[PATCH net v1] net: Fix tuntap uninitialized value

[Jiayuan Chen]

Then tun/tap allocates an skb, it additionally allocates a prepad size
(usually equal to NET_SKB_PAD) but leaves it uninitialized.

bpf_xdp_adjust_head() may move skb->data forward, which may lead to an
issue.

Since the linear address is likely to be allocated from kmem_cache, it's
unlikely to trigger a KMSAN warning. We need some tricks, such as forcing
kmem_cache_shrink in __do_kmalloc_node, to reproduce the issue and trigger
a KMSAN warning.

Reported-by: syzbot+0e6ddb1ef80986bdfe64@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/00000000000067f65105edbd295d@xxxxxxxxxx/T/
Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>
---
 drivers/net/tun.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index f75f912a0225..111f83668b5e 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1463,6 +1463,7 @@ static struct sk_buff *tun_alloc_skb(struct tun_file *tfile,
 	if (!skb)
 		return ERR_PTR(err);
 
+	memset(skb->data, 0, prepad);
 	skb_reserve(skb, prepad);
 	skb_put(skb, linear);
 	skb->data_len = len - linear;
@@ -1621,6 +1622,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 		return ERR_PTR(-ENOMEM);
 
 	buf = (char *)page_address(alloc_frag->page) + alloc_frag->offset;
+	memset(buf, 0, pad);
 	copied = copy_page_from_iter(alloc_frag->page,
 				     alloc_frag->offset + pad,
 				     len, from);
-- 
2.47.1