[PATCH] misc: ibmasm: fix OOB MMIO read in ibmasm_handle_mouse_interrupt()

[Tyllis Xu <livelycarpet87@gmail.com>]

ibmasm_handle_mouse_interrupt() performs an out-of-bounds MMIO read
when the queue reader or writer index from hardware exceeds
REMOTE_QUEUE_SIZE (60).

A compromised service processor can trigger this by writing an
out-of-range value to the reader or writer MMIO register before
asserting an interrupt. Since writer is re-read from hardware on
every loop iteration, it can also be set to an out-of-range value
after the loop has already started.

The root cause is that get_queue_reader() and get_queue_writer() return
raw readl() values that are passed directly into get_queue_entry(),
which computes:

  queue_begin + reader * sizeof(struct remote_input)

with no bounds check. This unchecked MMIO address is then passed to
memcpy_fromio(), reading 8 bytes from unintended device registers.
For sufficiently large values the address falls outside the PCI BAR
mapping entirely, triggering a machine check exception.

Fix by checking both indices against REMOTE_QUEUE_SIZE at the top of
the loop body, before any call to get_queue_entry(). On an out-of-range
value, reset the reader register to 0 via set_queue_reader() before
breaking, so that normal queue operation can resume if the corrupted
hardware state is transient.

Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Fixes: 278d72ae8803 ("[PATCH] ibmasm driver: redesign handling of remote control events")
Cc: stable@vger.kernel.org
Cc: ychen@northwestern.edu
Signed-off-by: Tyllis Xu <LivelyCarpet87@gmail.com>
---
 drivers/misc/ibmasm/remote.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/misc/ibmasm/remote.c b/drivers/misc/ibmasm/remote.c
index ec816d3b38cb..521531738c9a 100644
--- a/drivers/misc/ibmasm/remote.c
+++ b/drivers/misc/ibmasm/remote.c
@@ -177,6 +177,11 @@ void ibmasm_handle_mouse_interrupt(struct service_processor *sp)
 	writer = get_queue_writer(sp);
 
 	while (reader != writer) {
+		if (reader >= REMOTE_QUEUE_SIZE || writer >= REMOTE_QUEUE_SIZE) {
+			set_queue_reader(sp, 0);
+			break;
+		}
+
 		memcpy_fromio(&input, get_queue_entry(sp, reader),
 				sizeof(struct remote_input));
 
-- 
2.43.0