KASAN: slab-out-of-bounds Read in erspan_xmit

[syzbot]

Hello,

syzbot hit the following crash on net-next commit
9d6474e458b13a94a0d5b141f2b8f38adf1991ae (Mon Jan 22 02:55:38 2018 +0000)
tun: add missing rcu annotation

So far this crash happened 5 times on net-next.
C reproducer is attached.
syzkaller reproducer is attached.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.

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Reported-by: syzbot+9723f2...@syzkaller.appspotmail.com
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==================================================================
BUG: KASAN: slab-out-of-bounds in erspan_xmit+0x22d4/0x2430
net/ipv4/ip_gre.c:735
Read of size 2 at addr ffff8801c50bb08b by task syzkaller525754/3647

CPU: 0 PID: 3647 Comm: syzkaller525754 Not tainted 4.15.0-rc8+ #203
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x194/0x257 lib/dump_stack.c:53
print_address_description+0x73/0x250 mm/kasan/report.c:252
kasan_report_error mm/kasan/report.c:351 [inline]
kasan_report+0x25b/0x340 mm/kasan/report.c:409
__asan_report_load_n_noabort+0xf/0x20 mm/kasan/report.c:440
erspan_xmit+0x22d4/0x2430 net/ipv4/ip_gre.c:735
__netdev_start_xmit include/linux/netdevice.h:4053 [inline]
netdev_start_xmit include/linux/netdevice.h:4062 [inline]
packet_direct_xmit+0x3ad/0x790 net/packet/af_packet.c:267
packet_snd net/packet/af_packet.c:2944 [inline]
packet_sendmsg+0x3aed/0x60b0 net/packet/af_packet.c:2969
sock_sendmsg_nosec net/socket.c:630 [inline]
sock_sendmsg+0xca/0x110 net/socket.c:640
SYSC_sendto+0x361/0x5c0 net/socket.c:1721
SyS_sendto+0x40/0x50 net/socket.c:1689
entry_SYSCALL_64_fastpath+0x29/0xa0
RIP: 0033:0x445649
RSP: 002b:00007ffe82dde5b8 EFLAGS: 00000217 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: ffffffffffffffff RCX: 0000000000445649
RDX: 0000000000000000 RSI: 0000000020003fd9 RDI: 0000000000000004
RBP: 00000000004a78c5 R08: 0000000020008000 R09: 000000000000001c
R10: 0000000000000000 R11: 0000000000000217 R12: 0000000000402720
R13: 00000000004027b0 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 3221:
save_stack+0x43/0xd0 mm/kasan/kasan.c:447
set_track mm/kasan/kasan.c:459 [inline]
kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:551
kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489
kmem_cache_alloc+0x12e/0x760 mm/slab.c:3544
getname_flags+0xcb/0x580 fs/namei.c:138
getname+0x19/0x20 fs/namei.c:209
do_sys_open+0x2e7/0x6d0 fs/open.c:1053
SYSC_open fs/open.c:1077 [inline]
SyS_open+0x2d/0x40 fs/open.c:1072
entry_SYSCALL_64_fastpath+0x29/0xa0

Freed by task 3221:
save_stack+0x43/0xd0 mm/kasan/kasan.c:447
set_track mm/kasan/kasan.c:459 [inline]
kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
__cache_free mm/slab.c:3488 [inline]
kmem_cache_free+0x83/0x2a0 mm/slab.c:3746
putname+0xee/0x130 fs/namei.c:258
do_sys_open+0x31b/0x6d0 fs/open.c:1068
SYSC_open fs/open.c:1077 [inline]
SyS_open+0x2d/0x40 fs/open.c:1072
entry_SYSCALL_64_fastpath+0x29/0xa0

The buggy address belongs to the object at ffff8801c50ba000
which belongs to the cache names_cache of size 4096
The buggy address is located 139 bytes to the right of
4096-byte region [ffff8801c50ba000, ffff8801c50bb000)
The buggy address belongs to the page:
page:ffffea0007142e80 count:1 mapcount:0 mapping:ffff8801c50ba000 index:0x0
compound_mapcount: 0
flags: 0x2fffc0000008100(slab|head)
raw: 02fffc0000008100 ffff8801c50ba000 0000000000000000 0000000100000001
raw: ffffea0007145320 ffffea00071433a0 ffff8801dae2c600 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8801c50baf80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8801c50bb000: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801c50bb080: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
^
ffff8801c50bb100: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8801c50bb180: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


---
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[David Ahern]

[ cc William Tu ]

[William Tu]

Thanks. I'm working on it.

[William Tu]

Hi,

I'm new to kasan and trying to follow this instruction to reproduce the issue:
https://github.com/google/syzkaller/blob/master/docs/executing_syzkaller_programs.md

After re-compile my kernel with KASAN related config enable, I run
$ ./syz-execprog -cover=0 -repeat=0 -procs=16 program

I wonder does the "program" mean the repro.c.txt? or I should compile
it to binary?
# gcc -o program repro.c.txt
# ./syz-execprog myprogram
2018/01/23 10:45:19 parsed 0 programs

And how to use the "repro.syz.txt"?
It seems to have some command like "syz_emit_ethernet" to generate packet.
but I have no clue where to run it. Maybe I'm still missing something?

Thanks a lot
William

[David Ahern]

On 1/23/18 11:50 AM, William Tu wrote:
> Hi,
>
> I'm new to kasan and trying to follow this instruction to reproduce the issue:
> https://github.com/google/syzkaller/blob/master/docs/executing_syzkaller_programs.md
>
> After re-compile my kernel with KASAN related config enable, I run
> $ ./syz-execprog -cover=0 -repeat=0 -procs=16 program
>
> I wonder does the "program" mean the repro.c.txt? or I should compile
> it to binary?
> # gcc -o program repro.c.txt
> # ./syz-execprog myprogram
> 2018/01/23 10:45:19 parsed 0 programs
>
> And how to use the "repro.syz.txt"?
> It seems to have some command like "syz_emit_ethernet" to generate packet.
> but I have no clue where to run it. Maybe I'm still missing something?
>

In the past I have only compiled a kernel with KASAN, compiled the
reproducer program and run it in a VM. No need for the syzbot overhead.

[Dmitry Vyukov]

Yes, if C program reproducer the crash then it's easier to use.
repro.c.txt is the C program, you need to rename it to repro.c,
compile with gcc and run just as ./a.out.
But make sure that you have a gcc that supports KASAN (kernel build
does not in the beginning on compiler not supporting KASAN). I think
it's at least gcc 5+, but gcc 7+ would be better.

You can also run the syzkaller reproducer as:
./syz-execprog -cover=0 -repeat=0 -procs=16 repro.syz.txt

[William Tu]

Thanks for the reply.

I was using gcc 5+ and "gcc repro.c".
Running ./a.out does not show any issue on dmesg. Let me switch to gcc 7+.

>
> You can also run the syzkaller reproducer as:
> ./syz-execprog -cover=0 -repeat=0 -procs=16 repro.syz.txt

When using repro.syz.txt, which binary or what tests does it execute?
I didn't see it uses/compiles the repro.c.txt.
But it seems to run something...
~/net-next# ./syz-execprog -cover=0 -repeat=0 -procs=2 repro.syz.txt
2018/01/23 11:15:24 parsed 1 programs
2018/01/23 11:15:24 executed programs: 0
2018/01/23 11:15:29 executed programs: 210
2018/01/23 11:15:34 executed programs: 422
..

Thanks
William

[Dmitry Vyukov]

It interprets the program in syzkaller notation in repro.syz.txt file.
It should be more of less equivalent to repro.c.txt C program in
behavior.

[William Tu]

thanks!. Now I can reproduce the issue.