[PATCH 5.7 107/112] dma-buf: Move dma_buf_release() from fops to dentry_ops

Greg Kroah-Hartman
2020-07-07T15:27:47Z

From: Sumit Semwal <sumit.semwal@linaro.org>

commit 4ab59c3c638c6c8952bf07739805d20eb6358a4d upstream.

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>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Tested-by: Charan Teja Reddy <charante@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200611114418.19852-1-sumit.semwal@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/dma-buf/dma-buf.c | 54 +++++++++++++++++++++-------------------------
1 file changed, 25 insertions(+), 29 deletions(-)

--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -54,37 +54,11 @@ static char *dmabuffs_dname(struct dentr
dentry->d_name.name, ret > 0 ? name : "");
}

-static const struct dentry_operations dma_buf_dentry_ops = {
- .d_dname = dmabuffs_dname,
-};
-
-static struct vfsmount *dma_buf_mnt;
-
-static int dma_buf_fs_init_context(struct fs_context *fc)
-{
- struct pseudo_fs_context *ctx;
-
- ctx = init_pseudo(fc, DMA_BUF_MAGIC);
- if (!ctx)
- return -ENOMEM;
- ctx->dops = &dma_buf_dentry_ops;
- return 0;
-}
-
-static struct file_system_type dma_buf_fs_type = {
- .name = "dmabuf",
- .init_fs_context = dma_buf_fs_init_context,
- .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;
-
- dmabuf = file->private_data;
+ dmabuf = dentry->d_fsdata;

BUG_ON(dmabuf->vmapping_counter);

@@ -110,9 +84,32 @@ static int dma_buf_release(struct inode
module_put(dmabuf->owner);
kfree(dmabuf->name);
kfree(dmabuf);
+}
+
+static const struct dentry_operations dma_buf_dentry_ops = {
+ .d_dname = dmabuffs_dname,
+ .d_release = dma_buf_release,
+};
+
+static struct vfsmount *dma_buf_mnt;
+
+static int dma_buf_fs_init_context(struct fs_context *fc)
+{
+ struct pseudo_fs_context *ctx;
+
+ ctx = init_pseudo(fc, DMA_BUF_MAGIC);
+ if (!ctx)
+ return -ENOMEM;
+ ctx->dops = &dma_buf_dentry_ops;
return 0;
}

+static struct file_system_type dma_buf_fs_type = {
+ .name = "dmabuf",
+ .init_fs_context = dma_buf_fs_init_context,
+ .kill_sb = kill_anon_super,
+};
+
static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
{
struct dma_buf *dmabuf;
@@ -412,7 +409,6 @@ static void dma_buf_show_fdinfo(struct s
}

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,