[PATCH v3] x86/mm/pti: remove dead logic during user pagetable population

[Jike Song]

Look at one of the code snippets:

 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.

To quote Dave Hansen:

 > 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 it 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.

v3: mark functions __init; add first-hand explanation
v2: add commit message.

Signed-off-by: Jike Song <albcamus@xxxxxxxxx>
Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
Cc: Alan Cox <gnomes@xxxxxxxxxxxxxxxxxxx>
Cc: Jiri Koshina <jikos@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Andi Lutomirski  <luto@xxxxxxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 arch/x86/mm/pti.c | 32 ++++++--------------------------
 1 file changed, 6 insertions(+), 26 deletions(-)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 43d4a4a29037..ce38f165489b 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -149,7 +149,7 @@ pgd_t __pti_set_user_pgd(pgd_t *pgdp, pgd_t pgd)
  *
  * 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_p4d(unsigned long address)
 		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_p4d(unsigned long address)
  *
  * 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_pmd(unsigned long address)
 		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_pmd(unsigned long address)
 		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_walk_pte(unsigned long address)
 		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);
-- 
2.14.3