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Date:   Fri, 9 Sep 2022 07:18:21 -0700
From:   Petar Penkov <>
To:     Ziyang Xuan <>
Subject: Re: [PATCH net] net: tun: limit first seg size to avoid oversized linearization

Thanks, this looks good to me!

On Tue, Sep 6, 2022 at 6:56 PM Ziyang Xuan
<> wrote:
> Recently, we found a syzkaller problem as following:
> ========================================================
> WARNING: CPU: 1 PID: 17965 at mm/page_alloc.c:5295 __alloc_pages+0x1308/0x16c4 mm/page_alloc.c:5295
> ...
> Call trace:
>  __alloc_pages+0x1308/0x16c4 mm/page_alloc.c:5295
>  __alloc_pages_node include/linux/gfp.h:550 [inline]
>  alloc_pages_node include/linux/gfp.h:564 [inline]
>  kmalloc_large_node+0x94/0x350 mm/slub.c:4038
>  __kmalloc_node_track_caller+0x620/0x8e4 mm/slub.c:4545
>  __kmalloc_reserve.constprop.0+0x1e4/0x2b0 net/core/skbuff.c:151
>  pskb_expand_head+0x130/0x8b0 net/core/skbuff.c:1654
>  __skb_grow include/linux/skbuff.h:2779 [inline]
>  tun_napi_alloc_frags+0x144/0x610 drivers/net/tun.c:1477
>  tun_get_user+0x31c/0x2010 drivers/net/tun.c:1835
>  tun_chr_write_iter+0x98/0x100 drivers/net/tun.c:2036
> It is because the first seg size of the iov_iter from user space is
> very big, it is 2147479538 which is bigger than the threshold value
> for bail out early in __alloc_pages(). And skb->pfmemalloc is true,
> __kmalloc_reserve() would use pfmemalloc reserves without __GFP_NOWARN
> flag. Thus we got a warning.
> I noticed that non-first segs size are required less than PAGE_SIZE in
> tun_napi_alloc_frags(). The first seg should not be a special case, and
> oversized linearization is also unreasonable. Limit the first seg size to
> PAGE_SIZE to avoid oversized linearization.
> Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")
> Signed-off-by: Ziyang Xuan <>
> ---
Acked-by: Petar Penkov <>

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