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Date:   Mon, 14 Dec 2020 11:09:02 +0100
From:   Marco Elver <elver@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Jann Horn <jannh@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Willem de Bruijn <willemb@...gle.com>,
        syzbot <syzbot+7b99aafdcc2eedea6178@...kaller.appspotmail.com>
Subject: Re: WARNING in sk_stream_kill_queues (5)

On Thu, 10 Dec 2020 at 20:01, Marco Elver <elver@...gle.com> wrote:
> On Thu, 10 Dec 2020 at 18:14, Eric Dumazet <edumazet@...gle.com> wrote:
> > On Thu, Dec 10, 2020 at 5:51 PM Marco Elver <elver@...gle.com> wrote:
> [...]
> > > So I started putting gdb to work, and whenever I see an allocation
> > > exactly like the above that goes through tso_fragment() a warning
> > > immediately follows.
> > >
> > > Long story short, I somehow synthesized this patch that appears to fix
> > > things, but I can't explain why exactly:
> > >
> > > | --- a/net/core/skbuff.c
> > > | +++ b/net/core/skbuff.c
> > > | @@ -1679,13 +1679,6 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
> > > |
> > > |       skb_metadata_clear(skb);
> > > |
> > > | -     /* It is not generally safe to change skb->truesize.
> > > | -      * For the moment, we really care of rx path, or
> > > | -      * when skb is orphaned (not attached to a socket).
> > > | -      */
> > > | -     if (!skb->sk || skb->destructor == sock_edemux)
> > > | -             skb->truesize += size - osize;
> > > | -
> > > |       return 0;
> > > |
> > > |  nofrags:
> > >
> > > Now, here are the breadcrumbs I followed:
> > >
> > >
> > > 1.      Breakpoint on kfence_ksize() -- first allocation that matches the above:
> > >
> > >         | #0  __kfence_ksize (s=18446612700164612096) at mm/kfence/core.c:726
> > >         | #1  0xffffffff816fbf30 in kfence_ksize (addr=0xffff888436856000) at mm/kfence/core.c:737
> > >         | #2  0xffffffff816217cf in ksize (objp=0xffff888436856000) at mm/slab_common.c:1178
> > >         | #3  0xffffffff84896911 in __alloc_skb (size=914710528, gfp_mask=2592, flags=0, node=-1) at net/core/skbuff.c:217
> > >         | #4  0xffffffff84d0ba73 in alloc_skb_fclone (priority=<optimized out>, size=<optimized out>) at ./include/linux/skbuff.h:1144
> > >         | #5  sk_stream_alloc_skb (sk=0xffff8881176cc000, size=0, gfp=2592, force_schedule=232) at net/ipv4/tcp.c:888
> > >         | #6  0xffffffff84d41c36 in tso_fragment (gfp=<optimized out>, mss_now=<optimized out>, len=<optimized out>,
> > >         |     skb=<optimized out>, sk=<optimized out>) at net/ipv4/tcp_output.c:2124
> > >         | #7  tcp_write_xmit (sk=0xffff8881176cc000, mss_now=21950, nonagle=3096, push_one=-1996874776, gfp=0)
> > >         |     at net/ipv4/tcp_output.c:2674
> > >         | #8  0xffffffff84d43e48 in __tcp_push_pending_frames (sk=0xffff8881176cc000, cur_mss=337, nonagle=0)
> > >         |     at ./include/net/sock.h:918
> > >         | #9  0xffffffff84d3259c in tcp_push_pending_frames (sk=<optimized out>) at ./include/net/tcp.h:1864
> > >         | #10 tcp_data_snd_check (sk=<optimized out>) at net/ipv4/tcp_input.c:5374
> > >         | #11 tcp_rcv_established (sk=0xffff8881176cc000, skb=0x0 <fixed_percpu_data>) at net/ipv4/tcp_input.c:5869
> > >         | #12 0xffffffff84d56731 in tcp_v4_do_rcv (sk=0xffff8881176cc000, skb=0xffff888117f52ea0) at net/ipv4/tcp_ipv4.c:1668
> > >         | [...]
> > >
> > >         Set watchpoint on skb->truesize:
> > >
> > >         | (gdb) frame 3
> > >         | #3  0xffffffff84896911 in __alloc_skb (size=914710528, gfp_mask=2592, flags=0, node=-1) at net/core/skbuff.c:217
> > >         | 217             size = SKB_WITH_OVERHEAD(ksize(data));
> > >         | (gdb) p &skb->truesize
> > >         | $5 = (unsigned int *) 0xffff888117f55f90
> > >         | (gdb) awatch *0xffff888117f55f90
> > >         | Hardware access (read/write) watchpoint 6: *0xffff888117f55f90
> > >
> > > 2.      Some time later, we see that the skb with kfence-allocated data
> > >         is cloned:
> > >
> > >         | Thread 7 hit Hardware access (read/write) watchpoint 6: *0xffff888117f55f90
> > >         |
> > >         | Value = 1570
> > >         | 0xffffffff84886947 in __skb_clone (n=0xffff888117f55fa0, skb=0xffff888117f55ec0) at net/core/skbuff.c:1002
> > >         | 1002            C(truesize);
> > >         | (gdb) bt
> > >         | #0  0xffffffff84886947 in __skb_clone (n=0xffff888117f55fa0, skb=0xffff888117f55ec0) at net/core/skbuff.c:1002
> > >         | #1  0xffffffff8488bfb9 in skb_clone (skb=0xffff888117f55ec0, gfp_mask=2592) at net/core/skbuff.c:1454
> > >         | #2  0xffffffff84d3cd1c in __tcp_transmit_skb (sk=0xffff8881176cc000, skb=0xffff888117f55ec0, clone_it=0, gfp_mask=2592,
> > >         |     rcv_nxt=0) at net/ipv4/tcp_output.c:1267
> > >         | #3  0xffffffff84d4125b in tcp_transmit_skb (gfp_mask=<optimized out>, clone_it=<optimized out>, skb=<optimized out>,
> > >         |     sk=<optimized out>) at ./include/linux/tcp.h:439
> > >         | #4  tcp_write_xmit (sk=0xffff8881176cc000, mss_now=392485600, nonagle=1326, push_one=-1996875104, gfp=0)
> > >         |     at net/ipv4/tcp_output.c:2688
> > >         | #5  0xffffffff84d43e48 in __tcp_push_pending_frames (sk=0xffff8881176cc000, cur_mss=337, nonagle=0)
> > >         |     at ./include/net/sock.h:918
> > >         | #6  0xffffffff84d3259c in tcp_push_pending_frames (sk=<optimized out>) at ./include/net/tcp.h:1864
> > >         | #7  tcp_data_snd_check (sk=<optimized out>) at net/ipv4/tcp_input.c:5374
> > >         | #8  tcp_rcv_established (sk=0xffff8881176cc000, skb=0x0 <fixed_percpu_data>) at net/ipv4/tcp_input.c:5869
> > >         | #9  0xffffffff84d56731 in tcp_v4_do_rcv (sk=0xffff8881176cc000, skb=0xffff888117f57820) at net/ipv4/tcp_ipv4.c:1668
> > >         | #10 0xffffffff8487bf67 in sk_backlog_rcv (skb=<optimized out>, sk=<optimized out>) at ./include/net/sock.h:1010
> > >         [...]
> > >
> > >
> > > 3.      The original skb (that was cloned) has its truesize adjusted
> > >         after a pskb_expand_head():
> > >
> > >         | Thread 2 hit Hardware access (read/write) watchpoint 6: *0xffff888117f55f90
> > >         |
> > >         | Old value = 1570
> > >         | New value = 1954
> > >
> > >         ^^ the difference between the old and the new value is exactly
> > >         384, which is also the final underflow of the sk_wmem_queued
> > >         that triggers the warning. Presumably if the original allocation
> > >         had been through kmalloc-1k and not KFENCE, the difference here
> > >         would have been 0, since ksize() of the original allocation in
> > >         step (1) would have been 1024, and not 640 (difference of 384).
> > >
> > >         | 0xffffffff8488d84b in pskb_expand_head (skb=0xffff888117f55ec0, nhead=401956752, ntail=1954, gfp_mask=2298092192)
> > >         |     at net/core/skbuff.c:1687
> > >         | 1687                    skb->truesize += size - osize;
> > >         | (gdb) bt
> > >         | #0  0xffffffff8488d84b in pskb_expand_head (skb=0xffff888117f55ec0, nhead=401956752, ntail=1954, gfp_mask=2298092192)
> > >         |     at net/core/skbuff.c:1687
> > >         | #1  0xffffffff8488de01 in skb_prepare_for_shift (skb=<optimized out>) at ./arch/x86/include/asm/atomic.h:29
> > >         | #2  skb_prepare_for_shift (skb=0xffff888117f55ec0) at net/core/skbuff.c:3276
> > >         | #3  0xffffffff848936b1 in skb_shift (tgt=0xffff888117f549c0, skb=0xffff888117f55ec0, shiftlen=674) at net/core/skbuff.c:3351
> > >         | #4  0xffffffff84d264de in tcp_skb_shift (shiftlen=<optimized out>, pcount=<optimized out>, from=<optimized out>,
> > >         |     to=<optimized out>) at net/ipv4/tcp_input.c:1497
> > >         | #5  tcp_shift_skb_data (dup_sack=<optimized out>, end_seq=<optimized out>, start_seq=<optimized out>, state=<optimized out>,
> > >         |     skb=<optimized out>, sk=<optimized out>) at net/ipv4/tcp_input.c:1605
> > >         | #6  tcp_sacktag_walk (skb=0xffff888117f55ec0, sk=0xffff8881176cc000, next_dup=0x894,
> > >         |     state=0xffffffff88fa1aa0 <watchpoints+192>, start_seq=0, end_seq=401956752, dup_sack_in=false)
> > >         |     at net/ipv4/tcp_input.c:1670
> > >         | #7  0xffffffff84d276de in tcp_sacktag_write_queue (sk=0xffff888117f55f90, ack_skb=0x1888117f55f90, prior_snd_una=2196,
> > >         |     state=0xffffffff88fa1aa0 <watchpoints+192>) at net/ipv4/tcp_input.c:1931
> > >         | #8  0xffffffff84d2ca1d in tcp_ack (sk=0xffff8881176cc000, skb=0x1888117f55f90, flag=16643) at net/ipv4/tcp_input.c:3758
> > >         | #9  0xffffffff84d32387 in tcp_rcv_established (sk=0xffff8881176cc000, skb=0xffff888117f54020) at net/ipv4/tcp_input.c:5858
> > >         | #10 0xffffffff84d56731 in tcp_v4_do_rcv (sk=0xffff8881176cc000, skb=0xffff888117f54020) at net/ipv4/tcp_ipv4.c:1668
> > >         [...]
> > >
> > >
> > > Any of this make sense?
> >
> > Very nice debugging !
> >
> > I guess we could fix this in skb_prepare_for_shift(), eventually
> > caring for the truesize manipulation
> > (or reverting the change done in pskb_expand_head(), since only kfence
> > is having this issue.
>
> Phew, good to hear I finally got lucky. :-)
>
> Either option is fine, as long as it avoids this problem in future.
> Hopefully it can be fixed for 5.11.
>
> > (All TCP skbs in output path have the same allocation size for skb->head)
> >
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index e578544b2cc7110ec2f6bcf4c29d93e4b4b1ad14..798b51eeeaa4fbed65d41d9eab207dbbf438dab3
> > 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -3270,7 +3270,14 @@ EXPORT_SYMBOL(skb_split);
> >   */
> >  static int skb_prepare_for_shift(struct sk_buff *skb)
> >  {
> > -       return skb_cloned(skb) && pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
> > +       unsigned int ret = 0, save;
> > +
> > +       if (skb_cloned(skb)) {
> > +               save = skb->truesize;
> > +               ret = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
> > +               skb->truesize = save;
> > +       }
> > +       return ret;
> >  }
>
> FWIW,
>
>     Tested-by: Marco Elver <elver@...gle.com>

Has this patch, or similar, already been sent?

Thanks,
-- Marco

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