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

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2022-09-13 at 20:07 +0800, Ziyang Xuan (William) wrote:
> > On Tue, Sep 6, 2022 at 6:56 PM Ziyang Xuan
> > <william.xuanziyang@huawei.com> 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@huawei.com>
> > > ---
> > >  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.

I'm guessing that what Eric is suggesting here is to use a bigger limit
for 'linear'. Possibly ETH_MAX_MTU could fit. @Eric, fell free to
correct me :)

Thanks!

Paolo