ARM: 8780/1: ftrace: Only set kernel memory back to read-only after boot

rostedt · Russell King · commit b4c7e2bd2eb4 · 2018-07-11T22:57:57.000+01:00
Dynamic ftrace requires modifying the code segments that are usually set to read-only. To do this, a per arch function is called both before and after the ftrace modifications are performed. The "before" function will set kernel code text to read-write to allow for ftrace to make the modifications, and the "after" function will set the kernel code text back to "read-only" to keep the kernel code text protected. The issue happens when dynamic ftrace is tested at boot up. The test is done before the kernel code text has been set to read-only. But the "before" and "after" calls are still performed. The "after" call will change the kernel code text to read-only prematurely, and other boot code that expects this code to be read-write will fail. The solution is to add a variable that is set when the kernel code text is expected to be converted to read-only, and make the ftrace "before" and "after" calls do nothing if that variable is not yet set. This is similar to the x86 solution from commit 1623963 ("ftrace, x86: make kernel text writable only for conversions"). Link: http://lkml.kernel.org/r/20180620212906.24b7b66e@vmware.local.home Reported-by: Stefan Agner <stefan@agner.ch> Tested-by: Stefan Agner <stefan@agner.ch> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org> Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
@@ -736,20 +736,29 @@ static int __mark_rodata_ro(void *unused)
 	return 0;
 }
 
+static int kernel_set_to_readonly __read_mostly;
+
 void mark_rodata_ro(void)
 {
+	kernel_set_to_readonly = 1;
 	stop_machine(__mark_rodata_ro, NULL, NULL);
 	debug_checkwx();
 }
 
 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);
 }

Original file line number	Diff line number	Diff line change
`@@ -736,20 +736,29 @@ static int __mark_rodata_ro(void *unused)`
`736`	`736`	`return 0;`
`737`	`737`	`}`
`738`	`738`
	`739`	`+static int kernel_set_to_readonly __read_mostly;`
	`740`	`+`
`739`	`741`	`void mark_rodata_ro(void)`
`740`	`742`	`{`
	`743`	`+ kernel_set_to_readonly = 1;`
`741`	`744`	`stop_machine(__mark_rodata_ro, NULL, NULL);`
`742`	`745`	`debug_checkwx();`
`743`	`746`	`}`
`744`	`747`
`745`	`748`	`void set_kernel_text_rw(void)`
`746`	`749`	`{`
	`750`	`+ if (!kernel_set_to_readonly)`
	`751`	`+ return;`
	`752`	`+`
`747`	`753`	`set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), false,`
`748`	`754`	`current->active_mm);`
`749`	`755`	`}`
`750`	`756`
`751`	`757`	`void set_kernel_text_ro(void)`
`752`	`758`	`{`
	`759`	`+ if (!kernel_set_to_readonly)`
	`760`	`+ return;`
	`761`	`+`
`753`	`762`	`set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), true,`
`754`	`763`	`current->active_mm);`
`755`	`764`	`}`