[PATCH net-next v5 7/7] net: skb: isolate skb data area allocations into a separate bucket

[Kees Cook <kees@kernel.org>]

From: 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.

Ask for the accounted kmalloc type as well as the normal one. AF_UNIX
sets sk_allocation to GFP_KERNEL_ACCOUNT, so without it every AF_UNIX
skb data area would fall back to the general caches, and those are the
ones most worth isolating. GFP_DMA is left to fall back, being passed to
an skb allocator only by rare devices.

Link: https://github.com/google/security-research/blob/master/pocs/linux/kernelctf/CVE-2023-4207_lts_cos_mitigation_2/docs/exploit.md [0]
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Kees Cook <kees@kernel.org>
---
 net/core/skbuff.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 966af3beed94..e0660b2dbc19 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -586,6 +586,8 @@ struct sk_buff *napi_build_skb(void *data, unsigned int frag_size)
 }
 EXPORT_SYMBOL(napi_build_skb);
 
+static kmem_buckets *skb_data_buckets __ro_after_init;
+
 static void *kmalloc_pfmemalloc(size_t obj_size, gfp_t flags, int node)
 {
 	if (!gfp_pfmemalloc_allowed(flags))
@@ -593,7 +595,8 @@ static void *kmalloc_pfmemalloc(size_t obj_size, gfp_t flags, int node)
 	if (!obj_size)
 		return kmem_cache_alloc_node(net_hotdata.skb_small_head_cache,
 					     flags, node);
-	return kmalloc_node_track_caller(obj_size, flags, node);
+	return kmem_buckets_alloc_node_track_caller(skb_data_buckets, obj_size,
+						    flags, node);
 }
 
 /*
@@ -634,7 +637,7 @@ 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))
@@ -5235,6 +5238,10 @@ void __init skb_init(void)
 						0,
 						SKB_SMALL_HEAD_HEADROOM,
 						NULL);
+	skb_data_buckets = kmem_buckets_create_types("skb_data", 0, SLAB_PANIC,
+						     0, INT_MAX, NULL,
+						     BIT(KMEM_BUCKET_NORMAL) |
+						     BIT(KMEM_BUCKET_CGROUP));
 	skb_extensions_init();
 }
 
-- 
2.34.1