[PATCH 4.9 52/74] ARM: 8874/1: mm: only adjust sections of valid mm structures

[Greg Kroah-Hartman]

From: Doug Berger <opendmb@xxxxxxxxx>

[ Upstream commit c51bc12d06b3a5494fbfcbd788a8e307932a06e9 ]

A timing hazard exists when an early fork/exec thread begins
exiting and sets its mm pointer to NULL while a separate core
tries to update the section information.

This commit ensures that the mm pointer is not NULL before
setting its section parameters. The arguments provided by
commit 11ce4b33aedc ("ARM: 8672/1: mm: remove tasklist locking
from update_sections_early()") are equally valid for not
requiring grabbing the task_lock around this check.

Fixes: 08925c2f124f ("ARM: 8464/1: Update all mm structures with section adjustments")
Signed-off-by: Doug Berger <opendmb@xxxxxxxxx>
Acked-by: Laura Abbott <labbott@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Florian Fainelli <f.fainelli@xxxxxxxxx>
Cc: Rob Herring <robh@xxxxxxxxxx>
Cc: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
Cc: Peng Fan <peng.fan@xxxxxxx>
Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/arm/mm/init.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 1565d6b671636..4fb1474141a61 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -698,7 +698,8 @@ static void update_sections_early(struct section_perm perms[], int n)
 		if (t->flags & PF_KTHREAD)
 			continue;
 		for_each_thread(t, s)
-			set_section_perms(perms, n, true, s->mm);
+			if (s->mm)
+				set_section_perms(perms, n, true, s->mm);
 	}
 	read_unlock(&tasklist_lock);
 	set_section_perms(perms, n, true, current->active_mm);
-- 
2.20.1