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

[Chris Wagenaar <c.c.j.wagenaar@vu.nl>]

On 8/4/26 12:02, Greg KH wrote:
> 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>

Thanks Dmitry and Greg, I appreciate the quick fix and the attribution!