Re: [PATCH 1/3] mm: Add unsafe_follow_pfn

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Mar 16, 2021 at 04:33:01PM +0100, Daniel Vetter wrote:
> 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.
> 
> Also mark up follow_pfn as EXPORT_SYMBOL_GPL. The only safe way to use
> that by drivers/modules is together with an mmu_notifier, and that's
> all _GPL stuff.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Jérôme Glisse <jglisse@redhat.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: linux-mm@kvack.org
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-samsung-soc@vger.kernel.org
> Cc: linux-media@vger.kernel.org
> Cc: kvm@vger.kernel.org
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> --
> v5: Suggestions from Christoph
> - reindent for less weirdness
> - use IS_ENABLED instead of #ifdef
> - same checks for nommu, for consistency
> - EXPORT_SYMBOL_GPL for follow_pfn.
> - kerneldoc was already updated in previous versions to explain when
>   follow_pfn can be used safely
> ---
>  include/linux/mm.h |  2 ++
>  mm/memory.c        | 34 ++++++++++++++++++++++++++++++++--
>  mm/nommu.c         | 27 ++++++++++++++++++++++++++-
>  security/Kconfig   | 13 +++++++++++++
>  4 files changed, 73 insertions(+), 3 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason