[PATCH] usb: gadget: f_fs: check ffs->gadget in the DMABUF detach/find paths

[Yu Zhang <yuz08559@gmail.com>]

ffs_dmabuf_detach() and ffs_dmabuf_find_attachment() dereference
ffs->gadget->dev.parent as their very first statement, without checking
that ffs->gadget is non-NULL:

	static int ffs_dmabuf_detach(struct file *file, int fd)
	{
		...
		struct device *dev = ffs->gadget->dev.parent;

ffs->gadget is only set between functionfs_bind() and
functionfs_unbind() (UDC bind/unbind). The epfile ioctls, however, are
gated only on ffs->state == FFS_ACTIVE, which userspace reaches by
writing the descriptors and strings and which is independent of the
gadget lifetime: an FFS instance is FFS_ACTIVE before it has ever been
bound to a UDC, and stays FFS_ACTIVE after unbind. So
FUNCTIONFS_DMABUF_DETACH (and the DMABUF transfer path, through
ffs_dmabuf_find_attachment()) can be issued while ffs->gadget is NULL,
dereferencing NULL:

	BUG: kernel NULL pointer dereference, address: 00000000000000b0
	RIP: 0010:ffs_epfile_ioctl+0x402/0x1700
	 __x64_sys_ioctl+0x120/0x170
	 do_syscall_64+0xf9/0x540

Reproduced by mounting functionfs, writing a descriptor+string blob so
the state becomes FFS_ACTIVE without ever binding a UDC, then issuing
FUNCTIONFS_DMABUF_DETACH on an endpoint file.

ffs_dmabuf_attach() already guards this with
"if (!gadget || !gadget->sg_supported) return -EPERM". Do the same in
ffs_dmabuf_detach() and ffs_dmabuf_find_attachment() (the latter also
covers ffs_dmabuf_transfer()): snapshot ffs->gadget, return -EPERM if it
is NULL, and use the snapshot.

Fixes: 7b07a2a7ca02 ("usb: gadget: functionfs: Add DMABUF import interface")
Cc: stable@vger.kernel.org
Signed-off-by: Yu Zhang <yuz08559@gmail.com>
---
 drivers/usb/gadget/function/f_fs.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 44218be..0976db7 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -1473,9 +1473,15 @@ static int ffs_dma_resv_lock(struct dma_buf *dmabuf, bool nonblock)
 static struct dma_buf_attachment *
 ffs_dmabuf_find_attachment(struct ffs_epfile *epfile, struct dma_buf *dmabuf)
 {
-	struct device *dev = epfile->ffs->gadget->dev.parent;
+	struct usb_gadget *gadget = epfile->ffs->gadget;
 	struct dma_buf_attachment *attach = NULL;
 	struct ffs_dmabuf_priv *priv;
+	struct device *dev;
+
+	if (!gadget)
+		return ERR_PTR(-EPERM);
+
+	dev = gadget->dev.parent;
 
 	mutex_lock(&epfile->dmabufs_mutex);
 
@@ -1570,11 +1576,17 @@ static int ffs_dmabuf_detach(struct file *file, int fd)
 {
 	struct ffs_epfile *epfile = file->private_data;
 	struct ffs_data *ffs = epfile->ffs;
-	struct device *dev = ffs->gadget->dev.parent;
+	struct usb_gadget *gadget = ffs->gadget;
 	struct ffs_dmabuf_priv *priv, *tmp;
 	struct dma_buf *dmabuf;
+	struct device *dev;
 	int ret = -EPERM;
 
+	if (!gadget)
+		return -EPERM;
+
+	dev = gadget->dev.parent;
+
 	dmabuf = dma_buf_get(fd);
 	if (IS_ERR(dmabuf))
 		return PTR_ERR(dmabuf);
-- 
2.43.0