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Patch "x86/mm/pti: Remove dead logic in pti_user_pagetable_walk*()" has been added to the 4.14-stable tree

PATCH [email protected] 1 participant 1 message Jan 15, 2018

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This is a note to let you know that I've just added the patch titled

x86/mm/pti: Remove dead logic in pti_user_pagetable_walk*()

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-mm-pti-remove-dead-logic-in-pti_user_pagetable_walk.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

Quoted 1 line

From 8d56eff266f3e41a6c39926269c4c3f58f881a8e Mon Sep 17 00:00:00 2001

From: Jike Song <[email protected]>
Date: Tue, 9 Jan 2018 00:03:41 +0800
Subject: x86/mm/pti: Remove dead logic in pti_user_pagetable_walk*()

From: Jike Song <[email protected]>

commit 8d56eff266f3e41a6c39926269c4c3f58f881a8e upstream.

The following code contains dead logic:

162 if (pgd_none(*pgd)) {
163 unsigned long new_p4d_page = __get_free_page(gfp);
164 if (!new_p4d_page)
165 return NULL;
166
167 if (pgd_none(*pgd)) {
168 set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
169 new_p4d_page = 0;
170 }
171 if (new_p4d_page)
172 free_page(new_p4d_page);
173 }

There can't be any difference between two pgd_none(*pgd) at L162 and L167, so it's always false at L171.

Dave Hansen explained:

Yes, the double-test was part of an optimization where we attempted to avoid using a global spinlock in the fork() path. We would check for unallocated mid-level page tables without the lock. The lock was only taken when we needed to *make* an entry to avoid collisions.

Now that it is all single-threaded, there is no chance of a collision, no need for a lock, and no need for the re-check.

As all these functions are only called during init, mark them __init as well.

Fixes 03f4424f348e ("x86/mm/pti: Add functions to clone kernel PMDs")
Signed-off-by Jike Song <[email protected]>
Signed-off-by Thomas Gleixner <[email protected]>
Cc Alan Cox <[email protected]>
Cc Andi Kleen <[email protected]>
Cc Tom Lendacky <[email protected]>
Cc Peter Zijlstra <[email protected]>
Cc Tim Chen <[email protected]>
Cc Jiri Koshina <[email protected]>
Cc Dave Hansen <[email protected]>
Cc Borislav Petkov <[email protected]>
Cc Kees Cook <[email protected]>
Cc Andi Lutomirski <[email protected]>
Cc Linus Torvalds <[email protected]>
Cc Greg KH <[email protected]>
Cc David Woodhouse <[email protected]>
Cc Paul Turner <[email protected]>
Link https://lkml.kernel.org/r/[email protected] open here
Signed-off-by Greg Kroah-Hartman <[email protected]>
---
 arch/x86/mm/pti.c |   32 ++++++--------------------------
 1 file changed, 6 insertions(+), 26 deletions(-)
arch/x86/mm/pti.c +6 -26
@@ -149,7 +149,7 @@ pgd_t __pti_set_user_pgd(pgd_t *pgdp, pg
  *
  * Returns a pointer to a P4D on success, or NULL on failure.
  */
-static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
+static __init p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
 {
 	pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));
 	gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);

@@ -164,12 +164,7 @@ static p4d_t *pti_user_pagetable_walk_p4
 		if (!new_p4d_page)
 			return NULL;
 
-		if (pgd_none(*pgd)) {
-			set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
-			new_p4d_page = 0;
-		}
-		if (new_p4d_page)
-			free_page(new_p4d_page);
+		set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
 	}
 	BUILD_BUG_ON(pgd_large(*pgd) != 0);
 

@@ -182,7 +177,7 @@ static p4d_t *pti_user_pagetable_walk_p4
  *
  * Returns a pointer to a PMD on success, or NULL on failure.
  */
-static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
+static __init pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
 {
 	gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
 	p4d_t *p4d = pti_user_pagetable_walk_p4d(address);

@@ -194,12 +189,7 @@ static pmd_t *pti_user_pagetable_walk_pm
 		if (!new_pud_page)
 			return NULL;
 
-		if (p4d_none(*p4d)) {
-			set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
-			new_pud_page = 0;
-		}
-		if (new_pud_page)
-			free_page(new_pud_page);
+		set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
 	}
 
 	pud = pud_offset(p4d, address);

@@ -213,12 +203,7 @@ static pmd_t *pti_user_pagetable_walk_pm
 		if (!new_pmd_page)
 			return NULL;
 
-		if (pud_none(*pud)) {
-			set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
-			new_pmd_page = 0;
-		}
-		if (new_pmd_page)
-			free_page(new_pmd_page);
+		set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
 	}
 
 	return pmd_offset(pud, address);

@@ -251,12 +236,7 @@ static __init pte_t *pti_user_pagetable_
 		if (!new_pte_page)
 			return NULL;
 
-		if (pmd_none(*pmd)) {
-			set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
-			new_pte_page = 0;
-		}
-		if (new_pte_page)
-			free_page(new_pte_page);
+		set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
 	}
 
 	pte = pte_offset_kernel(pmd, address);

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/x86-mm-pti-remove-dead-logic-in-pti_user_pagetable_walk.patch