From: Julian Stecklina <jsteckli@xxxxxxxxx>
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@xxxxxxxxx>
Signed-off-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
Cc: Khalid Aziz <khalid@xxxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
Cc: Vasileios P. Kemerlis <vpk@xxxxxxxxxxxxxxx>
Cc: Juerg Haefliger <juerg.haefliger@xxxxxxxxxxxxx>
Cc: Tycho Andersen <tycho@xxxxxxxx>
Cc: Marco Benatto <marco.antonio.780@xxxxxxxxx>
Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
---
include/linux/xpfo.h | 24 +++++++++++++++---------
1 file changed, 15 insertions(+), 9 deletions(-)
diff --git a/include/linux/xpfo.h b/include/linux/xpfo.h
index 2318c7eb5fb7..37e7f52fa6ce 100644
--- a/include/linux/xpfo.h
+++ b/include/linux/xpfo.h
@@ -61,6 +61,7 @@ static inline void xpfo_kmap(void *kaddr, struct page *page)
static inline void xpfo_kunmap(void *kaddr, struct page *page)
{
unsigned long flags;
+ bool flush_tlb = false;
if (!static_branch_unlikely(&xpfo_inited))
return;
@@ -72,18 +73,23 @@ static inline void xpfo_kunmap(void *kaddr, struct page *page)
* 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.
*/
- spin_lock_irqsave(&page->xpfo_lock, flags);
-
if (atomic_dec_return(&page->xpfo_mapcount) == 0) {
-#ifdef CONFIG_XPFO_DEBUG
- WARN_ON(PageXpfoUnmapped(page));
-#endif
- SetPageXpfoUnmapped(page);
- set_kpte(kaddr, page, __pgprot(0));
- xpfo_flush_kernel_tlb(page, 0);
+ spin_lock_irqsave(&page->xpfo_lock, flags);
+
+ /*
+ * 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));
+ flush_tlb = true;
+ }
+ spin_unlock_irqrestore(&page->xpfo_lock, flags);
}
- spin_unlock_irqrestore(&page->xpfo_lock, flags);
+ if (flush_tlb)
+ xpfo_flush_kernel_tlb(page, 0);
}
void xpfo_alloc_pages(struct page *page, int order, gfp_t gfp, bool will_map);
--
2.17.1
