linux-kernel - [PATCH v5 03/32] x86/cpa: In populate_pgd, don't set the pgd entry until it's populated

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Message-Id: <fb3254e25e38790c538cca0f46c4a7a714c77ddf.1468270393.git.luto@kernel.org>
Date:	Mon, 11 Jul 2016 13:53:36 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	linux-arch@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v5 03/32] x86/cpa: In populate_pgd, don't set the pgd entry until it's populated

This avoids pointless races in which another CPU or task might see a
partially populated global pgd entry.  These races should normally
be harmless, but, if another CPU propagates the entry via
vmalloc_fault and then populate_pgd fails (due to memory allocation
failure, for example), this prevents a use-after-free of the pgd
entry.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/mm/pageattr.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 7a1f7bbf4105..6088aa03de63 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -1104,8 +1104,6 @@ static int populate_pgd(struct cpa_data *cpa, unsigned long addr)
 		pud = (pud_t *)get_zeroed_page(GFP_KERNEL | __GFP_NOTRACK);
 		if (!pud)
 			return -1;
-
-		set_pgd(pgd_entry, __pgd(__pa(pud) | _KERNPG_TABLE));
 	}
 
 	pgprot_val(pgprot) &= ~pgprot_val(cpa->mask_clr);
@@ -1113,11 +1111,16 @@ static int populate_pgd(struct cpa_data *cpa, unsigned long addr)
 
 	ret = populate_pud(cpa, addr, pgd_entry, pgprot);
 	if (ret < 0) {
-		unmap_pgd_range(cpa->pgd, addr,
+		if (pud)
+			free_page((unsigned long)pud);
+		unmap_pud_range(pgd_entry, addr,
 				addr + (cpa->numpages << PAGE_SHIFT));
 		return ret;
 	}
 
+	if (pud)
+		set_pgd(pgd_entry, __pgd(__pa(pud) | _KERNPG_TABLE));
+
 	cpa->numpages = ret;
 	return 0;
 }
-- 
2.7.4