linux-kernel - Re: [PATCH v3 29/30] luo: allow preserving memfd

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Message-ID: <2025081341-essence-unheard-63a1@gregkh>
Date: Wed, 13 Aug 2025 15:53:51 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: Pratyush Yadav <pratyush@...nel.org>, Jason Gunthorpe <jgg@...dia.com>,
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Subject: Re: [PATCH v3 29/30] luo: allow preserving memfd

On Wed, Aug 13, 2025 at 01:41:51PM +0000, Pasha Tatashin wrote:
> On Wed, Aug 13, 2025 at 1:37 PM Pratyush Yadav <pratyush@...nel.org> wrote:
> >
> > On Wed, Aug 13 2025, Greg KH wrote:
> >
> > > On Wed, Aug 13, 2025 at 09:41:40AM -0300, Jason Gunthorpe wrote:
> > [...]
> > >> Use the warn ons. Make sure they can't be triggered by userspace. Use
> > >> them to detect corruption/malfunction in the kernel.
> > >>
> > >> In this case if kho_unpreserve_folio() fails in this call chain it
> > >> means some error unwind is wrongly happening out of sequence, and we
> > >> are now forced to leak memory. Unwind is not something that userspace
> > >> should be controlling, so of course we want a WARN_ON here.
> > >
> > > "should be" is the key here.  And it's not obvious from this patch if
> > > that's true or not, which is why I mentioned it.
> > >
> > > I will keep bringing this up, given the HUGE number of CVEs I keep
> > > assigning each week for when userspace hits WARN_ON() calls until that
> > > flow starts to die out either because we don't keep adding new calls, OR
> > > we finally fix them all.  Both would be good...
> >
> > Out of curiosity, why is hitting a WARN_ON() considered a vulnerability?
> > I'd guess one reason is overwhelming system console which can cause a
> > denial of service, but what about WARN_ON_ONCE() or WARN_RATELIMIT()?
> 
> My understanding that it is vulnerability only if it can be triggered
> from userspace, otherwise it is a preferred method to give a notice
> that something is very wrong.
> 
> Given the large number of machines that have panic_on_warn, a reliable
> kernel crash that is triggered from userspace is a vulnerability(?).

Yes, and so is a unreliable one :)