Re: [PATCH v3] net/core/skbuff: fix passing wrong size to __alloc_skb

[Alexander Lobakin <alobakin@pm.me>]

From: Pavel Skripkin <paskripkin@gmail.com>
Date: Sat, 27 Feb 2021 20:51:14 +0300

Hi,

> syzbot found WARNING in __alloc_pages_nodemask()[1] when order >= MAX_ORDER.
> It was caused by __netdev_alloc_skb(), which doesn't check len value after adding NET_SKB_PAD.
> Order will be >= MAX_ORDER and passed to __alloc_pages_nodemask() if size > KMALLOC_MAX_SIZE.
> Same happens in __napi_alloc_skb.
>
> static void *kmalloc_large_node(size_t size, gfp_t flags, int node)
> {
> 	struct page *page;
> 	void *ptr = NULL;
> 	unsigned int order = get_order(size);
> ...
> 	page = alloc_pages_node(node, flags, order);
> ...
>
> [1] WARNING in __alloc_pages_nodemask+0x5f8/0x730 mm/page_alloc.c:5014
> Call Trace:
>  __alloc_pages include/linux/gfp.h:511 [inline]
>  __alloc_pages_node include/linux/gfp.h:524 [inline]
>  alloc_pages_node include/linux/gfp.h:538 [inline]
>  kmalloc_large_node+0x60/0x110 mm/slub.c:3999
>  __kmalloc_node_track_caller+0x319/0x3f0 mm/slub.c:4496
>  __kmalloc_reserve net/core/skbuff.c:150 [inline]
>  __alloc_skb+0x4e4/0x5a0 net/core/skbuff.c:210
>  __netdev_alloc_skb+0x70/0x400 net/core/skbuff.c:446
>  netdev_alloc_skb include/linux/skbuff.h:2832 [inline]
>  qrtr_endpoint_post+0x84/0x11b0 net/qrtr/qrtr.c:442
>  qrtr_tun_write_iter+0x11f/0x1a0 net/qrtr/tun.c:98
>  call_write_iter include/linux/fs.h:1901 [inline]
>  new_sync_write+0x426/0x650 fs/read_write.c:518
>  vfs_write+0x791/0xa30 fs/read_write.c:605
>  ksys_write+0x12d/0x250 fs/read_write.c:658
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Ah, by the way. Have you tried to seek for the root cause, why
a request for such insanely large (at least 4 Mib) skb happens
in QRTR? I don't believe it's intended to be like this.
Now I feel that silencing this error with early return isn't
really correct approach for this.

> Reported-by: syzbot+80dccaee7c6630fa9dcf@syzkaller.appspotmail.com
> Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
>
> ---
> Changes from v3:
> * Removed Change-Id and extra tabs in net/core/skbuff.c
>
> Changes from v2:
> * Added length check to __napi_alloc_skb
> * Added unlikely() in checks
>
> Change from v1:
> * Added length check to __netdev_alloc_skb
> ---
>  net/core/skbuff.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 785daff48030..ec7ba8728b61 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -443,6 +443,9 @@ struct sk_buff *__netdev_alloc_skb(struct net_device *dev, unsigned int len,
>  	if (len <= SKB_WITH_OVERHEAD(1024) ||
>  	    len > SKB_WITH_OVERHEAD(PAGE_SIZE) ||
>  	    (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) {
> +		if (unlikely(len > KMALLOC_MAX_SIZE))
> +			return NULL;
> +
>  		skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX, NUMA_NO_NODE);
>  		if (!skb)
>  			goto skb_fail;
> @@ -517,6 +520,9 @@ struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
>  	if (len <= SKB_WITH_OVERHEAD(1024) ||
>  	    len > SKB_WITH_OVERHEAD(PAGE_SIZE) ||
>  	    (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) {
> +		if (unlikely(len > KMALLOC_MAX_SIZE))
> +			return NULL;
> +
>  		skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX, NUMA_NO_NODE);
>  		if (!skb)
>  			goto skb_fail;
> --
> 2.25.1

Thanks,
Al