[PATCH] Arm: mm: ftrace: Only set text back to ro after kernel has been marked ro

[Steven Rostedt]

ftrace needs to modify the kernel text in order to enable function tracing.
For security reasons, the kernel text is marked to read-only (ro) at the end
of system bootup. When enabling function tracing after that, ftrace calls
arch specific code that needs to enable the modification of kernel text
while ftrace does the update, and reset it back again when finished.

The issue arises when function tracing is enabled during system bootup. The
text hasn't been marked as read-only yet, but the same code to modify the
kernel is executed, and when it is finished, it will cause the kernel to
become read-only. This causes issues for other init code that requires
modification of kernel text during system bootup. This appears to cause
issue with Raspberry Pi 2.

By implementing the feature that is used in x86 to deal with this issue, it
fixes the problem. The solution is simple. Have a variable
(kernel_set_to_readonly) get set when the system finished boot and marks the
kernel to readonly. If that variable is not set, both
kernel_set_to_readonly() and kernel_set_to_rw() return without doing any
modifications. Those functions are used by ftrace to change the permissions
of the kernel text. By not doing anything, ftrace will not mess with the
permissions when it is enabled at system bootup.

Link: http://lkml.kernel.org/r/20170821153402.7so2u364htvt6tnf@camel2.lan
Link: https://github.com/raspberrypi/linux/issues/2166#issuecomment-323355145
Reported-by: Matthias Reichl <hias at horus.com>
Cc: Russell King <linux at armlinux.org.uk>
Cc: Kees Cook <keescook at chromium.org>
Cc: Eric Anholt <eric at anholt.net>
Cc: Stefan Wahren <stefan.wahren at i2se.com>
Cc: Phil Elwell <phil at raspberrypi.org>
Cc: linux-rpi-kernel at lists.infradead.org
Cc: linux-arm-kernel at lists.infradead.org
Cc: stable at vger.kernel.org
Fixes: 80d6b0c2ee ("ARM: mm: allow text and rodata sections to be read-only")
Signed-off-by: Steven Rostedt (VMware) <rostedt at goodmis.org>
---
 arch/arm/mm/init.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index ad80548..fd75f38 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -745,19 +745,29 @@ static int __mark_rodata_ro(void *unused)
 	return 0;
 }
 
+static int kernel_set_to_readonly;
+
 void mark_rodata_ro(void)
 {
+	kernel_set_to_readonly = 1;
+
 	stop_machine(__mark_rodata_ro, NULL, NULL);
 }
 
 void set_kernel_text_rw(void)
 {
+	if (!kernel_set_to_readonly)
+		return;
+
 	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), false,
 				current->active_mm);
 }
 
 void set_kernel_text_ro(void)
 {
+	if (!kernel_set_to_readonly)
+		return;
+
 	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), true,
 				current->active_mm);
 }
-- 
2.9.5