Re: net/sched: GPF in qdisc_hash_add

On Thu, Mar 23, 2017 at 12:10 PM, Eric Dumazet [off-list ref] wrote:

On Thu, Mar 23, 2017 at 12:06 PM, Dmitry Vyukov [off-list ref] wrote:

quoted

On Thu, Mar 23, 2017 at 8:00 PM, Cong Wang [off-list ref] wrote:

quoted

On Thu, Mar 23, 2017 at 9:06 AM, Dmitry Vyukov [off-list ref] wrote:

quoted

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 2 PID: 12732 Comm: syz-executor6 Not tainted 4.11.0-rc3+ #365
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff880062b7a2c0 task.stack: ffff880033480000
RIP: 0010:qdisc_hash_add.part.19+0xb6/0x3c0 net/sched/sch_api.c:280
RSP: 0018:ffff880033487820 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: ffffffff85357e00 RCX: ffffc90002b24000
RDX: 000000000000007a RSI: ffffffff835a523a RDI: 00000000000003d0
RBP: ffff8800334878b8 R08: fffffbfff0a6afeb R09: fffffbfff0a6afeb
R10: 0000000000000001 R11: fffffbfff0a6afea R12: ffffffff85357e48
R13: 1ffff10006690f06 R14: ffff880033487890 R15: 0000000000000000
FS:  00007f68665d0700(0000) GS:ffff88006e200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004c2d44 CR3: 000000003c6f8000 CR4: 00000000000026e0
Call Trace:
 qdisc_hash_add+0x76/0x90 net/sched/sch_api.c:279
 attach_default_qdiscs net/sched/sch_generic.c:798 [inline]
 dev_activate+0x6ca/0x920 net/sched/sch_generic.c:829
 __dev_open+0x25b/0x360 net/core/dev.c:1348
 __dev_change_flags+0x159/0x3d0 net/core/dev.c:6460
 dev_change_flags+0x88/0x140 net/core/dev.c:6525
 dev_ifsioc+0x51f/0x9b0 net/core/dev_ioctl.c:254
 dev_ioctl+0x1fe/0x1030 net/core/dev_ioctl.c:532
 sock_do_ioctl+0x94/0xb0 net/socket.c:902
 sock_ioctl+0x2c2/0x440 net/socket.c:993
 vfs_ioctl fs/ioctl.c:45 [inline]
 do_vfs_ioctl+0x1af/0x16d0 fs/ioctl.c:685
 SYSC_ioctl fs/ioctl.c:700 [inline]
 SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691
 entry_SYSCALL_64_fastpath+0x1f/0xc2

The interesting part is why the NULL dereference is in
qdisc_hash_add(), since we have a check before calling
it:

#ifdef CONFIG_NET_SCHED
        if (dev->qdisc)
                qdisc_hash_add(dev->qdisc);
#endif


When attach_one_default_qdisc() fails, we should trigger
the NULL pointer dereference bug at:

    atomic_inc(&dev->qdisc->refcnt);

I think qdisc is not NULL, it's something _in_ qdisc that is NULL. The
crash happens here:

                struct Qdisc *root = qdisc_dev(q)->qdisc;

so it's probably device.

Looks like this bug came with commit 59cc1f61f09c
("net: sched: convert qdisc linked list to hashtable")

I would simply guard qdisc_hash_add()

(Against &noop_qdisc)

Yeah, I missed that dev_init_scheduler() could assign noop_qdisc
to each tx queue. Then the check in attach_default_qdiscs()
is always false? If so we need...

 #ifdef CONFIG_NET_SCHED
-       if (dev->qdisc)
+       if (dev->qdisc != &noop_qdisc)
                qdisc_hash_add(dev->qdisc);
 #endif