[PATCH 2/2] net: skb: isolate skb data area allocations into a separate bucket

[Pedro Falcato <pfalcato@suse.de>]

SKB data area allocations (as done from alloc_skb()) use kmalloc().
These allocations can be variably sized and their contents can be more
or less controlled from userspace, which makes them useful for attackers
that want to overwrite a use-after-free'd object from the same kmalloc slab
(which often just requires the sizes to roughly match into the same kmalloc
bucket). [0] is an easy example of an exploit that uses netlink skb
allocation to target another similarly-sized accidentally freed object.

While other mitigations like CONFIG_RANDOM_KMALLOC_CACHES exist, these are
probabilistic. Use the existing kmem buckets API to further isolate these
allocations in a guaranteed fashion, when CONFIG_SLAB_BUCKETS=y.

Link: https://github.com/google/security-research/blob/master/pocs/linux/kernelctf/CVE-2023-4207_lts_cos_mitigation_2/docs/exploit.md [0]
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
---
 net/core/skbuff.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 44a7f8401468..1f6c6b531ece 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -594,6 +594,8 @@ static void *kmalloc_pfmemalloc(size_t obj_size, gfp_t flags, int node)
 	return kmalloc_node_track_caller(obj_size, flags, node);
 }
 
+static kmem_buckets *skb_data_buckets __ro_after_init;
+
 /*
  * kmalloc_reserve is a wrapper around kmalloc_node_track_caller that tells
  * the caller if emergency pfmemalloc reserves are being used. If it is and
@@ -632,7 +634,7 @@ static void *kmalloc_reserve(unsigned int *size, gfp_t flags, int node,
 	 * Try a regular allocation, when that fails and we're not entitled
 	 * to the reserves, fail.
 	 */
-	obj = kmalloc_node_track_caller(obj_size,
+	obj = kmem_buckets_alloc_node_track_caller(skb_data_buckets, obj_size,
 					flags | __GFP_NOMEMALLOC | __GFP_NOWARN,
 					node);
 	if (likely(obj))
@@ -5213,6 +5215,7 @@ void __init skb_init(void)
 						0,
 						SKB_SMALL_HEAD_HEADROOM,
 						NULL);
+	skb_data_buckets = kmem_buckets_create("skb_data", SLAB_PANIC, 0, INT_MAX, NULL);
 	skb_extensions_init();
 }
 
-- 
2.54.0