Re: [PATCH] Input: evdev - sanitize event type index when fetching event masks

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Aug 03, 2026 at 06:41:49PM -0700, Dmitry Torokhov wrote:
> 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
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> ---
>  drivers/input/evdev.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
> index 5764c98b4f1f..796ac7ac2b8c 100644
> --- a/drivers/input/evdev.c
> +++ b/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 */
> -- 
> 2.55.0.629.g250fe7f194-goog
> 
> 
> -- 
> Dmitry

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>