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From: Axel Rasmussen <axelrasmussen@google.com>
Date: Wed, 15 May 2024 10:33:03 -0700
Message-ID: <CAJHvVchGGJkEX=qroW=+N-RJDMDGuxM4xoGe7iOtRu9YcfxEEw@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] arch/fault: don't print logs for pte marker poison errors
To: Oscar Salvador <osalvador@suse.de>
Cc: Borislav Petkov <bp@alien8.de>, Andrew Morton <akpm@linux-foundation.org>, 
	Andy Lutomirski <luto@kernel.org>, "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>, 
	Christophe Leroy <christophe.leroy@csgroup.eu>, Dave Hansen <dave.hansen@linux.intel.com>, 
	David Hildenbrand <david@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, Helge Deller <deller@gmx.de>, 
	Ingo Molnar <mingo@redhat.com>, 
	"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>, John Hubbard <jhubbard@nvidia.com>, 
	Liu Shixin <liushixin2@huawei.com>, "Matthew Wilcox (Oracle)" <willy@infradead.org>, 
	Michael Ellerman <mpe@ellerman.id.au>, Muchun Song <muchun.song@linux.dev>, 
	"Naveen N. Rao" <naveen.n.rao@linux.ibm.com>, Nicholas Piggin <npiggin@gmail.com>, 
	Peter Xu <peterx@redhat.com>, Peter Zijlstra <peterz@infradead.org>, 
	Suren Baghdasaryan <surenb@google.com>, Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org, 
	linux-mm@kvack.org, linux-parisc@vger.kernel.org, 
	linuxppc-dev@lists.ozlabs.org, x86@kernel.org
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On Wed, May 15, 2024 at 3:54=E2=80=AFAM Oscar Salvador <osalvador@suse.de> =
wrote:
>
> On Wed, May 15, 2024 at 12:41:42PM +0200, Borislav Petkov wrote:
> > On Fri, May 10, 2024 at 11:29:26AM -0700, Axel Rasmussen wrote:
> > > @@ -3938,7 +3938,7 @@ static vm_fault_t handle_pte_marker(struct vm_f=
ault *vmf)
> > >
> > >     /* Higher priority than uffd-wp when data corrupted */
> > >     if (marker & PTE_MARKER_POISONED)
> > > -           return VM_FAULT_HWPOISON;
> > > +           return VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_SILENT;
> >
> > If you know here that this poisoning should be silent, why do you have
> > to make it all complicated and propagate it into arch code, waste
> > a separate VM_FAULT flag just for that instead of simply returning here
> > a VM_FAULT_COMPLETED or some other innocuous value which would stop
> > processing the fault?
>
> AFAIK, He only wants it to be silent wrt. the arch fault handler not scre=
aming,
> but he still wants to be able to trigger force_sig_mceerr().

Right, the goal is to still have the process get a SIGBUS, but to
avoid the "MCE error" log message. The basic issue is, unprivileged
users can set these markers up, and thereby completely spam up the
log.

Also since this is a process-specific thing, and it's not a real
hardware poison event, it's unclear system admins care at all at a
global level (this is why we didn't want to switch to just
printk_ratelimited for example). Better to let the process handle the
SIGBUS however it likes for its use case (logging a message elsewhere,
etc.).

That said, one thing I'm not sure about is whether or not
VM_FAULT_SIGBUS is a viable alternative (returned for a new PTE marker
type specific to simulated poison). The goal of the simulated poison
feature is to "closely simulate" a real hardware poison event. If you
live migrate a VM from a host with real poisoned memory, to a new
host: you'd want to keep the same behavior if the guest accessed those
addresses again, so as not to confuse the guest about why it suddenly
became "un-poisoned". At a basic level I think VM_FAULT_SIGBUS gives
us what we want (send SIGBUS to the process, don't log about MCEs),
but I'm not confident I know all the differences vs. VM_FAULT_HWPOISON
on all the arches.

>
>
> --
> Oscar Salvador
> SUSE Labs