[tip:x86/mm] x86/mm/cpa: In populate_pgd(), don't set the PGD entry until it's populated

[tip-bot for Andy Lutomirski <tipbot@zytor.com>]

Commit-ID:  360cb4d15567a7eca07a5f3ade6de308bbfb4e70
Gitweb:     http://git.kernel.org/tip/360cb4d15567a7eca07a5f3ade6de308bbfb4e70
Author:     Andy Lutomirski <luto@kernel.org>
AuthorDate: Thu, 14 Jul 2016 13:22:50 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 15 Jul 2016 10:26:25 +0200

x86/mm/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@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/bf99df27eac6835f687005364bd1fbd89130946c.1468527351.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.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 7514215..26aa487 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;
 }