Re: [PATCH net] net: tun: limit first seg size to avoid oversized linearization

[Ziyang Xuan (William)]

> On Tue, Sep 6, 2022 at 6:56 PM Ziyang Xuan
> <william.xuanziyang@xxxxxxxxxx> 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 <william.xuanziyang@xxxxxxxxxx>
>> ---
>>  drivers/net/tun.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index 259b2b84b2b3..7db515f94667 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1454,12 +1454,12 @@ static struct sk_buff *tun_napi_alloc_frags(struct tun_file *tfile,
>>                                             size_t len,
>>                                             const struct iov_iter *it)
>>  {
>> +       size_t linear = iov_iter_single_seg_count(it);
>>         struct sk_buff *skb;
>> -       size_t linear;
>>         int err;
>>         int i;
>>
>> -       if (it->nr_segs > MAX_SKB_FRAGS + 1)
>> +       if (it->nr_segs > MAX_SKB_FRAGS + 1 || linear > PAGE_SIZE)
>>                 return ERR_PTR(-EMSGSIZE);
>>
> 
> This does not look good to me.
> 
> Some drivers allocate 9KB+ for 9000 MTU, in a single allocation,
> because the hardware is not SG capable in RX.

So, do you mean that it does not matter and keep current status, or give a bigger size but PAGE_SIZE (usually 4KB size)?

Would like to hear your advice.

Thank you.

> 
>>         local_bh_disable();
>> @@ -1468,7 +1468,6 @@ static struct sk_buff *tun_napi_alloc_frags(struct tun_file *tfile,
>>         if (!skb)
>>                 return ERR_PTR(-ENOMEM);
>>
>> -       linear = iov_iter_single_seg_count(it);
>>         err = __skb_grow(skb, linear);
>>         if (err)
>>                 goto free;
>> --
>> 2.25.1
>>
> .
>