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Date:   Sat, 27 Feb 2021 11:03:22 +0000
From:   Alexander Lobakin <alobakin@...me>
To:     Pavel Skripkin <paskripkin@...il.com>
Cc:     Alexander Lobakin <alobakin@...me>, davem@...emloft.net,
        kuba@...nel.org, linmiaohe@...wei.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot+80dccaee7c6630fa9dcf@...kaller.appspotmail.com
Subject: Re: [PATCH] net/core/skbuff.c: __netdev_alloc_skb fix when len is greater than KMALLOC_MAX_SIZE

From: Pavel Skripkin <paskripkin@...il.com>
Date: Fri, 26 Feb 2021 22:11:06 +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.
>
> 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
>
> Reported-by: syzbot+80dccaee7c6630fa9dcf@...kaller.appspotmail.com
> Signed-off-by: Pavel Skripkin <paskripkin@...il.com>
> Change-Id: I480a6d6f818a4c0a387db0cd3f230b68a7daeb16
> ---
>  net/core/skbuff.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 785daff48030..dc28c8f7bf5f 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 (len > KMALLOC_MAX_SIZE)
> +			return NULL;

I'd use unlikely() for this as it's very very rare condition on the
very hot path.

Also, I'd add the same check below into __napi_alloc_skb() as it has
the same fallback.

>  		skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX, NUMA_NO_NODE);
>  		if (!skb)
>  			goto skb_fail;
> --
> 2.25.1

Thanks,
Al

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