Jann Horn identified a racy access to p->mm in the global expedited command of the membarrier system call.
The suggested fix is to hold the task_lock() around the accesses to p->mm and to the mm_struct membarrier_state field to guarantee the existence of the mm_struct. Link: https://lore.kernel.org/lkml/cag48ez2g8ctf8dhs42tf37pthfr3y0rnooytmxvacm4u8yu...@mail.gmail.com Signed-off-by: Mathieu Desnoyers <mathieu.desnoy...@efficios.com> CC: Jann Horn <ja...@google.com> CC: Thomas Gleixner <t...@linutronix.de> CC: Peter Zijlstra (Intel) <pet...@infradead.org> CC: Ingo Molnar <mi...@kernel.org> CC: Andrea Parri <parri.and...@gmail.com> CC: Andrew Hunter <a...@google.com> CC: Andy Lutomirski <l...@kernel.org> CC: Avi Kivity <a...@scylladb.com> CC: Benjamin Herrenschmidt <b...@kernel.crashing.org> CC: Boqun Feng <boqun.f...@gmail.com> CC: Dave Watson <davejwat...@fb.com> CC: David Sehr <s...@google.com> CC: Greg Hackmann <ghackm...@google.com> CC: H. Peter Anvin <h...@zytor.com> CC: Linus Torvalds <torva...@linux-foundation.org> CC: Maged Michael <maged.mich...@gmail.com> CC: Michael Ellerman <m...@ellerman.id.au> CC: Paul E. McKenney <paul...@linux.vnet.ibm.com> CC: Paul Mackerras <pau...@samba.org> CC: Russell King <li...@armlinux.org.uk> CC: Will Deacon <will.dea...@arm.com> CC: sta...@vger.kernel.org # v4.16+ CC: linux-...@vger.kernel.org --- kernel/sched/membarrier.c | 27 +++++++++++++++++++++------ 1 file changed, 21 insertions(+), 6 deletions(-) diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c index 76e0eaf4654e..305fdcc4c5f7 100644 --- a/kernel/sched/membarrier.c +++ b/kernel/sched/membarrier.c @@ -81,12 +81,27 @@ static int membarrier_global_expedited(void) rcu_read_lock(); p = task_rcu_dereference(&cpu_rq(cpu)->curr); - if (p && p->mm && (atomic_read(&p->mm->membarrier_state) & - MEMBARRIER_STATE_GLOBAL_EXPEDITED)) { - if (!fallback) - __cpumask_set_cpu(cpu, tmpmask); - else - smp_call_function_single(cpu, ipi_mb, NULL, 1); + /* + * Skip this CPU if the runqueue's current task is NULL or if + * it is a kernel thread. + */ + if (p && READ_ONCE(p->mm)) { + bool mm_match; + + /* + * Read p->mm and access membarrier_state while holding + * the task lock to ensure existence of mm. + */ + task_lock(p); + mm_match = p->mm && (atomic_read(&p->mm->membarrier_state) & + MEMBARRIER_STATE_GLOBAL_EXPEDITED); + task_unlock(p); + if (mm_match) { + if (!fallback) + __cpumask_set_cpu(cpu, tmpmask); + else + smp_call_function_single(cpu, ipi_mb, NULL, 1); + } } rcu_read_unlock(); } -- 2.17.1
