[PATCH] skbuff: Reallocate to ksize() in __build_skb_around()

[Kees Cook]

When build_skb() is passed a frag_size of 0, it means the buffer came
from kmalloc. In these cases, ksize() is used to find its actual size,
but since the allocation may not have been made to that size, actually
perform the krealloc() call so that all the associated buffer size
checking will be correctly notified. For example, syzkaller reported:

  BUG: KASAN: slab-out-of-bounds in __build_skb_around+0x235/0x340 net/core/skbuff.c:294
  Write of size 32 at addr ffff88802aa172c0 by task syz-executor413/5295

For bpf_prog_test_run_skb(), which uses a kmalloc()ed buffer passed to
build_skb().

Reported-by: syzbot+fda18eaa8c12534ccb3b@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://groups.google.com/g/syzkaller-bugs/c/UnIKxTtU5-0/m/-wbXinkgAQAJ
Fixes: 38931d8989b5 ("mm: Make ksize() a reporting-only function")
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: Pavel Begunkov <asml.silence@xxxxxxxxx>
Cc: pepsipu <soopthegoop@xxxxxxxxx>
Cc: syzbot+fda18eaa8c12534ccb3b@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: kasan-dev <kasan-dev@xxxxxxxxxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: ast@xxxxxxxxxx
Cc: bpf <bpf@xxxxxxxxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: Hao Luo <haoluo@xxxxxxxxxx>
Cc: Jesper Dangaard Brouer <hawk@xxxxxxxxxx>
Cc: John Fastabend <john.fastabend@xxxxxxxxx>
Cc: jolsa@xxxxxxxxxx
Cc: KP Singh <kpsingh@xxxxxxxxxx>
Cc: martin.lau@xxxxxxxxx
Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>
Cc: song@xxxxxxxxxx
Cc: Yonghong Song <yhs@xxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 net/core/skbuff.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 1d9719e72f9d..b55d061ed8b4 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -274,7 +274,23 @@ static void __build_skb_around(struct sk_buff *skb, void *data,
 			       unsigned int frag_size)
 {
 	struct skb_shared_info *shinfo;
-	unsigned int size = frag_size ? : ksize(data);
+	unsigned int size = frag_size;
+
+	/* When frag_size == 0, the buffer came from kmalloc, so we
+	 * must find its true allocation size (and grow it to match).
+	 */
+	if (unlikely(size == 0)) {
+		void *resized;
+
+		size = ksize(data);
+		/* krealloc() will immediate return "data" when
+		 * "ksize(data)" is requested: it is the existing upper
+		 * bounds. As a result, GFP_ATOMIC will be ignored.
+		 */
+		resized = krealloc(data, size, GFP_ATOMIC);
+		if (WARN_ON(resized != data))
+			data = resized;
+	}
 
 	size -= SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 
-- 
2.34.1