[PATCH v2 2/2] slab: Achieve better kmalloc caches randomization in kvmalloc

[GONG Ruiqi]

As revealed by this writeup[1], due to the fact that __kmalloc_node (now
renamed to __kmalloc_node_noprof) is an exported symbol and will never
get inlined, using it in kvmalloc_node (now is __kvmalloc_node_noprof)
would make the RET_IP inside always point to the same address:

    upper_caller
        kvmalloc
        kvmalloc_node
        kvmalloc_node_noprof
        __kvmalloc_node_noprof	<-- all macros all the way down here
            __kmalloc_node_noprof
                __do_kmalloc_node(.., _RET_IP_)
            ...			<-- _RET_IP_ points to

That literally means all kmalloc invoked via kvmalloc would use the same
seed for cache randomization (CONFIG_RANDOM_KMALLOC_CACHES), which makes
this hardening unfunctional.

The root cause of this problem, IMHO, is that using RET_IP only cannot
identify the actual allocation site in case of kmalloc being called
inside wrappers or helper functions. And I believe there could be
similar cases in other functions. Nevertheless, I haven't thought of
any good solution for this. So for now let's solve this specific case
first.

For __kvmalloc_node_noprof, replace __kmalloc_node_noprof and call
__do_kmalloc_node directly instead, so that RET_IP can take the return
address of kvmalloc and differentiate each kvmalloc invocation:

    upper_caller
        kvmalloc
        kvmalloc_node
        kvmalloc_node_noprof
        __kvmalloc_node_noprof	<-- all macros all the way down here
            __do_kmalloc_node(.., _RET_IP_)
        ...			<-- _RET_IP_ points to

Thanks to Tamás Koczka for the report and discussion!

Link: https://github.com/google/security-research/pull/83/files#diff-1604319b55a48c39a210ee52034ed7ff5b9cdc3d704d2d9e34eb230d19fae235R200 [1]
Reported-by: Tamás Koczka <poprdi@xxxxxxxxxx>
Signed-off-by: GONG Ruiqi <gongruiqi1@xxxxxxxxxx>
---
 mm/slub.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 0830894bb92c..46e884b77dca 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4903,9 +4903,9 @@ void *__kvmalloc_node_noprof(DECL_BUCKET_PARAMS(size, b), gfp_t flags, int node)
 	 * It doesn't really make sense to fallback to vmalloc for sub page
 	 * requests
 	 */
-	ret = __kmalloc_node_noprof(PASS_BUCKET_PARAMS(size, b),
-				    kmalloc_gfp_adjust(flags, size),
-				    node);
+	ret = __do_kmalloc_node(size, PASS_BUCKET_PARAM(b),
+				kmalloc_gfp_adjust(flags, size),
+				node, _RET_IP_);
 	if (ret || size <= PAGE_SIZE)
 		return ret;
 
-- 
2.25.1