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Subject: [PATCH 1/2] mm: mmap: fix fput in error path v2
Date:   Mon, 12 Oct 2020 10:52:02 +0200
Message-Id: <20201012085203.56119-1-christian.koenig@amd.com>
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Patch "495c10cc1c0c CHROMIUM: dma-buf: restore args..."
adds a workaround for a bug in mmap_region.

As the comment states ->mmap() callback can change
vma->vm_file and so we might call fput() on the wrong file.

Revert the workaround and proper fix this in mmap_region.

v2: drop the extra if in dma_buf_mmap as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
---
 drivers/dma-buf/dma-buf.c | 20 +++-----------------
 mm/mmap.c                 |  2 +-
 2 files changed, 4 insertions(+), 18 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index a6ba4d598f0e..08630d057cf2 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -1143,9 +1143,6 @@ EXPORT_SYMBOL_GPL(dma_buf_end_cpu_access);
 int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
 		 unsigned long pgoff)
 {
-	struct file *oldfile;
-	int ret;
-
 	if (WARN_ON(!dmabuf || !vma))
 		return -EINVAL;
 
@@ -1163,22 +1160,11 @@ int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
 		return -EINVAL;
 
 	/* readjust the vma */
-	get_file(dmabuf->file);
-	oldfile = vma->vm_file;
-	vma->vm_file = dmabuf->file;
+	fput(vma->vm_file);
+	vma->vm_file = get_file(dmabuf->file);
 	vma->vm_pgoff = pgoff;
 
-	ret = dmabuf->ops->mmap(dmabuf, vma);
-	if (ret) {
-		/* restore old parameters on failure */
-		vma->vm_file = oldfile;
-		fput(dmabuf->file);
-	} else {
-		if (oldfile)
-			fput(oldfile);
-	}
-	return ret;
-
+	return dmabuf->ops->mmap(dmabuf, vma);
 }
 EXPORT_SYMBOL_GPL(dma_buf_mmap);
 
diff --git a/mm/mmap.c b/mm/mmap.c
index 40248d84ad5f..3a2670d73355 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1852,8 +1852,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	return addr;
 
 unmap_and_free_vma:
+	fput(vma->vm_file);
 	vma->vm_file = NULL;
-	fput(file);
 
 	/* Undo any partial mapping done by a device driver. */
 	unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);
-- 
2.17.1