Messages in this thread | | | From | Dan Williams <> | Date | Thu, 15 Feb 2018 06:39:13 -0800 | Subject | Re: [PATCH] posix-timers: Protect posix clock array access against speculation |
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On Thu, Feb 15, 2018 at 6:05 AM, Rasmus Villemoes <rasmus.villemoes@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 <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?
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];
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