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

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 <redacted>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kees Cook <redacted>
Cc: Dan Williams <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jérôme Glisse <redacted>
Cc: Jan Kara <jack@suse.cz>
Cc: Dan Williams <redacted>
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 <redacted>
--
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

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