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>

1 files changed, 6 insertions, 3 deletions

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 75142159b0a5..26aa487ae4ef 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;
 }