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

[Dan Williams]

On Thu, Feb 15, 2018 at 6:05 AM, Rasmus Villemoes
<rasmus.villemoes@xxxxxxxxx> 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 <tglx@xxxxxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
>> ---
>>  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];