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

[Thomas Gleixner]

On Thu, 15 Feb 2018, Dan Williams wrote:
> On Thu, Feb 15, 2018 at 6:05 AM, Rasmus Villemoes
>  wrote:
> > (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:

Indeed. Missed that.

> 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];

The !posix_clocks[idx] check is pointless and always was.

if (id >= ARRAY_SIZE(posix_clocks))
return NULL;

idx = array_index_nospec(idx, ARRAY_SIZE(posix_clocks));
return posix_clocks[idx];

is sufficient. It returns NULL for !posix_clocks[idx] anyway.

Thanks,

tglx

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

[Thomas Gleixner]

On Thu, 15 Feb 2018, Dan Williams wrote:
> On Thu, Feb 15, 2018 at 6:05 AM, Rasmus Villemoes
>  wrote:
> > (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:

Indeed. Missed that.

> 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];

The !posix_clocks[idx] check is pointless and always was.

if (id >= ARRAY_SIZE(posix_clocks))
return NULL;

idx = array_index_nospec(idx, ARRAY_SIZE(posix_clocks));
return posix_clocks[idx];

is sufficient. It returns NULL for !posix_clocks[idx] anyway.

Thanks,

tglx

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

[Dan Williams]

On Thu, Feb 15, 2018 at 6:05 AM, Rasmus Villemoes
 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 
>> 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 
>>  #include 
>>  #include 
>> +#include 
>>
>>  #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 ?
>>   _posix_dynamic : _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];

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

[Dan Williams]

On Thu, Feb 15, 2018 at 6:05 AM, Rasmus Villemoes
 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 
>> 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 
>>  #include 
>>  #include 
>> +#include 
>>
>>  #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 ?
>>   _posix_dynamic : _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];

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

[Rasmus Villemoes]

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 
> 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 
>  #include 
>  #include 
> +#include 
>  
>  #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 ?
>   _posix_dynamic : _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

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

[Rasmus Villemoes]

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 
> 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 
>  #include 
>  #include 
> +#include 
>  
>  #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 ?
>   _posix_dynamic : _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