Re: [PATCH] posix-timers: Protect posix clock array access against speculation

[Rasmus Villemoes <rasmus.villemoes@prevas.dk>]

On 2018-02-15 14:27, Thomas Gleixner wrote:
> The (clock) id argument of clockid_to_kclock() comes straight from user
> space via various syscalls and is used as index into the posix_clocks
> array.
> 
> Protect it against spectre v1 array out of bounds speculation.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: stable@vger.kernel.org
> ---
>  kernel/time/posix-timers.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> --- a/kernel/time/posix-timers.c
> +++ b/kernel/time/posix-timers.c
> @@ -50,6 +50,7 @@
>  #include <linux/export.h>
>  #include <linux/hashtable.h>
>  #include <linux/compat.h>
> +#include <linux/nospec.h>
>  
>  #include "timekeeping.h"
>  #include "posix-timers.h"
> @@ -1346,11 +1347,14 @@ static const struct k_clock * const posi
>  
>  static const struct k_clock *clockid_to_kclock(const clockid_t id)
>  {
> +	clockid_t idx = id;
> +
>  	if (id < 0)
>  		return (id & CLOCKFD_MASK) == CLOCKFD ?
>  			&clock_posix_dynamic : &clock_posix_cpu;
>  
>  	if (id >= ARRAY_SIZE(posix_clocks) || !posix_clocks[id])
>  		return NULL;
> -	return posix_clocks[id];
> +
> +	return posix_clocks[array_index_nospec(idx, ARRAY_SIZE(posix_clocks))];
>  }
> 

Stupid questions from someone trying to learn what the rules for when
and how to apply these _nospec macros:

(1) why introduce the idx var? There's no assignment to it other than
the initialization. Is it some magic in array_index_nospec that prevents
the use of a const-qualified expression?

(2) The line "if (id >= ARRAY_SIZE(posix_clocks) || !posix_clocks[id])"
still seems to allow speculatively accessing posix_clocks[id]. Is that
ok, and even if so, wouldn't it be cleaner to elide the
!posix_clocks[id] check and just return the NULL safely fetched from the
array in the following line?

Rasmus