[PATCH v2] bus: mhi: host: Detect events pointing to unexpected TREs

[Youssef Samir <quic_yabdulra@quicinc.com>]

When a remote device sends a completion event to the host, it contains a
pointer to the consumed TRE. The host uses this pointer to process all of
the TREs between it and the host's local copy of the ring's read pointer.
This works when processing completion for chained transactions, but can
lead to nasty results if the device sends an event for a single-element
transaction with a read pointer that is multiple elements ahead of the
host's read pointer.

For instance, if the host accesses an event ring while the device is
updating it, the pointer inside of the event might still point to an old
TRE. If the host uses the channel's xfer_cb() to directly free the buffer
pointed to by the TRE, the buffer will be double-freed.

This behavior was observed on an ep that used drivers/bus/mhi/ep/ without
'commit 6f18d174b73d ("bus: mhi: ep: Update read pointer only after buffer is written")'.
Where the device updated the events ring pointer before updating the event
contents, so it left a window where the host was able to access the stale
data the event pointed to, before the device had the chance to update them.
The usual pattern was that the host received an event pointing to a TRE
that is not immediately after the last processed one, so it got treated as
if it was a chained transaction, processing all of the TREs in between the
two read pointers.

This patch aims to harden the host by ensuring transactions where the event
points to a TRE that isn't local_rp + 1 are chained.

Fixes: 1d3173a3bae7 ("bus: mhi: core: Add support for processing events from client device")
Signed-off-by: Youssef Samir <quic_yabdulra@quicinc.com>
---
Changes in v2:
- Update the commit message
- Link to v1: https://lore.kernel.org/mhi/20250521163110.571893-1-quic_yabdulra@quicinc.com/
---
 drivers/bus/mhi/host/main.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
index aa8a0ef697c7..57dc9c5c0d84 100644
--- a/drivers/bus/mhi/host/main.c
+++ b/drivers/bus/mhi/host/main.c
@@ -602,7 +602,7 @@ static int parse_xfer_event(struct mhi_controller *mhi_cntrl,
 	{
 		dma_addr_t ptr = MHI_TRE_GET_EV_PTR(event);
 		struct mhi_ring_element *local_rp, *ev_tre;
-		void *dev_rp;
+		void *dev_rp, *next_rp;
 		struct mhi_buf_info *buf_info;
 		u16 xfer_len;
 
@@ -621,6 +621,16 @@ static int parse_xfer_event(struct mhi_controller *mhi_cntrl,
 		result.dir = mhi_chan->dir;
 
 		local_rp = tre_ring->rp;
+
+		next_rp = local_rp + 1;
+		if (next_rp >= tre_ring->base + tre_ring->len)
+			next_rp = tre_ring->base;
+		if (dev_rp != next_rp && !MHI_TRE_DATA_GET_CHAIN(local_rp)) {
+			dev_err(&mhi_cntrl->mhi_dev->dev,
+				"Event element points to an unexpected tre\n");
+			break;
+		}
+
 		while (local_rp != dev_rp) {
 			buf_info = buf_ring->rp;
 			/* If it's the last TRE, get length from the event */
-- 
2.25.1