Subject: Re: [PATCH] net: sched: sch_netem: Fix a divide error in netem_enqueue during randomized corruption.

From: Stephen Hemminger <[email-masked]>
Date: Fri, 19 Nov 2021 09:01:10 -0800

On Fri, 19 Nov 2021 07:49:59 -0800
Eric Dumazet <[email-masked]> wrote:

> On 11/19/21 12:42 AM, Harshit Mogalapalli wrote:
> > In netem_enqueue function the value of skb_headlen(skb) can be zero
> > which leads to a division error during randomized corruption of the packet.
> > This fix  adds a check to skb_headlen(skb) to prevent the division error.
> > 
> > Crash report:
> > [  343.170349] netdevsim netdevsim0 netdevsim3: set [1, 0] type 2 family
> > 0 port 6081 - 0
> > [  343.216110] netem: version 1.3
> > [  343.235841] divide error: 0000 [#1] PREEMPT SMP KASAN NOPTI
> > [  343.236680] CPU: 3 PID: 4288 Comm: reproducer Not tainted 5.16.0-rc1+
> > [  343.237569] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> > BIOS 1.11.0-2.el7 04/01/2014
> > [  343.238707] RIP: 0010:netem_enqueue+0x1590/0x33c0 [sch_netem]
> > [  343.239499] Code: 89 85 58 ff ff ff e8 5f 5d e9 d3 48 8b b5 48 ff ff
> > ff 8b 8d 50 ff ff ff 8b 85 58 ff ff ff 48 8b bd 70 ff ff ff 31 d2 2b 4f
> > 74 <f7> f1 48 b8 00 00 00 00 00 fc ff df 49 01 d5 4c 89 e9 48 c1 e9 03
> > [  343.241883] RSP: 0018:ffff88800bcd7368 EFLAGS: 00010246
> > [  343.242589] RAX: 00000000ba7c0a9c RBX: 0000000000000001 RCX:
> > 0000000000000000
> > [  343.243542] RDX: 0000000000000000 RSI: ffff88800f8edb10 RDI:
> > ffff88800f8eda40
> > [  343.244474] RBP: ffff88800bcd7458 R08: 0000000000000000 R09:
> > ffffffff94fb8445
> > [  343.245403] R10: ffffffff94fb8336 R11: ffffffff94fb8445 R12:
> > 0000000000000000
> > [  343.246355] R13: ffff88800a5a7000 R14: ffff88800a5b5800 R15:
> > 0000000000000020
> > [  343.247291] FS:  00007fdde2bd7700(0000) GS:ffff888109780000(0000)
> > knlGS:0000000000000000
> > [  343.248350] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [  343.249120] CR2: 00000000200000c0 CR3: 000000000ef4c000 CR4:
> > 00000000000006e0
> > [  343.250076] Call Trace:
> > [  343.250423]  <TASK>
> > [  343.250713]  ? memcpy+0x4d/0x60
> > [  343.251162]  ? netem_init+0xa0/0xa0 [sch_netem]
> > [  343.251795]  ? __sanitizer_cov_trace_pc+0x21/0x60
> > [  343.252443]  netem_enqueue+0xe28/0x33c0 [sch_netem]
> > [  343.253102]  ? stack_trace_save+0x87/0xb0
> > [  343.253655]  ? filter_irq_stacks+0xb0/0xb0
> > [  343.254220]  ? netem_init+0xa0/0xa0 [sch_netem]
> > [  343.254837]  ? __kasan_check_write+0x14/0x20
> > [  343.255418]  ? _raw_spin_lock+0x88/0xd6
> > [  343.255953]  dev_qdisc_enqueue+0x50/0x180
> > [  343.256508]  __dev_queue_xmit+0x1a7e/0x3090
> > [  343.257083]  ? netdev_core_pick_tx+0x300/0x300
> > [  343.257690]  ? check_kcov_mode+0x10/0x40
> > [  343.258219]  ? _raw_spin_unlock_irqrestore+0x29/0x40
> > [  343.258899]  ? __kasan_init_slab_obj+0x24/0x30
> > [  343.259529]  ? setup_object.isra.71+0x23/0x90
> > [  343.260121]  ? new_slab+0x26e/0x4b0
> > [  343.260609]  ? kasan_poison+0x3a/0x50
> > [  343.261118]  ? kasan_unpoison+0x28/0x50
> > [  343.261637]  ? __kasan_slab_alloc+0x71/0x90
> > [  343.262214]  ? memcpy+0x4d/0x60
> > [  343.262674]  ? write_comp_data+0x2f/0x90
> > [  343.263209]  ? __kasan_check_write+0x14/0x20
> > [  343.263802]  ? __skb_clone+0x5d6/0x840
> > [  343.264329]  ? __sanitizer_cov_trace_pc+0x21/0x60
> > [  343.264958]  dev_queue_xmit+0x1c/0x20
> > [  343.265470]  netlink_deliver_tap+0x652/0x9c0
> > [  343.266067]  netlink_unicast+0x5a0/0x7f0
> > [  343.266608]  ? netlink_attachskb+0x860/0x860
> > [  343.267183]  ? __sanitizer_cov_trace_pc+0x21/0x60
> > [  343.267820]  ? write_comp_data+0x2f/0x90
> > [  343.268367]  netlink_sendmsg+0x922/0xe80
> > [  343.268899]  ? netlink_unicast+0x7f0/0x7f0
> > [  343.269472]  ? __sanitizer_cov_trace_pc+0x21/0x60
> > [  343.270099]  ? write_comp_data+0x2f/0x90
> > [  343.270644]  ? netlink_unicast+0x7f0/0x7f0
> > [  343.271210]  sock_sendmsg+0x155/0x190
> > [  343.271721]  ____sys_sendmsg+0x75f/0x8f0
> > [  343.272262]  ? kernel_sendmsg+0x60/0x60
> > [  343.272788]  ? write_comp_data+0x2f/0x90
> > [  343.273332]  ? write_comp_data+0x2f/0x90
> > [  343.273869]  ___sys_sendmsg+0x10f/0x190
> > [  343.274405]  ? sendmsg_copy_msghdr+0x80/0x80
> > [  343.274984]  ? slab_post_alloc_hook+0x70/0x230
> > [  343.275597]  ? futex_wait_setup+0x240/0x240
> > [  343.276175]  ? security_file_alloc+0x3e/0x170
> > [  343.276779]  ? write_comp_data+0x2f/0x90
> > [  343.277313]  ? __sanitizer_cov_trace_pc+0x21/0x60
> > [  343.277969]  ? write_comp_data+0x2f/0x90
> > [  343.278515]  ? __fget_files+0x1ad/0x260
> > [  343.279048]  ? __sanitizer_cov_trace_pc+0x21/0x60
> > [  343.279685]  ? write_comp_data+0x2f/0x90
> > [  343.280234]  ? __sanitizer_cov_trace_pc+0x21/0x60
> > [  343.280874]  ? sockfd_lookup_light+0xd1/0x190
> > [  343.281481]  __sys_sendmsg+0x118/0x200
> > [  343.281998]  ? __sys_sendmsg_sock+0x40/0x40
> > [  343.282578]  ? alloc_fd+0x229/0x5e0
> > [  343.283070]  ? write_comp_data+0x2f/0x90
> > [  343.283610]  ? write_comp_data+0x2f/0x90
> > [  343.284135]  ? __sanitizer_cov_trace_pc+0x21/0x60
> > [  343.284776]  ? ktime_get_coarse_real_ts64+0xb8/0xf0
> > [  343.285450]  __x64_sys_sendmsg+0x7d/0xc0
> > [  343.285981]  ? syscall_enter_from_user_mode+0x4d/0x70
> > [  343.286664]  do_syscall_64+0x3a/0x80
> > [  343.287158]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > [  343.287850] RIP: 0033:0x7fdde24cf289
> > [  343.288344] Code: 01 00 48 81 c4 80 00 00 00 e9 f1 fe ff ff 0f 1f 00
> > 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f
> > 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d b7 db 2c 00 f7 d8 64 89 01 48
> > [  343.290729] RSP: 002b:00007fdde2bd6d98 EFLAGS: 00000246 ORIG_RAX:
> > 000000000000002e
> > [  343.291730] RAX: ffffffffffffffda RBX: 0000000000000000 RCX:
> > 00007fdde24cf289
> > [  343.292673] RDX: 0000000000000000 RSI: 00000000200000c0 RDI:
> > 0000000000000004
> > [  343.293618] RBP: 00007fdde2bd6e20 R08: 0000000100000001 R09:
> > 0000000000000000
> > [  343.294557] R10: 0000000100000001 R11: 0000000000000246 R12:
> > 0000000000000000
> > [  343.295493] R13: 0000000000021000 R14: 0000000000000000 R15:
> > 00007fdde2bd7700
> > [  343.296432]  </TASK>
> > [  343.296735] Modules linked in: sch_netem ip6_vti ip_vti ip_gre ipip
> > sit ip_tunnel geneve macsec macvtap tap ipvlan macvlan 8021q garp mrp
> > hsr wireguard libchacha20poly1305 chacha_x86_64 poly1305_x86_64
> > ip6_udp_tunnel udp_tunnel libblake2s blake2s_x86_64 libblake2s_generic
> > curve25519_x86_64 libcurve25519_generic libchacha xfrm_interface
> > xfrm6_tunnel tunnel4 veth netdevsim psample batman_adv nlmon dummy team
> > bonding tls vcan ip6_gre ip6_tunnel tunnel6 gre tun ip6t_rpfilter
> > ipt_REJECT nf_reject_ipv4 ip6t_REJECT nf_reject_ipv6 xt_conntrack ip_set
> > ebtable_nat ebtable_broute ip6table_nat ip6table_mangle
> > ip6table_security ip6table_raw iptable_nat nf_nat nf_conntrack
> > nf_defrag_ipv6 nf_defrag_ipv4 iptable_mangle iptable_security
> > iptable_raw ebtable_filter ebtables rfkill ip6table_filter ip6_tables
> > iptable_filter ppdev bochs drm_vram_helper drm_ttm_helper ttm
> > drm_kms_helper cec parport_pc drm joydev floppy parport sg syscopyarea
> > sysfillrect sysimgblt i2c_piix4 qemu_fw_cfg fb_sys_fops pcspkr
> > [  343.297459]  ip_tables xfs virtio_net net_failover failover sd_mod
> > sr_mod cdrom t10_pi ata_generic pata_acpi ata_piix libata virtio_pci
> > virtio_pci_legacy_dev serio_raw virtio_pci_modern_dev dm_mirror
> > dm_region_hash dm_log dm_mod
> > [  343.311074] Dumping ftrace buffer:
> > [  343.311532]    (ftrace buffer empty)
> > [  343.312040] ---[ end trace a2e3db5a6ae05099 ]---
> > [  343.312691] RIP: 0010:netem_enqueue+0x1590/0x33c0 [sch_netem]
> > [  343.313481] Code: 89 85 58 ff ff ff e8 5f 5d e9 d3 48 8b b5 48 ff ff
> > ff 8b 8d 50 ff ff ff 8b 85 58 ff ff ff 48 8b bd 70 ff ff ff 31 d2 2b 4f
> > 74 <f7> f1 48 b8 00 00 00 00 00 fc ff df 49 01 d5 4c 89 e9 48 c1 e9 03
> > [  343.315893] RSP: 0018:ffff88800bcd7368 EFLAGS: 00010246
> > [  343.316622] RAX: 00000000ba7c0a9c RBX: 0000000000000001 RCX:
> > 0000000000000000
> > [  343.317585] RDX: 0000000000000000 RSI: ffff88800f8edb10 RDI:
> > ffff88800f8eda40
> > [  343.318549] RBP: ffff88800bcd7458 R08: 0000000000000000 R09:
> > ffffffff94fb8445
> > [  343.319503] R10: ffffffff94fb8336 R11: ffffffff94fb8445 R12:
> > 0000000000000000
> > [  343.320455] R13: ffff88800a5a7000 R14: ffff88800a5b5800 R15:
> > 0000000000000020
> > [  343.321414] FS:  00007fdde2bd7700(0000) GS:ffff888109780000(0000)
> > knlGS:0000000000000000
> > [  343.322489] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [  343.323283] CR2: 00000000200000c0 CR3: 000000000ef4c000 CR4:
> > 00000000000006e0
> > [  343.324264] Kernel panic - not syncing: Fatal exception in interrupt
> > [  343.333717] Dumping ftrace buffer:
> > [  343.334175]    (ftrace buffer empty)
> > [  343.334653] Kernel Offset: 0x13600000 from 0xffffffff81000000
> > (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
> > [  343.336027] Rebooting in 86400 seconds..
> > 
> > Reported-by: syzkaller <[email-masked]>
> > Signed-off-by: Harshit Mogalapalli <[email-masked]>
> > ---
> >  net/sched/sch_netem.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
> > index ecbb10db1111..e1e1a00fedda 100644
> > --- a/net/sched/sch_netem.c
> > +++ b/net/sched/sch_netem.c
> > @@ -513,8 +513,14 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
> >  			goto finish_segs;
> >  		}
> >  
> > -		skb->data[prandom_u32() % skb_headlen(skb)] ^=
> > -			1<<(prandom_u32() % 8);
> > +		if (unlikely(!skb_headlen(skb))) {
> > +			qdisc_drop(skb, sch, to_free);
> > +			skb = NULL;
> > +			goto finish_segs;
> > +		} else {
> > +			skb->data[prandom_u32() % skb_headlen(skb)] ^=
> > +				1<<(prandom_u32() % 8);
> > +		}
> >  	}
> >  
> >  	if (unlikely(sch->q.qlen >= sch->limit)) {
> >   
> 
> 
> If we accept the fact that a packet can reach qdisc with nothing in skb->head,
> then we have other serious issues.
> 
> Why dropping the packet ?
> 
> I would rather pull headers here.
> 

Agree with Eric, would be to just linearize befor the corruption step.
There is also the issue of checksum offload here.