[PATCH] staging: nvec: kbd: bound keycode table lookups

[Mohamed Anis Ben Azza <benazzaanis783@gmail.com>]

nvec_keys_notifier() indexes code_tabs[_size][code] using values taken
directly from the embedded controller's event message without checking
them against the tables:

 - _size is a 2-bit field (0..3) from the message header, but code_tabs[]
   only has entries for NVEC_2BYTES and NVEC_3BYTES. NVEC_VAR_SIZE is
   rejected, but the reserved encoding 3 is not, so code_tabs[3] reads a
   pointer past the end of the array and then dereferences it.

 - code is msg[1] & 0x7f (0..127), but code_tab_102us[] has only 126
   entries and extcode_tab_us102[] only 110, so scancodes at or above
   those sizes read past the end of the tables.

Reject any size encoding without a table and any scancode beyond the
end of the selected table, dropping the event as is already done for
the power button.

This only matters for an EC that sends malformed or out-of-spec
events, so it is a robustness fix rather than a security issue.

The issue was found and the fix drafted with the help of an AI coding
assistant (Claude). It was compile-tested with
ARCH=arm LLVM=1 tegra_defconfig + MFD_NVEC/KEYBOARD_NVEC at W=1 with no
new warnings, and the lookup logic was exercised in a userspace harness
under AddressSanitizer for all 4 size encodings x 256 scancodes: the old
code reports a global-buffer-overflow, the new code none. It has not
been tested on real Tegra/AC100 hardware.

Fixes: 32890b983086 ("Staging: initial version of the nvec driver")
Signed-off-by: Mohamed Anis Ben Azza <benazzaanis783@gmail.com>
---
 drivers/staging/nvec/nvec_kbd.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/nvec/nvec_kbd.c b/drivers/staging/nvec/nvec_kbd.c
index d2b91318f..9b976f0f5 100644
--- a/drivers/staging/nvec/nvec_kbd.c
+++ b/drivers/staging/nvec/nvec_kbd.c
@@ -28,6 +28,11 @@ enum kbd_subcmds {
 static unsigned char keycodes[ARRAY_SIZE(code_tab_102us)
 			      + ARRAY_SIZE(extcode_tab_us102)];
 
+static const unsigned int code_tab_sizes[] = {
+	ARRAY_SIZE(code_tab_102us),
+	ARRAY_SIZE(extcode_tab_us102),
+};
+
 struct nvec_keys {
 	struct input_dev *input;
 	struct notifier_block notifier;
@@ -59,8 +64,11 @@ static int nvec_keys_notifier(struct notifier_block *nb,
 	if (event_type == NVEC_KB_EVT) {
 		int _size = (msg[0] & (3 << 5)) >> 5;
 
-/* power on/off button */
-		if (_size == NVEC_VAR_SIZE)
+		/*
+		 * NVEC_VAR_SIZE is the power on/off button; the fourth
+		 * encoding is reserved. Neither has a keycode table.
+		 */
+		if (_size >= ARRAY_SIZE(code_tabs))
 			return NOTIFY_STOP;
 
 		if (_size == NVEC_3BYTES)
@@ -69,6 +77,9 @@ static int nvec_keys_notifier(struct notifier_block *nb,
 		code = msg[1] & 0x7f;
 		state = msg[1] & 0x80;
 
+		if (code >= code_tab_sizes[_size])
+			return NOTIFY_STOP;
+
 		if (code_tabs[_size][code] == KEY_CAPSLOCK && state)
 			nvec_kbd_toggle_led();
 
-- 
2.56.0