[commit 6647e76ab623b2b3fb2efe03a86e9c9046c52c33 upstream]

The V4L2_MEMORY_USERPTR interface is long deprecated and shouldn't be used (and is discouraged for any modern v4l drivers). And Seth Jenkins points out that the fallback to VM_PFNMAP/VM_IO is fundamentally racy and dangerous.

Note that it's not even a case that should trigger, since any normal user pointer logic ends up just using the pin_user_pages_fast() call that does the proper page reference counting. That's not the problem case, only if you try to use special device mappings do you have any issues.

Normally I'd just remove this during the merge window, but since Seth pointed out the problem cases, we really want to know as soon as possible if there are actually any users of this odd special case of a legacy interface. Neither Hans nor Mauro seem to think that such mis-uses of the old legacy interface should exist. As Mauro says:

"See, V4L2 has actually 4 streaming APIs: - Kernel-allocated mmap (usually referred simply as just mmap); - USERPTR mmap; - read(); - dmabuf;

The USERPTR is one of the oldest way to use it, coming from V4L version 1 times, and by far the least used one"

And Hans chimed in on the USERPTR interface:

"To be honest, I wouldn't mind if it goes away completely, but that's a bit of a pipe dream right now"

but while removing this legacy interface entirely may be a pipe dream we can at least try to remove the unlikely (and actively broken) case of using special device mappings for USERPTR accesses.

This replaces it with a WARN_ONCE() that we can remove once we've hopefully confirmed that no actual users exist.

NOTE! Longer term, this means that a 'struct frame_vector' only ever contains proper page pointers, and all the games we have with converting them to pages can go away (grep for 'frame_vector_to_pages()' and the uses of 'vec->is_pfns'). But this is just the first step, to verify that this code really is all dead, and do so as quickly as possible.

Reported-by: Seth Jenkins sethjenkins@google.com Acked-by: Hans Verkuil hverkuil@xs4all.nl Acked-by: Mauro Carvalho Chehab mchehab@kernel.org Cc: David Hildenbrand david@redhat.com Cc: Jan Kara jack@suse.cz Signed-off-by: Linus Torvalds torvalds@linux-foundation.org CC: stable@vger.kernel.org # 4.9 Signed-off-by: Sergey Senozhatsky senozhatsky@chromium.org --- mm/frame_vector.c | 31 ++++++------------------------- 1 file changed, 6 insertions(+), 25 deletions(-)

diff --git a/mm/frame_vector.c b/mm/frame_vector.c index d73eed0443f6..aa5526e62c5e 100644 --- a/mm/frame_vector.c +++ b/mm/frame_vector.c @@ -36,7 +36,6 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames, struct mm_struct *mm = current->mm; struct vm_area_struct *vma; int ret = 0; - int err; int locked;

if (nr_frames == 0) @@ -71,32 +70,14 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames, vec->is_pfns = false; ret = get_user_pages_locked(start, nr_frames, gup_flags, (struct page **)(vec->ptrs), &locked); - goto out; + if (likely(ret > 0)) + goto out; }

- vec->got_ref = false; - vec->is_pfns = true; - do { - unsigned long *nums = frame_vector_pfns(vec); - - while (ret < nr_frames && start + PAGE_SIZE <= vma->vm_end) { - err = follow_pfn(vma, start, &nums[ret]); - if (err) { - if (ret == 0) - ret = err; - goto out; - } - start += PAGE_SIZE; - ret++; - } - /* - * We stop if we have enough pages or if VMA doesn't completely - * cover the tail page. - */ - if (ret >= nr_frames || start < vma->vm_end) - break; - vma = find_vma_intersection(mm, start, start + 1); - } while (vma && vma->vm_flags & (VM_IO | VM_PFNMAP)); + /* This used to (racily) return non-refcounted pfns. Let people know */ + WARN_ONCE(1, "get_vaddr_frames() cannot follow VM_IO mapping"); + vec->nr_frames = 0; + out: if (locked) up_read(&mm->mmap_sem);