Input: evdev - sanitize event type index when fetching event masks

The user-supplied event type index passed to EVIOCGMASK / EVIOCSMASK
ioctls is used to index the static counts array in evdev_get_mask_cnt()
and client evmasks array in evdev_get_mask().

While the event type is architecturally bounded by EV_CNT, speculative
execution may mispredict bounds checks and perform out-of-bounds loads.

Sanitize the event type index in evdev_get_mask_cnt() branchlessly using
array_index_mask_nospec(). This clamps the index to 0 for safe array
access and forces the returned count to 0 speculatively when the index
is out of bounds.

We do not need additional array_index_nospec() calls in evdev_get_mask()
because evdev_get_mask_cnt() speculatively forces the count (and
resulting xfer_size) to 0 for out-of-bounds types, preventing any
speculative memory access to client evmasks array.

Reported-by: "Wagenaar, C.C.J. (Chris)" <c.c.j.wagenaar@vu.nl>
Cc: stable@vger.kernel.org
Assisted-by: Antigravity:gemini-3.6-flash
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/anFCAfvxwXB5eJF1@google.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/evdev.c

@@ -21,6 +21,7 @@
 #include <linux/init.h>
 #include <linux/input/mt.h>
 #include <linux/major.h>
+#include <linux/nospec.h>
 #include <linux/device.h>
 #include <linux/cdev.h>
 #include "input-compat.h"
@@ -67,8 +68,10 @@ static size_t evdev_get_mask_cnt(unsigned int type)
 		[EV_SND]	= SND_CNT,
 		[EV_FF]		= FF_CNT,
 	};
+	unsigned long mask = array_index_mask_nospec(type, EV_CNT);
 
-	return (type < EV_CNT) ? counts[type] : 0;
+	/* Returns 0 for out-of-bounds types, including speculatively */
+	return counts[type & mask] & mask;
 }
 
 /* requires the buffer lock to be held */