[PATCH 6.6.y 2/2] firmware: arm_scmi: Unwind TX receiver mailbox setup failure

[Artem Dinaburg <artem@trailofbits.com>]

From: Sudeep Holla <sudeep.holla@kernel.org>

[ Upstream commit 6f7c06744d53dc8e047725d411d7f915d9ec35ae ]

mailbox_chan_setup() can request an additional unidirectional TX
receiver channel after successfully acquiring the primary channel. If
that second request fails, the function returns immediately and leaves
the primary channel allocated.

Unwind the primary mailbox channel before returning the error so probe
deferral or other setup failures do not leave the channel busy for later
probe attempts.

[ Backport to 6.6.y: apply the same failure unwind to the pre-transport-
  split mailbox source. ]

Fixes: 9f68ff79ec2c ("firmware: arm_scmi: Add support for unidirectional mailbox channels")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://patch.msgid.link/20260714-scmi_core_fixes-v6-13-3afe499d46e3@kernel.org
Signed-off-by: Sudeep Holla <sudeep.holla@kernel.org>
Assisted-by: LLM
Signed-off-by: Artem Dinaburg <artem@trailofbits.com>
---
This is patch 2 of 2 in the ordered 6.6.y backport series.
This change addresses CVE-2026-93083. The receiver-mailbox setup can fail
after callbacks become reachable; the unwind has real effect, but it is
safe only after channel setup state is published in the order fixed by
CVE-2026-93093.
This needed a target-specific adjustment; I called it out in the bracketed
backport note above.

The fix is already present in 6.12.y, 6.18.y, and 7.2.y, but not in 6.6.y.

 drivers/firmware/arm_scmi/mailbox.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/arm_scmi/mailbox.c b/drivers/firmware/arm_scmi/mailbox.c
index 80b67f46a4d1..a34a3c693e15 100644
--- a/drivers/firmware/arm_scmi/mailbox.c
+++ b/drivers/firmware/arm_scmi/mailbox.c
@@ -230,14 +230,17 @@ static int mailbox_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
 		smbox->chan_receiver = mbox_request_channel(cl, a2p_rx_chan);
 		if (IS_ERR(smbox->chan_receiver)) {
 			ret = PTR_ERR(smbox->chan_receiver);
+			smbox->chan_receiver = NULL;
 			if (ret != -EPROBE_DEFER)
 				dev_err(cdev, "failed to request SCMI Tx Receiver mailbox\n");
-			return ret;
+			goto err_free_chan;
 		}
 	}
 
 	return 0;
 
+err_free_chan:
+	mbox_free_channel(smbox->chan);
 err_clear_cinfo:
 	cinfo->transport_info = NULL;
 	smbox->cinfo = NULL;
-- 
2.39.5