Re: [PATCH v2 09/17] mm: Add unsafe_follow_pfn

[Mauro Carvalho Chehab]

Em Fri, 9 Oct 2020 14:37:23 +0200
Mauro Carvalho Chehab <mchehab+huawei@xxxxxxxxxx> escreveu:

> Em Fri, 9 Oct 2020 09:21:11 -0300
> Jason Gunthorpe <jgg@xxxxxxxx> escreveu:
> 
> > On Fri, Oct 09, 2020 at 12:34:21PM +0200, Mauro Carvalho Chehab wrote:  
> > > Hi,
> > > 
> > > Em Fri,  9 Oct 2020 09:59:26 +0200
> > > Daniel Vetter <daniel.vetter@xxxxxxxx> escreveu:
> > >     
> > > > Way back it was a reasonable assumptions that iomem mappings never
> > > > change the pfn range they point at. But this has changed:
> > > > 
> > > > - gpu drivers dynamically manage their memory nowadays, invalidating
> > > > ptes with unmap_mapping_range when buffers get moved
> > > > 
> > > > - contiguous dma allocations have moved from dedicated carvetouts to
> > > > cma regions. This means if we miss the unmap the pfn might contain
> > > > pagecache or anon memory (well anything allocated with GFP_MOVEABLE)
> > > > 
> > > > - even /dev/mem now invalidates mappings when the kernel requests that
> > > > iomem region when CONFIG_IO_STRICT_DEVMEM is set, see 3234ac664a87
> > > > ("/dev/mem: Revoke mappings when a driver claims the region")
> > > > 
> > > > Accessing pfns obtained from ptes without holding all the locks is
> > > > therefore no longer a good idea.
> > > > 
> > > > Unfortunately there's some users where this is not fixable (like v4l
> > > > userptr of iomem mappings) or involves a pile of work (vfio type1
> > > > iommu). For now annotate these as unsafe and splat appropriately.
> > > > 
> > > > This patch adds an unsafe_follow_pfn, which later patches will then
> > > > roll out to all appropriate places.    
> > > 
> > > NACK, as this breaks an existing userspace API on media.    
> > 
> > It doesn't break it. You get a big warning the thing is broken and it
> > keeps working.
> > 
> > We can't leave such a huge security hole open - it impacts other
> > subsystems, distros need to be able to run in a secure mode.  
> 
> Well, if distros disable it, then apps will break.
> 
> > > While I agree that using the userptr on media is something that
> > > new drivers may not support, as DMABUF is a better way of
> > > handling it, changing this for existing ones is a big no, 
> > > as it may break usersapace.    
> > 
> > media community needs to work to fix this, not pretend it is OK to
> > keep going as-is.  
> 
> > Dealing with security issues is the one case where an uABI break might
> > be acceptable.
> > 
> > If you want to NAK it then you need to come up with the work to do
> > something here correctly that will support the old drivers without the
> > kernel taint.
> > 
> > Unfortunately making things uncomfortable for the subsystem is the big
> > hammer the core kernel needs to use to actually get this security work
> > done by those responsible.  
> 
> 
> I'm not pretending that this is ok. Just pointing that the approach
> taken is NOT OK.
> 
> I'm not a mm/ expert, but, from what I understood from Daniel's patch
> description is that this is unsafe *only if*  __GFP_MOVABLE is used.
> 
> Well, no drivers inside the media subsystem uses such flag, although
> they may rely on some infrastructure that could be using it behind
> the bars.
> 
> If this is the case, the proper fix seems to have a GFP_NOT_MOVABLE 
> flag that it would be denying the core mm code to set __GFP_MOVABLE.
> 
> Please let address the issue on this way, instead of broken an
> userspace API that it is there since 1991.

In time: I meant to say 1998.

Thanks,
Mauro