doing' stuff when it comes to
_userland_, but we obviously don't want to ship a known-broken kernel :)

>
> When it comes to unmapping vdso: this would break user space since
> commit df29a7440c4b ("s390/signal: switch to using vdso for sigreturn
> and syscall restart") - there haven't been any reports.

OK that seems to implicitly suggest that you're fine with sealing then?

I had a quick glance and it seemed fine for s390. I mean it's _weird_ to remap
any of this stuff so it'd be the odd one out that does it (ppc _seems_ to in
_some_ circumstances need it, for instance).

So I think we're good? :)