diff mbox series

[2/3] x86/mm/pti: Don't clear permissions in pti_clone_pmd()

Message ID 1533637471-30953-3-git-send-email-joro@8bytes.org (mailing list archive)
State New, archived
Headers show
Series PTI for x86-32 Fixes | expand

Commit Message

Joerg Roedel Aug. 7, 2018, 10:24 a.m. UTC 
From: Joerg Roedel <jroedel@suse.de>

The function sets the global-bit on cloned PMD entries,
which only makes sense when the permissions are identical
between the user and the kernel page-table.

Further, only write-permissions are cleared for entry-text
and kernel-text sections, which are not writeable anyway.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/mm/pti.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

Comments

Dave Hansen Aug. 7, 2018, 6:34 p.m. UTC | #1 
On 08/07/2018 03:24 AM, Joerg Roedel wrote:
> The function sets the global-bit on cloned PMD entries,
> which only makes sense when the permissions are identical
> between the user and the kernel page-table.
> 
> Further, only write-permissions are cleared for entry-text
> and kernel-text sections, which are not writeable anyway.

I think this patch is correct, but I'd be curious if Andy remembers why
we chose to clear _PAGE_RW on these things.  It might have been that we
were trying to say that the *entry* code shouldn't write to this stuff,
regardless of whether the normal kernel can.

But, either way, I agree with the logic here that Global pages must
share permissions between both mappings, so feel free to add my Ack.  I
just want to make sure Andy doesn't remember some detail I'm forgetting.
Andy Lutomirski Aug. 7, 2018, 7:38 p.m. UTC | #2 
On Tue, Aug 7, 2018 at 11:34 AM, Dave Hansen <dave.hansen@intel.com> wrote:
> On 08/07/2018 03:24 AM, Joerg Roedel wrote:
>> The function sets the global-bit on cloned PMD entries,
>> which only makes sense when the permissions are identical
>> between the user and the kernel page-table.
>>
>> Further, only write-permissions are cleared for entry-text
>> and kernel-text sections, which are not writeable anyway.
>
> I think this patch is correct, but I'd be curious if Andy remembers why
> we chose to clear _PAGE_RW on these things.  It might have been that we
> were trying to say that the *entry* code shouldn't write to this stuff,
> regardless of whether the normal kernel can.
>
> But, either way, I agree with the logic here that Global pages must
> share permissions between both mappings, so feel free to add my Ack.  I
> just want to make sure Andy doesn't remember some detail I'm forgetting.

I suspect it's because we used to (and maybe still do) initialize the
user tables before mark_read_only().
Thomas Gleixner Aug. 7, 2018, 8:21 p.m. UTC | #3 
On Tue, 7 Aug 2018, Andy Lutomirski wrote:

> On Tue, Aug 7, 2018 at 11:34 AM, Dave Hansen <dave.hansen@intel.com> wrote:
> > On 08/07/2018 03:24 AM, Joerg Roedel wrote:
> >> The function sets the global-bit on cloned PMD entries,
> >> which only makes sense when the permissions are identical
> >> between the user and the kernel page-table.
> >>
> >> Further, only write-permissions are cleared for entry-text
> >> and kernel-text sections, which are not writeable anyway.
> >
> > I think this patch is correct, but I'd be curious if Andy remembers why
> > we chose to clear _PAGE_RW on these things.  It might have been that we
> > were trying to say that the *entry* code shouldn't write to this stuff,
> > regardless of whether the normal kernel can.
> >
> > But, either way, I agree with the logic here that Global pages must
> > share permissions between both mappings, so feel free to add my Ack.  I
> > just want to make sure Andy doesn't remember some detail I'm forgetting.
> 
> I suspect it's because we used to (and maybe still do) initialize the
> user tables before mark_read_only().

We still do that because we need the entry stuff working for interrupts
early on. We now repeat the clone after mark_ro so the mask RW is not
longer required.

Thanks,

	tglx
Andy Lutomirski Aug. 7, 2018, 8:28 p.m. UTC | #4 
On Tue, Aug 7, 2018 at 1:21 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Tue, 7 Aug 2018, Andy Lutomirski wrote:
>
>> On Tue, Aug 7, 2018 at 11:34 AM, Dave Hansen <dave.hansen@intel.com> wrote:
>> > On 08/07/2018 03:24 AM, Joerg Roedel wrote:
>> >> The function sets the global-bit on cloned PMD entries,
>> >> which only makes sense when the permissions are identical
>> >> between the user and the kernel page-table.
>> >>
>> >> Further, only write-permissions are cleared for entry-text
>> >> and kernel-text sections, which are not writeable anyway.
>> >
>> > I think this patch is correct, but I'd be curious if Andy remembers why
>> > we chose to clear _PAGE_RW on these things.  It might have been that we
>> > were trying to say that the *entry* code shouldn't write to this stuff,
>> > regardless of whether the normal kernel can.
>> >
>> > But, either way, I agree with the logic here that Global pages must
>> > share permissions between both mappings, so feel free to add my Ack.  I
>> > just want to make sure Andy doesn't remember some detail I'm forgetting.
>>
>> I suspect it's because we used to (and maybe still do) initialize the
>> user tables before mark_read_only().
>
> We still do that because we need the entry stuff working for interrupts
> early on. We now repeat the clone after mark_ro so the mask RW is not
> longer required.

Agreed.
diff mbox series

Patch

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 113ba14..5164c98 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -291,7 +291,7 @@  static void __init pti_setup_vsyscall(void) { }
 #endif
 
 static void
-pti_clone_pmds(unsigned long start, unsigned long end, pmdval_t clear)
+pti_clone_pmds(unsigned long start, unsigned long end)
 {
 	unsigned long addr;
 
@@ -352,7 +352,7 @@  pti_clone_pmds(unsigned long start, unsigned long end, pmdval_t clear)
 		 * tables will share the last-level page tables of this
 		 * address range
 		 */
-		*target_pmd = pmd_clear_flags(*pmd, clear);
+		*target_pmd = *pmd;
 	}
 }
 
@@ -398,7 +398,7 @@  static void __init pti_clone_user_shared(void)
 	start = CPU_ENTRY_AREA_BASE;
 	end   = start + (PAGE_SIZE * CPU_ENTRY_AREA_PAGES);
 
-	pti_clone_pmds(start, end, 0);
+	pti_clone_pmds(start, end);
 }
 #endif /* CONFIG_X86_64 */
 
@@ -418,8 +418,7 @@  static void __init pti_setup_espfix64(void)
 static void pti_clone_entry_text(void)
 {
 	pti_clone_pmds((unsigned long) __entry_text_start,
-			(unsigned long) __irqentry_text_end,
-		       _PAGE_RW);
+		       (unsigned long) __irqentry_text_end);
 }
 
 /*
@@ -501,7 +500,7 @@  static void pti_clone_kernel_text(void)
 	 * pti_set_kernel_image_nonglobal() did to clear the
 	 * global bit.
 	 */
-	pti_clone_pmds(start, end_clone, _PAGE_RW);
+	pti_clone_pmds(start, end_clone);
 
 	/*
 	 * pti_clone_pmds() will set the global bit in any PMDs