[PATCH v4 04/29] x86/cpa: In populate_pgd, don't set the pgd entry until it's populated

[Andy Lutomirski]

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@xxxxxxxxxx>
---
 arch/x86/mm/pageattr.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 7a1f7bbf4105..6a8026918bf6 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -1113,7 +1113,9 @@ 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;
 	}
-- 
2.7.4