From: Pedro Falcato <[email protected]> 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. AF_UNIX sets sk_allocation to GFP_KERNEL_ACCOUNT, and those skb data areas, the ones most worth isolating, stay in the set, where memcg charges them as it would in the general caches. GFP_DMA falls back to the general caches, 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 <[email protected]> Signed-off-by: Pedro Falcato <[email protected]> Acked-by: Paolo Abeni <[email protected]> Signed-off-by: Kees Cook <[email protected]> --- net/core/skbuff.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/net/core/skbuff.c b/net/core/skbuff.c index 966af3beed94..6f6b5f4cb39f 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)) @@ -593,11 +595,12 @@ 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); } /* - * kmalloc_reserve is a wrapper around kmalloc_node_track_caller that tells + * kmalloc_reserve is a wrapper around a caller-tracked kmalloc that tells * the caller if emergency pfmemalloc reserves are being used. If it is and * the socket is later found to be SOCK_MEMALLOC then PFMEMALLOC reserves * may be used. Otherwise, the packet data may be discarded until enough @@ -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 +5238,7 @@ void __init skb_init(void) 0, SKB_SMALL_HEAD_HEADROOM, NULL); + skb_data_buckets = kmem_buckets_create("skb_data", 0, INT_MAX); skb_extensions_init(); } -- 2.55.0
