[RFC PATCH v7 13/16] xpfo, mm: optimize spinlock usage in xpfo_kunmap

[Khalid Aziz <khalid.aziz@oracle.com>]

From: Julian Stecklina <jsteckli@amazon.de>

Only the xpfo_kunmap call that needs to actually unmap the page
needs to be serialized. We need to be careful to handle the case,
where after the atomic decrement of the mapcount, a xpfo_kmap
increased the mapcount again. In this case, we can safely skip
modifying the page table.

Model-checked with up to 4 concurrent callers with Spin.

Signed-off-by: Julian Stecklina <jsteckli@amazon.de>
Cc: x86@kernel.org
Cc: kernel-hardening@lists.openwall.com
Cc: Vasileios P. Kemerlis <vpk@cs.columbia.edu>
Cc: Juerg Haefliger <juerg.haefliger@canonical.com>
Cc: Tycho Andersen <tycho@docker.com>
Cc: Marco Benatto <marco.antonio.780@gmail.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
---
 mm/xpfo.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/mm/xpfo.c b/mm/xpfo.c
index cbfeafc2f10f..dbf20efb0499 100644
--- a/mm/xpfo.c
+++ b/mm/xpfo.c
@@ -149,22 +149,24 @@ void xpfo_kunmap(void *kaddr, struct page *page)
 	if (!PageXpfoUser(page))
 		return;
 
-	spin_lock(&page->xpfo_lock);
-
 	/*
 	 * The page is to be allocated back to user space, so unmap it from the
 	 * kernel, flush the TLB and tag it as a user page.
 	 */
 	if (atomic_dec_return(&page->xpfo_mapcount) == 0) {
-#ifdef CONFIG_XPFO_DEBUG
-		BUG_ON(PageXpfoUnmapped(page));
-#endif
-		SetPageXpfoUnmapped(page);
-		set_kpte(kaddr, page, __pgprot(0));
-		xpfo_cond_flush_kernel_tlb(page, 0);
-	}
+		spin_lock(&page->xpfo_lock);
 
-	spin_unlock(&page->xpfo_lock);
+		/*
+		 * In the case, where we raced with kmap after the
+		 * atomic_dec_return, we must not nuke the mapping.
+		 */
+		if (atomic_read(&page->xpfo_mapcount) == 0) {
+			SetPageXpfoUnmapped(page);
+			set_kpte(kaddr, page, __pgprot(0));
+			xpfo_cond_flush_kernel_tlb(page, 0);
+		}
+		spin_unlock(&page->xpfo_lock);
+	}
 }
 EXPORT_SYMBOL(xpfo_kunmap);
 
-- 
2.17.1