linux-kernel - [PATCH v2] x86/mm/pti: remove dead logic during user pagetable population

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Message-Id: <20180107103317.22613-1-albcamus@gmail.com>
Date:   Sun,  7 Jan 2018 18:33:17 +0800
From:   Jike Song <albcamus@...il.com>
To:     tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org, Jike Song <albcamus@...il.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Koshina <jikos@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Lutomirski <luto@...capital.net>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul Turner <pjt@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Greg KH <gregkh@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Kees Cook <keescook@...gle.com>, stable@...r.kernel.org
Subject: [PATCH v2] x86/mm/pti: remove dead logic during user pagetable population

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.

v2: add the commit message above.

Signed-off-by: Jike Song <albcamus@...il.com>
Cc: David Woodhouse <dwmw@...zon.co.uk>
Cc: Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc: Jiri Koshina <jikos@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Andi Lutomirski  <luto@...capital.net>
Cc: Andi Kleen <ak@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Paul Turner <pjt@...gle.com>
Cc: Tom Lendacky <thomas.lendacky@....com>
Cc: Greg KH <gregkh@...ux-foundation.org>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Kees Cook <keescook@...gle.com>
Cc: stable@...r.kernel.org
Signed-off-by: Jike Song <albcamus@...il.com>
---
 arch/x86/mm/pti.c | 28 ++++------------------------
 1 file changed, 4 insertions(+), 24 deletions(-)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index 43d4a4a29037..dc611d039bd5 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -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);
 
@@ -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