[v5,03/32] x86/cpa: In populate_pgd, don't set the pgd entry until it's populated
diff mbox

Message ID fb3254e25e38790c538cca0f46c4a7a714c77ddf.1468270393.git.luto@kernel.org
State New
Headers show

Commit Message

Andy Lutomirski July 11, 2016, 8:53 p.m. UTC
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>
---
 arch/x86/mm/pageattr.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Comments

Valdis Kl ē tnieks July 22, 2016, 4:43 a.m. UTC | #1
On Mon, 11 Jul 2016 13:53:36 -0700, Andy Lutomirski said:
> 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>
> ---
>  arch/x86/mm/pageattr.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)

I just bisected a failure to boot down to this patch.  On my Dell Latitude
laptop, it results in the kernel being loaded and then just basically sitting
there dead in the water - as far as I can tell, it dies before the kernel
ever gets going far enough to do any console I/O (even with ignore_loglevel).
Nothing in /sys/fs/pstore either.  I admit not understanding the VM code
at all, so I don't have a clue *why* this causes indigestion...

CPU is an Intel Core i5-3340M in case that matters....

Patch
diff mbox

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;
 }