Re: [PATCH] mm/fadvise: validate offset in generic_fadvise

[Kevin Lourenco <klourencodev@gmail.com>]

Thank you, Andrew, for taking a look and for queueing in -next.

I'll keep an eye on it and will adjust the patch if needed.

Kevin

Le lun. 22 déc. 2025 à 18:52, Andrew Morton
<akpm@linux-foundation.org> a écrit :
>
> On Mon, 22 Dec 2025 15:18:17 +0100 klourencodev@gmail.com wrote:
>
> > From: Kevin Lourenco <klourencodev@gmail.com>
> >
> > When converted to (u64) for page calculations, a negative offset
> > can produce extremely large page indices. This may lead to issues in certain advice modes (excessive readahead or
> > cache invalidation)
> >
> > offsets are normally non-negative, but the API does not guarantee this. Since 'len' is already
> > validated, checking 'offset' here is reasonable to prevent potential system instability.
> >
>
> Yeah, seems this code really didn't think about negative offsets.
>
>         fadvise(fd, -20, 40, advice);
>
> does make some sense I guess - equivalent to
>
>         fadvise(fd, 0, 20, advice);
>
> but whether this is how the implementation actually treats -20,40 is
> unclear.  Probably not, and what will filemap_flush_range() make of
> this.
>
> ugh.
>
> I suppose I'll toss this into -next for now, see if it breaks any tests.
> But more thought is needed.
>