Re: [PATCH] tun: Fix use-after-free in tun_net_xmit

From: Jason Wang
Date: Sun Apr 28 2019 - 03:52:01 EST


On 2019/4/28 äå1:40, weiyongjun (A) wrote:
Hi Jason,

On 2019/4/28 äå11:24, Jason Wang wrote:
On 2019/4/28 äå11:05, Yue Haibing wrote:
From: YueHaibing <yuehaibing@xxxxxxxxxx>

KASAN report this:

BUG: KASAN: use-after-free in tun_net_xmit+0x1670/0x1750
drivers/net/tun.c:1104
Read of size 8 at addr ffff88836cc26a70 by task swapper/3/0

CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.19.32 #6
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.10.2-1ubuntu1 04/01/2014
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0xca/0x13e lib/dump_stack.c:113
 print_address_description+0x79/0x330 mm/kasan/report.c:253
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report+0x18a/0x2d0 mm/kasan/report.c:409
 tun_net_xmit+0x1670/0x1750 drivers/net/tun.c:1104
 __netdev_start_xmit include/linux/netdevice.h:4300 [inline]
 netdev_start_xmit include/linux/netdevice.h:4309 [inline]
 xmit_one net/core/dev.c:3243 [inline]
 dev_hard_start_xmit+0x17c/0x780 net/core/dev.c:3259
 sch_direct_xmit+0x24f/0x8a0 net/sched/sch_generic.c:327
 qdisc_restart net/sched/sch_generic.c:390 [inline]
 __qdisc_run+0x45b/0x1590 net/sched/sch_generic.c:398
 qdisc_run include/net/pkt_sched.h:120 [inline]
 __dev_xmit_skb net/core/dev.c:3438 [inline]
 __dev_queue_xmit+0xa6b/0x2500 net/core/dev.c:3797
 neigh_output include/net/neighbour.h:501 [inline]
 ip6_finish_output2+0xa36/0x2290 net/ipv6/ip6_output.c:120
 ip6_finish_output+0x3e7/0xa20 net/ipv6/ip6_output.c:154
 NF_HOOK_COND include/linux/netfilter.h:278 [inline]
 ip6_output+0x1e2/0x720 net/ipv6/ip6_output.c:171
 dst_output include/net/dst.h:444 [inline]
 NF_HOOK include/linux/netfilter.h:289 [inline]
 mld_sendpack+0x740/0xf20 net/ipv6/mcast.c:1683
 mld_send_cr net/ipv6/mcast.c:1979 [inline]
 mld_ifc_timer_expire+0x3cc/0x7f0 net/ipv6/mcast.c:2478
 call_timer_fn+0x1ea/0x6a0 kernel/time/timer.c:1326
 expire_timers kernel/time/timer.c:1363 [inline]
 __run_timers kernel/time/timer.c:1682 [inline]
 run_timer_softirq+0x637/0x1070 kernel/time/timer.c:1695
 __do_softirq+0x26d/0xabd kernel/softirq.c:292
 invoke_softirq kernel/softirq.c:372 [inline]
 irq_exit+0x209/0x290 kernel/softirq.c:412
 exiting_irq arch/x86/include/asm/apic.h:536 [inline]
 smp_apic_timer_interrupt+0xf6/0x480 arch/x86/kernel/apic/apic.c:1056
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:864
 </IRQ>
RIP: 0010:native_safe_halt+0x2/0x10 arch/x86/include/asm/irqflags.h:58
Code: 01 f0 0f 82 bc fd ff ff 48 c7 c7 40 25 b1 83 e8 a1 72 03 ff e9
ab fd ff ff 4c 89 e7 e8 37 f5 a7 fe e9 6a ff ff ff 90 90 fb f4 <c3>
0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 f4 c3 90 90 90 90 90 90
RSP: 0018:ffff8883e0f2fd20 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff13
RAX: 0000000000000007 RBX: dffffc0000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8883e0f04f1c
RBP: 0000000000000003 R08: ffffed107c5dc77b R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff848b96e0
R13: 0000000000000003 R14: 1ffff1107c1e5fae R15: 0000000000000000
 arch_safe_halt arch/x86/include/asm/paravirt.h:94 [inline]
 default_idle+0x24/0x2b0 arch/x86/kernel/process.c:561
 cpuidle_idle_call kernel/sched/idle.c:153 [inline]
 do_idle+0x2ca/0x420 kernel/sched/idle.c:262
 cpu_startup_entry+0xcb/0xe0 kernel/sched/idle.c:368
 start_secondary+0x421/0x570 arch/x86/kernel/smpboot.c:271
 secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:243

Allocated by task 19764:
 set_track mm/kasan/kasan.c:460 [inline]
 kasan_kmalloc+0xa0/0xd0 mm/kasan/kasan.c:553
 __kmalloc+0x11b/0x2d0 mm/slub.c:3750
 kmalloc include/linux/slab.h:518 [inline]
 sk_prot_alloc+0xf6/0x290 net/core/sock.c:1469
 sk_alloc+0x3d/0xc00 net/core/sock.c:1523
 tun_chr_open+0x80/0x560 drivers/net/tun.c:3204
 misc_open+0x367/0x4e0 drivers/char/misc.c:141
 chrdev_open+0x212/0x580 fs/char_dev.c:417
 do_dentry_open+0x704/0x1050 fs/open.c:777
 do_last fs/namei.c:3418 [inline]
 path_openat+0x7ed/0x2ae0 fs/namei.c:3533
 do_filp_open+0x1aa/0x2b0 fs/namei.c:3564
 do_sys_open+0x307/0x430 fs/open.c:1069
 do_syscall_64+0xc8/0x580 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 19764:
 set_track mm/kasan/kasan.c:460 [inline]
 __kasan_slab_free+0x12e/0x180 mm/kasan/kasan.c:521
 slab_free_hook mm/slub.c:1370 [inline]
 slab_free_freelist_hook mm/slub.c:1397 [inline]
 slab_free mm/slub.c:2952 [inline]
 kfree+0xeb/0x2f0 mm/slub.c:3905
 sk_prot_free net/core/sock.c:1506 [inline]
 __sk_destruct+0x4e6/0x6a0 net/core/sock.c:1588
 sk_destruct+0x48/0x70 net/core/sock.c:1596
 __sk_free+0xa9/0x270 net/core/sock.c:1607
 sk_free+0x2a/0x30 net/core/sock.c:1618
 sock_put include/net/sock.h:1696 [inline]
 __tun_detach+0x464/0xf70 drivers/net/tun.c:735
 tun_detach drivers/net/tun.c:747 [inline]
 tun_chr_close+0xd8/0x190 drivers/net/tun.c:3241
 __fput+0x27f/0x7f0 fs/file_table.c:278
 task_work_run+0x136/0x1b0 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:193 [inline]
 exit_to_usermode_loop+0x1a7/0x1d0 arch/x86/entry/common.c:166
 prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
 do_syscall_64+0x461/0x580 arch/x86/entry/common.c:293
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff88836cc26600
 which belongs to the cache kmalloc-4096 of size 4096
The buggy address is located 1136 bytes inside of
 4096-byte region [ffff88836cc26600, ffff88836cc27600)
The buggy address belongs to the page:
page:ffffea000db30800 count:1 mapcount:0 mapping:ffff8883e280e600
index:0x0 compound_mapcount: 0
flags: 0x2fffff80008100(slab|head)
raw: 002fffff80008100 dead000000000100 dead000000000200
ffff8883e280e600
raw: 0000000000000000 0000000000070007 00000001ffffffff
0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88836cc26900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88836cc26980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff88836cc26a00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ^
 ffff88836cc26a80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88836cc26b00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

If tun driver have multiqueues, user close the last queue by
tun_detach, then tun->tfiles[index] is not cleared. Then a new
queue may add to the tun, which using rcu_assign_pointer
tun->tfiles[index] to the new tfile and increase the numqueues.
However if there send a packet during this time, which picking the last
queue, it may uses the old tun->tfiles[index], beacause there no
RCU grace period.

1) tun_chr_close //close the last queue
 --> __tun_detach //close the last queue, but tun->tfiles[index]
still exist

2) tun_chr_open //attach a new queue
ÂÂÂÂ --> tun_attach
-->rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
ÂÂÂÂÂ //there need a RCU grace period

ÂÂÂÂ -->tun->numqueues++;

3) tun_net_xmit //a new packet is sending, which pick the last queue
ÂÂÂÂÂ -->if (txq >= tun->numqueues)
ÂÂÂÂÂÂ //above check passed, but tfile still not renew
ÂÂÂÂÂ -->if (tfile->socket.sk->sk_filter ...
ÂÂÂÂÂÂ //use the old tfile,trigger use-after-free

Reported-by: Hulk Robot <hulkci@xxxxxxxxxx>
Fixes: c8d68e6be1c3 ("tuntap: multiqueue support")
Signed-off-by: YueHaibing <yuehaibing@xxxxxxxxxx>
---
 drivers/net/tun.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index e9ca1c0..3770aba 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -876,6 +876,7 @@ static int tun_attach(struct tun_struct *tun,
struct file *file,
ÂÂÂÂÂÂ */
ÂÂÂÂÂ rcu_assign_pointer(tfile->tun, tun);
ÂÂÂÂÂ rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
+ÂÂÂ synchronize_net();
ÂÂÂÂÂ tun->numqueues++;
ÂÂÂÂÂ tun_set_real_num_queues(tun);
 out:

Good catch, that's one of my suspicion as well. Assume that this has
been tested.

Acked-by: Jason Wang <jasowang@xxxxxxxxxx>

This patch is needed for -stable.

I will post another theoretical issue similar to this soon.

Thanks

Ok, a second thought. All evil come from the access of tun->numqueues
without sufficient synchronization. This is a partial fix, another
possible case is __tun_detach(), we need either
In __tun_detach(),tun->tfiles changes should always call synchronize_net(),
and free tfile after RCU grace period.


Exactly, and that's why I suggest to check pointers in tfiles against NULL for method 1) I mentioned below.


tun_net_xmit() doesn't have the chance to
access the change because it holding the rcu_read_lock().


The problem is the following codes:


ÂÂÂ ÂÂÂ --tun->numqueues;

ÂÂÂ ÂÂÂ ...

ÂÂÂ ÂÂÂ synchronize_net();

We need make sure the decrement of tun->numqueues be visible to readers after synchronize_net(). And in tun_net_xmit():


ÂÂÂ rcu_read_lock();
ÂÂÂ tfile = rcu_dereference(tun->tfiles[txq]);

ÂÂÂ /* Drop packet if interface is not attached */
ÂÂÂ if (txq >= tun->numqueues)
ÂÂÂ ÂÂÂ goto drop;


We should make sure tun->numqueues were read after tfiles array dereference.

Unfortunately, we don't have such guarantee even with this patch. So we should change to use smp_load_acquire()/smp_store_release() for those cases (method 2).

It looks to me checking file against NULL (and NULL out tfiles[tun->numqueues] in __tun_detach) is much more easier to be understand.

What do you think?

Thanks



So __tun_detach() path cannot cause the use after free, isn't it?

Regards

1) synchronize through pointers in tfiles

2) or smp_load_acquire()/smp_store_release() to solve it completely.

Let me post a complete fix.

Thanks