[PATCH] misc: fastrpc: Fix NULL pointer dereference in rpmsg callback

[Mukesh Ojha]

A NULL pointer dereference was observed on Hawi at boot when the DSP
sends a glink message before fastrpc_rpmsg_probe() has completed
initialization:

  Unable to handle kernel NULL pointer dereference at virtual address 0000000000000178
  pc : _raw_spin_lock_irqsave+0x34/0x8c
  lr : fastrpc_rpmsg_callback+0x3c/0xcc [fastrpc]
  ...
  Call trace:
   _raw_spin_lock_irqsave+0x34/0x8c (P)
   fastrpc_rpmsg_callback+0x3c/0xcc [fastrpc]
   qcom_glink_native_rx+0x538/0x6a4
   qcom_glink_smem_intr+0x14/0x24 [qcom_glink_smem]

The faulting address 0x178 corresponds to the lock variable inside
struct fastrpc_channel_ctx, confirming that cctx is NULL when
fastrpc_rpmsg_callback() attempts to take the spinlock.

There are two issues here. First, dev_set_drvdata() is called before
spin_lock_init() and idr_init(), leaving a window where the callback
can retrieve a valid cctx pointer but operate on an uninitialized
spinlock. Second, the rpmsg channel becomes live as soon as the driver
is bound, so fastrpc_rpmsg_callback() can fire before dev_set_drvdata()
is called at all, resulting in dev_get_drvdata() returning NULL.

Fix both issues by moving all cctx initialization ahead of
dev_set_drvdata() so the structure is fully initialized before it
becomes visible to the callback, and add a NULL check in
fastrpc_rpmsg_callback() as a guard against any remaining window.

Signed-off-by: Mukesh Ojha <mukesh.ojha@xxxxxxxxxxxxxxxx>
---
 drivers/misc/fastrpc.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index 1080f9acf70a..a1a54453bb7e 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -2431,7 +2431,6 @@ static int fastrpc_rpmsg_probe(struct rpmsg_device *rpdev)
 
 	kref_init(&data->refcount);
 
-	dev_set_drvdata(&rpdev->dev, data);
 	rdev->dma_mask = &data->dma_mask;
 	dma_set_mask_and_coherent(rdev, DMA_BIT_MASK(32));
 	INIT_LIST_HEAD(&data->users);
@@ -2440,6 +2439,7 @@ static int fastrpc_rpmsg_probe(struct rpmsg_device *rpdev)
 	idr_init(&data->ctx_idr);
 	data->domain_id = domain_id;
 	data->rpdev = rpdev;
+	dev_set_drvdata(&rpdev->dev, data);
 
 	err = of_platform_populate(rdev->of_node, NULL, NULL, rdev);
 	if (err)
@@ -2513,6 +2513,9 @@ static int fastrpc_rpmsg_callback(struct rpmsg_device *rpdev, void *data,
 	if (len < sizeof(*rsp))
 		return -EINVAL;
 
+	if (!cctx)
+		return -ENODEV;
+
 	ctxid = ((rsp->ctx & FASTRPC_CTXID_MASK) >> 4);
 
 	spin_lock_irqsave(&cctx->lock, flags);
-- 
2.53.0