Date:	Mon, 23 Mar 2015 17:07:15 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Takashi Iwai <tiwai@...e.de>
CC:	Andy Lutomirski <luto@...capital.net>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Stefan Seyfried <stefan.seyfried@...glemail.com>,
	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: PANIC: double fault, error_code: 0x0 in 4.0.0-rc3-2, kvm related?

On 03/23/2015 02:22 PM, Takashi Iwai wrote:
> At Mon, 23 Mar 2015 10:35:41 +0100,
> Takashi Iwai wrote:
>>
>> At Mon, 23 Mar 2015 10:02:52 +0100,
>> Takashi Iwai wrote:
>>>
>>> At Fri, 20 Mar 2015 19:16:53 +0100,
>>> Denys Vlasenko wrote:
>>>> Takashi, are you willing to reproduce the panic one more time,
>>>> with this patch? I would like to see whether oops messages
>>>> are more informative with it.
>>>
>>> It can't be applied to 4.0-rc5, unfortunately.
>>>
>>> arch/x86/kernel/entry_64.S: Assembler messages:
>>> arch/x86/kernel/entry_64.S:1725: Error: no such instruction: `alloc_pt_gpregs_on_stack'
>>> arch/x86/kernel/entry_64.S:1716: Error: invalid operands (*UND* and *UND* sections) for `+'
>>> scripts/Makefile.build:294: recipe for target 'arch/x86/kernel/entry_64.o' failed
>>
>> I pulled tip tree on top of 4.0-rc5, built with your patch and now
>> succeeded to get a better message:
>>
>>  kvm: zapping shadow pages for mmio generation wraparound
>>  kvm [5126]: vcpu0 disabled perfctr wrmsr: 0xc1 data 0xffff
>>  Exception on user stack 00007ffd22c23ef0: RSP: 0018:00007ffd22c23f28  EFLAGS: 00010006
>>  RIP: 0010:[<ffffffff8162681d>]  [<ffffffff8162681d>] netlink_attachskb+0x1d/0x1d0
>>  PANIC: double fault, error_code: 0x0
>>  CPU: 1 PID: 10819 Comm: cc1 Tainted: G        W       4.0.0-rc5-debug1+ #2
>>  Hardware name: Dell Inc. OptiPlex 9010/0M9KCM, BIOS A12 01/10/2013
>>  task: ffff8800d1b34b10 ti: ffff8800d1b30000 task.ti: ffff8800d1b30000
>>  RIP: 0010:[<ffffffff8162681d>]  [<ffffffff8162681d>] netlink_attachskb+0x1d/0x1d0
>>  RSP: 0018:00007ffd22c23f28  EFLAGS: 00010006
>>  RAX: 0000000000000000 RBX: 0000000000000005 RCX: 00000000c0000101
>>  RDX: 0000000000000000 RSI: 0000000000000001 RDI: 00007ffd22c23ef0
>>  RBP: 0000000000000ea7 R08: 0000000000001ea7 R09: ffffffffffffffff
>>  R10: 000000000309dbf8 R11: 0000000000000246 R12: 0000000000000001
>>  R13: 0000000000000000 R14: 0000000003026e40 R15: 000000000309cd50
>>  FS:  00007f89c83c2800(0000) GS:ffff88021d240000(0000) knlGS:0000000000000000
>>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>  CR2: 000000000000016d CR3: 00000000d90a0000 CR4: 00000000001427e0
>>  Stack:
>>   0000000000000ea7 0000000000000000 0000000003099c10 0000000000000ea7
>>   0000000000000ea7 0000000000000001 0000000003099c10 0000000000000ea7
>>   0000000000c84696 0000000003099c88 00007f0122c23fb8 000000000302f610
>>  Call Trace:
>>   <UNK> 
>>  Code: 
>>  10 75 ee f0 ff 42 6c 48 89 d0 5d c3 66 90 0f 1f 44 00 00 55 48 89 e5 41 56 41 55 49 89 d5 41 54 49 89 f4 53 48 89 fb 48 83 ec 30 <8b> 87 68 01 00 00 39 87 9c 01 00 00 7c 25 48 8b 87 88 04 00 00 
>>  Kernel panic - not syncing: Machine halted.
>>  CPU: 1 PID: 10819 Comm: cc1 Tainted: G        W       4.0.0-rc5-debug1+ #2
>>  Hardware name: Dell Inc. OptiPlex 9010/0M9KCM, BIOS A12 01/10/2013
>>   0000000000000000 ffff8800d1b33e28 ffffffff816f80d2 0000000000000000
>>   ffffffff81a22f81 ffff8800d1b33ea8 ffffffff816f2358 00000000000058d7
>>   0000000000000008 ffff8800d1b33eb8 ffff8800d1b33e58 ffff8800d1b33ea8
>>  Call Trace:
>>   [<ffffffff816f80d2>] dump_stack+0x4c/0x6e
>>   [<ffffffff816f2358>] panic+0xc0/0x1f3
>>   [<ffffffff81046e65>] df_debug+0x35/0x40
>>   [<ffffffff81003fe7>] do_double_fault+0x87/0x100
>>   [<ffffffff81004167>] do_userpsace_rsp_in_kernel+0x107/0x140
>>   [<ffffffff8162681d>] ? netlink_attachskb+0x1d/0x1d0
>>   [<ffffffff81703ca6>] userpsace_rsp_in_kernel+0x36/0x40
>>   [<ffffffff8162681d>] ? netlink_attachskb+0x1d/0x1d0
>>
>>
>> So, it seems hitting in netlink_attachskb().
>> I'd need to check whether this consistently hits there or just at
>> random.
> 
> I managed to reproduce the bug two more times, and all three show the
> very same stack trace like the above.  So, it's well reproducible.

FYI: the disassembly of netlink_attachskb (from "Code:" line) is:

   0:   0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
   5:   55                      push   %rbp
   6:   48 89 e5                mov    %rsp,%rbp
   9:   41 56                   push   %r14
   b:   41 55                   push   %r13
   d:   49 89 d5                mov    %rdx,%r13
  10:   41 54                   push   %r12
  12:   49 89 f4                mov    %rsi,%r12
  15:   53                      push   %rbx
  16:   48 89 fb                mov    %rdi,%rbx
  19:   48 83 ec 30             sub    $0x30,%rsp
  1d:   8b 87 68 01 00 00       mov    0x168(%rdi),%eax
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  23:   39 87 9c 01 00 00       cmp    %eax,0x19c(%rdi)
  29:   7c 25                   jl     50 <_start+0x50>
  2b:   48 8b 87 88 04 00 00    mov    0x488(%rdi),%rax

The ^^^^^ instruction is the one which faults. Since you said it
consistently happens here, this should be a page fault, not an external
hardware interrupt.

The code corresponds to the comparison in if():

int netlink_attachskb(struct sock *sk, struct sk_buff *skb,
                      long *timeo, struct sock *ssk)
{
        struct netlink_sock *nlk;

        nlk = nlk_sk(sk);

        if ((atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf ||

%rdi (which is 1st param, "struct sock *sk") is 00007ffd22c23ef0
(userspace address), but it's just because my patch clobbers %rdi,   :(
we don't know which value it had at that moment.

> I'm really puzzled now.  We have a few pieces of information:
> 
> - git bisection pointed the commit 96b6352c1271:
>     x86_64, entry: Remove the syscall exit audit and schedule optimizations
>   and reverting this "fixes" the problem indeed.  Even just moving two
>   lines
>     LOCKDEP_SYS_EXIT
>     DISABLE_INTERRUPTS(CLBR_NONE) 
>   at the beginning of ret_from_sys_call already fixes.  (Of course I
>   can't prove the fix but it stabilizes for a day without crash while
>   usually I hit the bug in 10 minutes in full test running.)

The commit 96b6352c1271 moved TIF_ALLWORK_MASK check from
interrupt-disabled region to interrupt-enabled:

        cmpq $__NR_syscall_max,%rax
        ja ret_from_sys_call
        movq %r10,%rcx
        call *sys_call_table(,%rax,8)  # XXX:    rip relative
        movq %rax,RAX-ARGOFFSET(%rsp)
ret_from_sys_call:
	testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	jnz int_ret_from_sys_call_fixup	/* Go the the slow path */
 	LOCKDEP_SYS_EXIT
 	DISABLE_INTERRUPTS(CLBR_NONE)
 	TRACE_IRQS_OFF
...
...
int_ret_from_sys_call_fixup:
        FIXUP_TOP_OF_STACK %r11, -ARGOFFSET
        jmp int_ret_from_sys_call
...
...
GLOBAL(int_ret_from_sys_call)
        DISABLE_INTERRUPTS(CLBR_NONE)
        TRACE_IRQS_OFF

You reverted that by moving this insn to be after first DISABLE_INTERRUPTS(CLBR_NONE).

I also don't see how moving that check (even if it is wrong in a more
benign way) can have such a drastic effect.


Shot-in-the-dark idea. At this code revision we did not yet
store user's %rsp in pt_regs->sp, we used a fixup to populate it:

        .macro FIXUP_TOP_OF_STACK tmp offset=0
        movq PER_CPU_VAR(old_rsp),\tmp
        movq \tmp,RSP+\offset(%rsp)

(There are pending patches to fix this mess).

If an interrupt interrupting *kernel code* would go into a code path
which does FIXUP_TOP_OF_STACK, it'd overwrite the correct saved %rsp
with a user's one. The iret from interrupt would work,
but the resulting CPU state would be inconsistent. But I don't see
such a code path from interrupts to FIXUP_TOP_OF_STACK...


> - Another piece is that the bug happens only when a KVM is running.
>   The kernel ran without problem over days with similar tasks
>   (compiling kernel, etc) when no KVM was used.

Conceivably virtualization support in CPUs can have nasty erratas.
However, you and other reporter have different CPUs - yours
is Ivy Bridge, his CPU is a Penryn.

I don't see the path how KVM helps to trigger this.

> - And now I get the trace as above, pointing netlink_attachskb().
> 
> I have a difficulty to imagine how all these pieces fit into a single
> picture.  Is something already screwed up before that?

Well, a tiny bit more info will be seen if you'd change %rdi
to, say, %r15 in these two lines in my patch:

       /* Save bogus RSP value */
       movq    %rsp,%rdi
...
       push    %rdi            /* pt_regs->sp */

Then original %rdi will be visible in the crash message.
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