[syzbot] WARNING in unsafe_follow_pfn

[Dan Carpenter]

On Tue, Apr 13, 2021 at 03:11:45PM -0300, Jason Gunthorpe wrote:
> On Tue, Apr 13, 2021 at 07:20:12PM +0200, Dmitry Vyukov wrote:
> > > > Plus users are going to be seeing this as well.  According to the commit
> > > > message for 69bacee7f9ad ("mm: Add unsafe_follow_pfn") "Unfortunately
> > > > there's some users where this is not fixable (like v4l userptr of iomem
> > > > mappings)".  It sort of seems crazy to dump this giant splat and then
> > > > tell users to ignore it forever because it can't be fixed...  0_0
> > >
> > > I think the discussion conclusion was that this interface should not
> > > be used by userspace anymore, it is obsolete by some new interface?
> > >
> > > It should be protected by some kconfig and the kconfig should be
> > > turned off for syzkaller runs.
> > 
> > If this is not a kernel bug, then it must not use WARN_ON[_ONCE]. It
> > makes the kernel untestable for both automated systems and humans:
> 
> It is a kernel security bug triggerable by userspace.
> 
> > And if it's a kernel bug reachable from user-space, then I think this
> > code should be removed entirely, not just on all testing systems. Or
> > otherwise if we are not removing it for some reason, then it needs to
> > be fixed.
> 
> Legacy embedded systems apparently require it.

Are legacy embedded systems ever going to update their kernel?  It might
be better to just remove it.  (I don't really have any details outside
of your email so I don't know).

regards,
dan carpenter