From: Dan Williams <dan.j.williams@intel.com> To: Rasmus Villemoes <rasmus.villemoes@prevas.dk> Cc: Thomas Gleixner <tglx@linutronix.de>, LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>, David Woodhouse <dwmw@amazon.co.uk>, Greg KH <gregkh@linuxfoundation.org> Subject: Re: [PATCH] posix-timers: Protect posix clock array access against speculation Date: Thu, 15 Feb 2018 06:39:13 -0800 Message-ID: <CAPcyv4i-bULZp1WTUgUVtdEY5i_fTGkFWKBsVLDu6dsuxUwoug@mail.gmail.com> (raw) In-Reply-To: <45f8dece-e235-0831-4fe5-89ee7d27b959@prevas.dk> 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];
next prev parent reply index Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-02-15 13:27 Thomas Gleixner 2018-02-15 14:05 ` Rasmus Villemoes 2018-02-15 14:39 ` Dan Williams [this message] 2018-02-15 14:53 ` Thomas Gleixner 2018-02-15 16:21 ` [PATCH V2] " Thomas Gleixner 2018-02-15 17:01 ` Peter Zijlstra 2018-03-07 18:13 ` Dan Williams 2018-03-22 11:34 ` [tip:timers/urgent] " tip-bot for Thomas Gleixner 2018-02-15 19:52 ` [PATCH] linux/nospec.h: allow index argument to have const-qualified type Rasmus Villemoes 2018-02-15 20:59 ` Linus Torvalds 2018-02-15 21:56 ` Dan Williams 2018-02-15 22:03 ` Linus Torvalds 2018-02-15 22:08 ` Dan Williams
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