[PATCH] RFC: dma-buf: Require VM_SPECIAL vma for mmap

[Daniel Vetter]

tldr; DMA buffers aren't normal memory, expecting that you can use
them like that (like calling get_user_pages works, or that they're
accounting like any other normal memory) cannot be guaranteed.

Since some userspace only runs on integrated devices, where all
buffers are actually all resident system memory, there's a huge
temptation to assume that a struct page is always present and useable
like for any more pagecache backed mmap. This has the potential to
result in a uapi nightmare.

To stop this gap require that DMA buffer mmaps are VM_SPECIAL, which
blocks get_user_pages and all the other struct page based
infrastructure for everyone. In spirit this is the uapi counterpart to
the kernel-internal CONFIG_DMABUF_DEBUG.

Motivated by a recent patch which wanted to swich the system dma-buf
heap to vm_insert_page instead of vm_insert_pfn.

References: https://lore.kernel.org/lkml/CAKMK7uHi+mG0z0HUmNt13QCCvutuRVjpcR0NjRL12k-WbWzkRg@mail.gmail.com/
Cc: Jason Gunthorpe <jgg at ziepe.ca>
Cc: Suren Baghdasaryan <surenb at google.com>
Cc: Matthew Wilcox <willy at infradead.org>
Cc: John Stultz <john.stultz at linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
Cc: Sumit Semwal <sumit.semwal at linaro.org>
Cc: "Christian König" <christian.koenig at amd.com>
Cc: linux-media at vger.kernel.org
Cc: linaro-mm-sig at lists.linaro.org
---
 drivers/dma-buf/dma-buf.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index f264b70c383e..d3081fc07056 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -127,6 +127,7 @@ static struct file_system_type dma_buf_fs_type = {
 static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
 {
 	struct dma_buf *dmabuf;
+	int ret;
 
 	if (!is_dma_buf_file(file))
 		return -EINVAL;
@@ -142,7 +143,11 @@ static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
 	    dmabuf->size >> PAGE_SHIFT)
 		return -EINVAL;
 
-	return dmabuf->ops->mmap(dmabuf, vma);
+	ret = dmabuf->ops->mmap(dmabuf, vma);
+
+	WARN_ON(!(vma->vm_flags & VM_SPECIAL));
+
+	return ret;
 }
 
 static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
@@ -1244,6 +1249,8 @@ EXPORT_SYMBOL_GPL(dma_buf_end_cpu_access);
 int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
 		 unsigned long pgoff)
 {
+	int ret;
+
 	if (WARN_ON(!dmabuf || !vma))
 		return -EINVAL;
 
@@ -1264,7 +1271,11 @@ int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
 	vma_set_file(vma, dmabuf->file);
 	vma->vm_pgoff = pgoff;
 
-	return dmabuf->ops->mmap(dmabuf, vma);
+	ret = dmabuf->ops->mmap(dmabuf, vma);
+
+	WARN_ON(!(vma->vm_flags & VM_SPECIAL));
+
+	return ret;
 }
 EXPORT_SYMBOL_GPL(dma_buf_mmap);
 
-- 
2.30.0