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

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Message-ID: <20250311140630.12846Eef-hca@linux.ibm.com>
Date: Tue, 11 Mar 2025 15:06:30 +0100
From: Heiko Carstens <hca@...ux.ibm.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 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.

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.