On Nov 27, 2018, at 4:07 PM, Rick Edgecombe [off-list ref] wrote:

Since vfree will lazily flush the TLB, but not lazily free the underlying pages,
it often leaves stale TLB entries to freed pages that could get re-used. This is
undesirable for cases where the memory being freed has special permissions such
as executable.

So I am trying to finish my patch-set for preventing transient W+X mappings
from taking space, by handling kprobes & ftrace that I missed (thanks again for
pointing it out).

But all of the sudden, I don’t understand why we have the problem that this
(your) patch-set deals with at all. We already change the mappings to make
the memory writable before freeing the memory, so why can’t we make it
non-executable at the same time? Actually, why do we make the module memory,
including its data executable before freeing it???

In other words: disable_ro_nx() is called by free_module() before freeing
the memory. Wouldn’t inverting the logic makes much more sense? I am
confused.

-- >8 --

From: Nadav Amit <redacted>
Subject: [PATCH] modules: disable_ro_nx() should enable nx 

---
 kernel/module.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 7cb207249437..e12d760ea3b0 100644
--- a/kernel/module.c
+++ b/kernel/module.c

@@ -2029,14 +2029,13 @@ void set_all_modules_text_ro(void)
 
 static void disable_ro_nx(const struct module_layout *layout)
 {
+	frob_text(layout, set_memory_nx);
+
 	if (rodata_enabled) {
 		frob_text(layout, set_memory_rw);
 		frob_rodata(layout, set_memory_rw);
 		frob_ro_after_init(layout, set_memory_rw);
 	}
-	frob_rodata(layout, set_memory_x);
-	frob_ro_after_init(layout, set_memory_x);
-	frob_writable_data(layout, set_memory_x);
 }
 
 #else

-- 
2.17.1