[tip:x86/urgent] x86/smpboot: Prevent false positive out of bounds cpumask access warning

[tip-bot for Thomas Gleixner]

Commit-ID:  427d77a32365d5f942d335248305a5c237baf63a
Gitweb:     http://git.kernel.org/tip/427d77a32365d5f942d335248305a5c237baf63a
Author:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Tue, 13 Dec 2016 19:32:28 +0100
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Thu, 15 Dec 2016 11:32:31 +0100

x86/smpboot: Prevent false positive out of bounds cpumask access warning

prefill_possible_map() reinitializes the cpu_possible_map by setting the
possible cpu bits and clearing all other bits up to NR_CPUS.

This is technically always correct because cpu_possible_map is statically
allocated and sized NR_CPUS. With CPUMASK_OFFSTACK and DEBUG_PER_CPU_MAPS
enabled the bounds check of cpu masks happens on nr_cpu_ids. nr_cpu_ids is
initialized to NR_CPUS and only limited after the set/clear bit loops have
been executed. 

But if the system was booted with "nr_cpus=N" on the command line, where N
is < NR_CPUS then nr_cpu_ids is limited in the parameter parsing function
before prefill_possible_map() is invoked. As a consequence the cpumask
bounds check triggers when clearing the bits past nr_cpu_ids.

Add a helper which allows to reset cpu_possible_map w/o the bounds check
and then set only the possible bits which are well inside bounds.

Reported-by: Dmitry Safonov <dsafonov@xxxxxxxxxxxxx>
Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Cc: 0x7f454c46@xxxxxxxxx
Cc: Jan Beulich <JBeulich@xxxxxxxxxx>
Link: alpine.DEB.2.20.1612131836050.3415@nanos">http://lkml.kernel.org/r/alpine.DEB.2.20.1612131836050.3415@nanos
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 arch/x86/kernel/smpboot.c | 8 ++++----
 include/linux/cpumask.h   | 5 +++++
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index e09aa58..46732dc 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1463,15 +1463,15 @@ __init void prefill_possible_map(void)
 		possible = i;
 	}
 
+	nr_cpu_ids = possible;
+
 	pr_info("Allowing %d CPUs, %d hotplug CPUs\n",
 		possible, max_t(int, possible - num_processors, 0));
 
+	reset_cpu_possible_mask();
+
 	for (i = 0; i < possible; i++)
 		set_cpu_possible(i, true);
-	for (; i < NR_CPUS; i++)
-		set_cpu_possible(i, false);
-
-	nr_cpu_ids = possible;
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index da7fbf1..c717f5e 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -722,6 +722,11 @@ void init_cpu_present(const struct cpumask *src);
 void init_cpu_possible(const struct cpumask *src);
 void init_cpu_online(const struct cpumask *src);
 
+static inline void reset_cpu_possible_mask(void)
+{
+	bitmap_zero(cpumask_bits(&__cpu_possible_mask), NR_CPUS);
+}
+
 static inline void
 set_cpu_possible(unsigned int cpu, bool possible)
 {