WARNING in bpf_xdp_adjust_tail

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 2ce578ca net: ipv4: Fix wrong type conversion from hint to..
git tree: net
console output: https://syzkaller.appspot.com/x/log.txt?x=1190cf23100000
kernel config: https://syzkaller.appspot.com/x/.config?x=bf3aec367b9ab569
dashboard link: https://syzkaller.appspot.com/bug?extid=c3157bda041952444952
compiler: gcc (GCC) 10.1.0-syz 20200507

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+c3157b...@syzkaller.appspotmail.com

WARNING: CPU: 0 PID: 22595 at net/core/filter.c:3463 ____bpf_xdp_adjust_tail net/core/filter.c:3463 [inline]
WARNING: CPU: 0 PID: 22595 at net/core/filter.c:3463 bpf_xdp_adjust_tail+0x18e/0x1e0 net/core/filter.c:3452
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 22595 Comm: syz-executor.4 Not tainted 5.8.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x18f/0x20d lib/dump_stack.c:118
panic+0x2e3/0x75c kernel/panic.c:231
__warn.cold+0x20/0x45 kernel/panic.c:600
report_bug+0x1bd/0x210 lib/bug.c:198
exc_invalid_op+0x24d/0x400 arch/x86/kernel/traps.c:235
asm_exc_invalid_op+0x12/0x20 arch/x86/include/asm/idtentry.h:563
RIP: 0010:____bpf_xdp_adjust_tail net/core/filter.c:3463 [inline]
RIP: 0010:bpf_xdp_adjust_tail+0x18e/0x1e0 net/core/filter.c:3452
Code: 37 fb 84 db 74 09 49 c7 c4 ea ff ff ff eb c7 e8 f8 f5 37 fb 44 89 e6 48 c7 c7 e0 fc fd 88 c6 05 dc f5 6d 04 01 e8 94 3b 09 fb <0f> 0b eb d8 e8 d9 48 77 fb e9 c5 fe ff ff e8 df 48 77 fb e9 92 fe
RSP: 0018:ffffc900018878e0 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000040000 RSI: ffffffff815ce8d7 RDI: fffff52000310f0e
RBP: 0000000000000000 R08: 0000000000000001 R09: ffff8880ae620fcb
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000020000
R13: ffff88804c20feef R14: ffff88804c20feef R15: 0000000000000000
bpf_prog_4add87e5301a4105+0x20/0x818
bpf_prog_run_xdp include/linux/filter.h:734 [inline]
netif_receive_generic_xdp+0x70f/0x1760 net/core/dev.c:4647
do_xdp_generic net/core/dev.c:4735 [inline]
do_xdp_generic+0x96/0x1a0 net/core/dev.c:4728
tun_get_user+0x22d2/0x35b0 drivers/net/tun.c:1905
tun_chr_write_iter+0xba/0x151 drivers/net/tun.c:1999
call_write_iter include/linux/fs.h:1907 [inline]
new_sync_write+0x422/0x650 fs/read_write.c:484
__vfs_write+0xc9/0x100 fs/read_write.c:497
vfs_write+0x268/0x5d0 fs/read_write.c:559
ksys_write+0x12d/0x250 fs/read_write.c:612
do_syscall_64+0x60/0xe0 arch/x86/entry/common.c:359
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x416661
Code: Bad RIP value.
RSP: 002b:00007f8bc3971c60 EFLAGS: 00000293 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00000000005095a0 RCX: 0000000000416661
RDX: 000000000000fdef RSI: 0000000020000080 RDI: 00000000000000f0
RBP: 000000000078bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 00007f8bc39729d0 R11: 0000000000000293 R12: 00000000ffffffff
R13: 0000000000000bfd R14: 00000000004ce559 R15: 00007f8bc39726d4
Kernel Offset: disabled


---
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[Jesper Dangaard Brouer]

On Sun, 05 Jul 2020 00:20:18 -0700
syzbot <syzbot+c3157b...@syzkaller.appspotmail.com> wrote:

> Hello,
>
> syzbot found the following crash on:

It is WARN that trigger this, due to panic_on_warn set.

It's great to see that syzbot report these, as the WARN_ONCE is meant
to catch drivers that forget to init xdp->frame_sz. In this case it is
XDP-generic that manage to get an oversized SKB run through this code
path.

> HEAD commit: 2ce578ca net: ipv4: Fix wrong type conversion from hint to..
> git tree: net
> console output: https://syzkaller.appspot.com/x/log.txt?x=1190cf23100000
> kernel config: https://syzkaller.appspot.com/x/.config?x=bf3aec367b9ab569
> dashboard link: https://syzkaller.appspot.com/bug?extid=c3157bda041952444952
> compiler: gcc (GCC) 10.1.0-syz 20200507
>
> Unfortunately, I don't have any reproducer for this crash yet.
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+c3157b...@syzkaller.appspotmail.com
>

It would be practical to get the WARN message here (from console output):
[ 511.164212][T22595] Too BIG xdp->frame_sz = 131072

As call-stack indicate this is XDP-generic (do_xdp_generic).
Thus the xdp->frame_sz calc comes from:

xdp->data_hard_start = skb->data - skb_headroom(skb);
/* SKB "head" area always have tailroom for skb_shared_info */
xdp->frame_sz = (void *)skb_end_pointer(skb) - xdp->data_hard_start;
xdp->frame_sz += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));

I'm surprised to see a 128KiB (128*1024) sized SKB here (in "head" area).
How can this happen?

--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer

[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.