[PATCH net-next v2 0/2] net: isolate SKB data area allocations

[PATCH net-next v2 0/2] net: isolate SKB data area allocations

[Pedro Falcato]

This is a rather simple series that attempts to address a possible
exploitation avenue - the allocation of skbs around the network stack, which
frequently get user-controlled contents. Found while doing some amateur
exploitation analysis for some other issue, elsewhere.

Patch 0 is a precursor patch that adds a slab allocation helper, patch 1
does the actual bucketing.

v2:
 - Pick up A-b, R-b tags (thanks everyone!)
 - Cover kmalloc_pfmemalloc as well (Harry)

Pedro Falcato (2):
  mm/slab: add a node-track-caller variant for kmem buckets allocation
  net: skb: isolate skb data area allocations into a separate bucket

 include/linux/slab.h | 7 +++++--
 net/core/skbuff.c    | 8 ++++++--
 2 files changed, 11 insertions(+), 4 deletions(-)

-- 
2.54.0

[PATCH net-next v2 1/2] mm/slab: add a node-track-caller variant for kmem buckets allocation

[Pedro Falcato]

This is required by users that want to use kmem buckets, but still
desire specifying the NUMA node.

Reviewed-by: Kees Cook <kees@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Acked-by: Harry Yoo (Oracle) <harry@kernel.org>
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
---
 include/linux/slab.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 7b46fa499b08..685a87d8f0c5 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -1153,8 +1153,11 @@ void *kmalloc_nolock(size_t size, gfp_t gfp_flags, int node);
 #define kmem_buckets_alloc(_b, _size, _flags)	\
 	alloc_hooks(__kmalloc_node_noprof(PASS_KMALLOC_PARAMS(_size, _b, __kmalloc_token(_size)), _flags, NUMA_NO_NODE))
 
-#define kmem_buckets_alloc_track_caller(_b, _size, _flags)	\
-	alloc_hooks(__kmalloc_node_track_caller_noprof(PASS_KMALLOC_PARAMS(_size, _b, __kmalloc_token(_size)), _flags, NUMA_NO_NODE, _RET_IP_))
+#define kmem_buckets_alloc_node_track_caller(_b, _size, _flags, _node)	\
+	alloc_hooks(__kmalloc_node_track_caller_noprof(PASS_KMALLOC_PARAMS(_size, _b, __kmalloc_token(_size)), _flags, _node, _RET_IP_))
+
+#define kmem_buckets_alloc_track_caller(_b, _size, _flags) \
+	kmem_buckets_alloc_node_track_caller(_b, _size, _flags, NUMA_NO_NODE)
 
 static __always_inline __alloc_size(1) void *_kmalloc_node_noprof(size_t size, gfp_t flags, int node, kmalloc_token_t token)
 {
-- 
2.54.0

[PATCH net-next v2 2/2] net: skb: isolate skb data area allocations into a separate bucket

[Pedro Falcato]

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]
Reviewed-by: Kees Cook <kees@kernel.org>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
---
 net/core/skbuff.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 44a7f8401468..ac9fed7461ba 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -584,6 +584,8 @@ 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))
@@ -591,7 +593,8 @@ 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);
 }
 
 /*
@@ -632,7 +635,7 @@ 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 +5216,7 @@ 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

Re: [PATCH net-next v2 0/2] net: isolate SKB data area allocations

[Jakub Kicinski]

On Thu, 11 Jun 2026 13:46:40 +0100 Pedro Falcato wrote:
> Subject: [PATCH net-next v2 0/2] net: isolate SKB data area allocations

This doesn't apply to net-next, does patch 2 not apply to mm?
If neither tree can take both - maybe MM can take the first patch by
itself and we will queue patch 2 after the changes propagate during 
the merge window?