[PATCH] mm/slab_common.c: fix possible spectre-v1 in kmalloc_slab()

[Dianzhang Chen]

The `size` in kmalloc_slab() is indirectly controlled by userspace via syscall: poll(defined in fs/select.c), hence leading to a potential exploitation of the Spectre variant 1 vulnerability.
The `size` can be controlled from: poll -> do_sys_poll -> kmalloc -> __kmalloc -> kmalloc_slab.

Fix this by sanitizing `size` before using it to index size_index.

Signed-off-by: Dianzhang Chen <dianzhangchen0@xxxxxxxxx>
---
 mm/slab_common.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 58251ba..41c7e34 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -21,6 +21,7 @@
 #include <asm/tlbflush.h>
 #include <asm/page.h>
 #include <linux/memcontrol.h>
+#include <linux/nospec.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/kmem.h>
@@ -1056,6 +1057,7 @@ struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags)
 		if (!size)
 			return ZERO_SIZE_PTR;
 
+		size = array_index_nospec(size, 193);
 		index = size_index[size_index_elem(size)];
 	} else {
 		if (WARN_ON_ONCE(size > KMALLOC_MAX_CACHE_SIZE))
-- 
2.7.4