[MODERATED] [PATCH v6 17/43] MDSv6

[Andi Kleen <andi@firstfloor.org>]

Assume that any code using these functions is sensitive and shouldn't
leak any data.

This handles clearing for key and cryptographic data used in the kernel.

It also schedules a clear after each get_random_bytes, which while
being slightly overkill, may protect some hash table key secrets.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
 lib/string.c     | 6 ++++++
 mm/slab_common.c | 5 ++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/lib/string.c b/lib/string.c
index 38e4ca08e757..9ce59dd86541 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -28,6 +28,7 @@
 #include <linux/bug.h>
 #include <linux/errno.h>
 #include <linux/slab.h>
+#include <linux/clearcpu.h>
 
 #include <asm/byteorder.h>
 #include <asm/word-at-a-time.h>
@@ -715,12 +716,17 @@ EXPORT_SYMBOL(memset);
  * necessary, memzero_explicit() should be used instead in
  * order to prevent the compiler from optimising away zeroing.
  *
+ * As a side effect this may also trigger extra cleaning
+ * of CPU state before the next kernel exit to avoid
+ * side channels.
+ *
  * memzero_explicit() doesn't need an arch-specific version as
  * it just invokes the one of memset() implicitly.
  */
 void memzero_explicit(void *s, size_t count)
 {
 	memset(s, 0, count);
+	lazy_clear_cpu();
 	barrier_data(s);
 }
 EXPORT_SYMBOL(memzero_explicit);
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 81732d05e74a..7b5e2e1318a2 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1576,6 +1576,9 @@ EXPORT_SYMBOL(krealloc);
  * Note: this function zeroes the whole allocated buffer which can be a good
  * deal bigger than the requested buffer size passed to kmalloc(). So be
  * careful when using this function in performance sensitive code.
+ *
+ * As a side effect this may also clear CPU state later before the
+ * next kernel exit to avoid side channels.
  */
 void kzfree(const void *p)
 {
@@ -1585,7 +1588,7 @@ void kzfree(const void *p)
 	if (unlikely(ZERO_OR_NULL_PTR(mem)))
 		return;
 	ks = ksize(mem);
-	memset(mem, 0, ks);
+	memzero_explicit(mem, ks);
 	kfree(mem);
 }
 EXPORT_SYMBOL(kzfree);
-- 
2.17.2