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

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Message-ID: <CA+CK2bDs9prKNSo=Ris-L7T43ZFU7ji3cBH3KD1=FxXg7hFbFA@mail.gmail.com>
Date: Wed, 13 Aug 2025 13:41:51 +0000
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Greg KH <gregkh@...uxfoundation.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 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(?).

Pasha

>
> --
> Regards,
> Pratyush Yadav