[PATCH] dma-buf: Move dma_buf_release() from fops to dentry_ops

[Sumit Semwal <sumit.semwal@linaro.org>]

Charan Teja reported a 'use-after-free' in dmabuffs_dname [1], which
happens if the dma_buf_release() is called while the userspace is
accessing the dma_buf pseudo fs's dmabuffs_dname() in another process,
and dma_buf_release() releases the dmabuf object when the last reference
to the struct file goes away.

I discussed with Arnd Bergmann, and he suggested that rather than tying
the dma_buf_release() to the file_operations' release(), we can tie it to
the dentry_operations' d_release(), which will be called when the last ref
to the dentry is removed.

The path exercised by __fput() calls f_op->release() first, and then calls
dput, which eventually calls d_op->d_release().

In the 'normal' case, when no userspace access is happening via dma_buf
pseudo fs, there should be exactly one fd, file, dentry and inode, so
closing the fd will kill of everything right away.

In the presented case, the dentry's d_release() will be called only when
the dentry's last ref is released.

Therefore, lets move dma_buf_release() from fops->release() to
d_ops->d_release().

Many thanks to Arnd for his FS insights :)

[1]: https://lore.kernel.org/patchwork/patch/1238278/

Fixes: bb2bb9030425 ("dma-buf: add DMA_BUF_SET_NAME ioctls")
Reported-by: syzbot+3643a18836bce555bff6@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org> [5.3+]
Cc: Arnd Bergmann <arnd@arndb.de>
Reported-by: Charan Teja Reddy <charante@codeaurora.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
---
 drivers/dma-buf/dma-buf.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 01ce125f8e8d..92ba4b6ef3e7 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -54,8 +54,11 @@ static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen)
 			     dentry->d_name.name, ret > 0 ? name : "");
 }
 
+static void dma_buf_release(struct dentry *dentry);
+
 static const struct dentry_operations dma_buf_dentry_ops = {
 	.d_dname = dmabuffs_dname,
+	.d_release = dma_buf_release,
 };
 
 static struct vfsmount *dma_buf_mnt;
@@ -77,14 +80,14 @@ static struct file_system_type dma_buf_fs_type = {
 	.kill_sb = kill_anon_super,
 };
 
-static int dma_buf_release(struct inode *inode, struct file *file)
+static void dma_buf_release(struct dentry *dentry)
 {
 	struct dma_buf *dmabuf;
 
-	if (!is_dma_buf_file(file))
-		return -EINVAL;
+	if (dentry->d_op != &dma_buf_dentry_ops)
+		return;
 
-	dmabuf = file->private_data;
+	dmabuf = dentry->d_fsdata;
 
 	BUG_ON(dmabuf->vmapping_counter);
 
@@ -110,7 +113,6 @@ static int dma_buf_release(struct inode *inode, struct file *file)
 	module_put(dmabuf->owner);
 	kfree(dmabuf->name);
 	kfree(dmabuf);
-	return 0;
 }
 
 static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
@@ -412,7 +414,6 @@ static void dma_buf_show_fdinfo(struct seq_file *m, struct file *file)
 }
 
 static const struct file_operations dma_buf_fops = {
-	.release	= dma_buf_release,
 	.mmap		= dma_buf_mmap_internal,
 	.llseek		= dma_buf_llseek,
 	.poll		= dma_buf_poll,
-- 
2.27.0