Re: general protection fault in show_timer

[Dmitry Vyukov]

On Thu, Nov 30, 2017 at 12:08 PM, Alexey Dobriyan <adobriyan@xxxxxxxxx> wrote:
> On 11/30/17, syzbot
> <bot+054c6cd125793643a90db21e4b9ddc71a881f797@xxxxxxxxxxxxxxxxxxxxxxxxx>
> wrote:
>> Hello,
>>
>> syzkaller hit the following crash on
>> 43570f0383d6d5879ae585e6c3cf027ba321546f
>> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/master
>> compiler: gcc (GCC) 7.1.1 20170620
>> .config is attached
>> Raw console output is attached.
>>
>> Unfortunately, I don't have any reproducer for this bug yet.
>>
>>
>> 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: 1 PID: 22618 Comm: syz-executor4 Not tainted 4.15.0-rc1+ #199
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>> Google 01/01/2011
>> task: ffff8801c048c400 task.stack: ffff8801cd968000
>> RIP: 0010:show_timer+0x1c7/0x2b0 fs/proc/base.c:2274
>> RSP: 0018:ffff8801cd96f9e0 EFLAGS: 00010006
>> RAX: dffffc0000000000 RBX: ffff8801cff22e40 RCX: ffffffff81ccb88e
>> RDX: 0000000030a68524 RSI: ffffc90002dea000 RDI: 0000000185342920
>> RBP: ffff8801cd96fa10 R08: ffffed003a0514c5 R09: ffffed003a0514c5
>> R10: ffff8801c048c400 R11: ffffed003a0514c4 R12: 0000000040000000
>> R13: ffff8801c79e7000 R14: ffffffff853419e0 R15: 0000000000000507
>> FS:  00007f19f811e700(0000) GS:ffff8801db500000(0000)
>> knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00007f4ab586bdb8 CR3: 00000001d1788000 CR4: 00000000001426e0
>> DR0: 0000000020000000 DR1: 0000000020000008 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000600
>> Call Trace:
>>   traverse+0x248/0xa00 fs/seq_file.c:111
>>   seq_read+0x96a/0x13d0 fs/seq_file.c:189
>>   do_loop_readv_writev fs/read_write.c:673 [inline]
>>   do_iter_read+0x3db/0x5b0 fs/read_write.c:897
>>   vfs_readv+0x121/0x1c0 fs/read_write.c:959
>>   do_preadv+0x11b/0x1a0 fs/read_write.c:1043
>>   SYSC_preadv fs/read_write.c:1093 [inline]
>>   SyS_preadv+0x30/0x40 fs/read_write.c:1088
>>   entry_SYSCALL_64_fastpath+0x1f/0x96
>> RIP: 0033:0x4529d9
>> RSP: 002b:00007f19f811dc58 EFLAGS: 00000212 ORIG_RAX: 0000000000000127
>> RAX: ffffffffffffffda RBX: 00007f19f811d950 RCX: 00000000004529d9
>> RDX: 0000000000000001 RSI: 00000000205e2ff0 RDI: 0000000000000013
>> RBP: 00007f19f811d940 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0001000000000000 R11: 0000000000000212 R12: 00000000004b7346
>> R13: 00007f19f811dac8 R14: 00000000004b7351 R15: 0000000000000000
>> Code: 89 c7 4c 0f 44 f1 41 83 e4 fb 4d 63 e4 e8 a2 2f a3 ff 4a 8d 3c e5 20
>>
>> 29 34 85 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00
>> 0f 85 a2 00 00 00 4a 8b 14 e5 20 29 34 85 4c 89 ef
>> RIP: show_timer+0x1c7/0x2b0 fs/proc/base.c:2274 RSP: ffff8801cd96f9e0
>
> This looks like ASAN problem
>
> ffffffff81a5f1d9:       e8 72 74 b7 ff          call
> ffffffff815d6650 <__sanitizer_cov_trace_pc>
> ffffffff81a5f1de:       4a 8d 3c e5 a0 9e b4    lea    rdi,[r12*8-0x7b4b6160]
> ffffffff81a5f1e5:       84
> ffffffff81a5f1e6:       48 b8 00 00 00 00 00    movabs rax,0xdffffc0000000000
> ffffffff81a5f1ed:       fc ff df
> ffffffff81a5f1f0:       48 89 fa                mov    rdx,rdi
> ffffffff81a5f1f3:       48 c1 ea 03             shr    rdx,0x3
> ffffffff81a5f1f7:       80 3c 02 00     ===>cmp    BYTE PTR
> [rdx+rax*1],0x0  <====
>
> This is code injected by KASAN_INLINE
>
> timer_show() looks seemingly fine:
> "timer" pointer is valid otherwise code would oopsed earlier
> tp->ns is valid otherwise it'd oopsed inside pid_nr_ns()


This is not KASAN bug. Kernel tries to dereference 0x0000000185342920.
Failure mode is just different.