On Thu, Feb 15, 2018 at 6:05 AM, Rasmus Villemoes
<rasmus.villem...@prevas.dk> wrote:
> 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 <t...@linutronix.de>
>> Cc: sta...@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?
It does currently, but perhaps it can be fixed.
>
> (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?
Right, this looks broken. I would expect:
if (id >= ARRAY_SIZE(posix_clocks))
return NULL;
idx = array_index_nospec(idx, ARRAY_SIZE(posix_clocks));
if (!posix_clocks[idx])
return NULL;
return posix_clocks[idx];
