diff mbox

[30/39] x86/mm/pti: Clone entry-text again in pti_finalize()

Message ID 1531308586-29340-31-git-send-email-joro@8bytes.org (mailing list archive)
State New, archived
Headers show

Commit Message

Joerg Roedel July 11, 2018, 11:29 a.m. UTC 
From: Joerg Roedel <jroedel@suse.de>

The mapping for entry-text might have changed in the kernel
after it was cloned to the user page-table. Clone again
to update the user page-table to bring the mapping in sync
with the kernel again.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/mm/pti.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

Comments

Andy Lutomirski July 13, 2018, 11:21 p.m. UTC | #1 
On Wed, Jul 11, 2018 at 4:29 AM, Joerg Roedel <joro@8bytes.org> wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> The mapping for entry-text might have changed in the kernel
> after it was cloned to the user page-table. Clone again
> to update the user page-table to bring the mapping in sync
> with the kernel again.

Can't we just defer pti_init() until after mark_readonly()?  What am I missing?
Joerg Roedel July 14, 2018, 5:04 a.m. UTC | #2 
On Fri, Jul 13, 2018 at 04:21:45PM -0700, Andy Lutomirski wrote:
> On Wed, Jul 11, 2018 at 4:29 AM, Joerg Roedel <joro@8bytes.org> wrote:
> > From: Joerg Roedel <jroedel@suse.de>
> >
> > The mapping for entry-text might have changed in the kernel
> > after it was cloned to the user page-table. Clone again
> > to update the user page-table to bring the mapping in sync
> > with the kernel again.
> 
> Can't we just defer pti_init() until after mark_readonly()?  What am I missing?

I tried that:

	https://lore.kernel.org/lkml/1530618746-23116-1-git-send-email-joro@8bytes.org/

But while testing it turned out that the kernel potentially executes
user-space code already before mark_readonly() has ran. This happens
when some initcall requests a module and the initrd is already
populated. Then usermode-helper kicks in and runs a userspace binary
already. When pti_init() has not run yet the user-space page-table is
completly empty, causing a triple fault when we switch to the user cr3
on the way to user-space.


Regards,

	Joerg
diff mbox

Patch

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 1825f30..b879ccd 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -404,7 +404,7 @@  static void __init pti_setup_espfix64(void)
 /*
  * Clone the populated PMDs of the entry and irqentry text and force it RO.
  */
-static void __init pti_clone_entry_text(void)
+static void pti_clone_entry_text(void)
 {
 	pti_clone_pmds((unsigned long) __entry_text_start,
 			(unsigned long) __irqentry_text_end,
@@ -528,13 +528,18 @@  void __init pti_init(void)
 }
 
 /*
- * Finalize the kernel mappings in the userspace page-table.
+ * Finalize the kernel mappings in the userspace page-table. Some of the
+ * mappings for the kernel image might have changed since pti_init()
+ * cloned them. This is because parts of the kernel image have been
+ * mapped RO and/or NX.  These changes need to be cloned again to the
+ * userspace page-table.
  */
 void pti_finalize(void)
 {
 	/*
-	 * Do this after all of the manipulation of the
-	 * kernel text page tables are complete.
+	 * We need to clone everything (again) that maps parts of the
+	 * kernel image.
 	 */
+	pti_clone_entry_text();
 	pti_clone_kernel_text();
 }