[PATCH] iio: adc: xilinx-ams: fix OOB read in ams_get_ext_chan()

["Manush Prajwal" <manushprajwal555@gmail.com>]

The PL external-channel "reg" property is only checked against its
upper bound (AMS_PL_MAX_EXT_CHANNEL + 30 == 50), matching the
'maximum: 50' constraint in the devicetree binding
(Documentation/devicetree/bindings/iio/adc/xlnx,zynqmp-ams.yaml), but
the binding also documents 'minimum: 20' which the driver never
enforces at runtime.

ext_chan is computed as 'reg + AMS_PL_MAX_FIXED_CHANNEL - 30' in
unsigned arithmetic. For any reg < 20 (e.g. a hand-written or
malformed devicetree overlay with reg = <0>), this underflows to a
huge unsigned value, and the following

	memcpy(chan, &ams_pl_channels[ext_chan], sizeof(*channels));

reads far outside the 31-entry ams_pl_channels[] array.

Reject any reg value that would produce an out-of-range ext_chan
before it is used to index ams_pl_channels[], instead of relying only
on the upper-bound check.

Signed-off-by: Manush Prajwal <manushprajwal555@gmail.com>
---
 drivers/iio/adc/xilinx-ams.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/adc/xilinx-ams.c b/drivers/iio/adc/xilinx-ams.c
index 158e6133a..cd778d053 100644
--- a/drivers/iio/adc/xilinx-ams.c
+++ b/drivers/iio/adc/xilinx-ams.c
@@ -1154,8 +1154,11 @@ static int ams_get_ext_chan(struct fwnode_handle *chan_node,
 		if (ret || reg > AMS_PL_MAX_EXT_CHANNEL + 30)
 			continue;
 
-		chan = &channels[num_channels];
 		ext_chan = reg + AMS_PL_MAX_FIXED_CHANNEL - 30;
+		if (ext_chan >= ARRAY_SIZE(ams_pl_channels))
+			continue;
+
+		chan = &channels[num_channels];
 		memcpy(chan, &ams_pl_channels[ext_chan], sizeof(*channels));
 
 		if (fwnode_property_read_bool(child, "xlnx,bipolar"))
-- 
2.46.2.windows.1