Subject: [PATCH] ARM: mm: only adjust sections of valid mm structures

From: Doug Berger <[email-masked]>
Date: Thu, 27 Jun 2019 14:32:48 -0700

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 <[email-masked]>
---
 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 be0b42937888..bdc70dff477b 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -616,7 +616,8 @@ 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);
 	}
 	set_section_perms(perms, n, true, current->active_mm);
 	set_section_perms(perms, n, true, &init_mm);
-- 
2.7.4

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