Re: [PATCH] x86, entry: Check for syscall exit work with IRQs disabled

[Andy Lutomirski]

On Mon, Mar 23, 2015 at 12:21 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> On 03/23/2015 08:10 PM, Andy Lutomirski wrote:
>> We currently have a race: if we're preempted during syscall exit, we
>> can fail to process syscall return work that is queued up while
>> we're preempted in ret_from_sys_call after checking ti.flags.
>>
>> Fix it by disabling interrupts before checking ti.flags.
>>
>> Fixes: 96b6352c1271 x86_64, entry: Remove the syscall exit audit and schedule optimizations
>> Reported-by: Stefan Seyfried <stefan.seyfried@xxxxxxxxxxxxxx>
>> Reported-by: Takashi Iwai <tiwai@xxxxxxx>
>> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
>> ---
>>
>> Ingo, I don't understand the LOCKDEP_SYS_EXIT stuff.  Can you take a quick
>> look to confirm that it's okay to call it more than once?
>>
>> arch/x86/kernel/entry_64.S | 18 ++++++++++++++----
>>  1 file changed, 14 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
>> index 1d74d161687c..2babb393915e 100644
>> --- a/arch/x86/kernel/entry_64.S
>> +++ b/arch/x86/kernel/entry_64.S
>> @@ -364,12 +364,21 @@ system_call_fastpath:
>>   * Has incomplete stack frame and undefined top of stack.
>>   */
>>  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
>> +
>> +     /*
>> +      * We must check ti flags with interrupts (or at least preemption)
>> +      * off because we must *never* return to userspace without
>> +      * processing exit work that is enqueued if we're preempted here.
>> +      * In particular, returning to userspace with any of the one-shot
>> +      * flags (TIF_NOTIFY_RESUME, TIF_USER_RETURN_NOTIFY, etc) set is
>> +      * very bad.
>> +      */
>> +     testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
>> +     jnz int_ret_from_sys_call_fixup /* Go the the slow path */
>                                            ^^^^^^^^^^^^^^^^^^^^
>
> typo here; s/the the/to the/

Whoops.

>
>
>> +
>>       CFI_REMEMBER_STATE
>>       /*
>>        * sysretq will re-enable interrupts:
>> @@ -386,7 +395,7 @@ ret_from_sys_call:
>>
>>  int_ret_from_sys_call_fixup:
>>       FIXUP_TOP_OF_STACK %r11, -ARGOFFSET
>> -     jmp int_ret_from_sys_call
>> +     jmp int_ret_from_sys_call_irqs_off
>>
>>       /* Do syscall tracing */
>>  tracesys:
>> @@ -432,6 +441,7 @@ tracesys_phase2:
>>  GLOBAL(int_ret_from_sys_call)
>>       DISABLE_INTERRUPTS(CLBR_NONE)
>>       TRACE_IRQS_OFF
>> +int_ret_from_sys_call_irqs_off:
>>       movl $_TIF_ALLWORK_MASK,%edi
>>       /* edi: mask to check */
>>  GLOBAL(int_with_check)
>
>
> You can avoid having to know LOCKDEP_SYS_EXIT    :)
> Just set %edi = $_TIF_ALLWORK_MASK, and jump a bit farther:
>
>
>         movl $_TIF_ALLWORK_MASK,%edi
>         testl %edi,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
>         jnz int_ret_from_sys_call_fixup /* Go to the slow path */
> ...
> ...
> GLOBAL(int_ret_from_sys_call)
>         DISABLE_INTERRUPTS(CLBR_NONE)
>         TRACE_IRQS_OFF
>         movl $_TIF_ALLWORK_MASK,%edi
>         /* edi: mask to check */
> GLOBAL(int_with_check)
>         LOCKDEP_SYS_EXIT_IRQ
> int_ret_from_sys_call_irqs_off:  <========== HERE
>

I didn't want to do that, because I really want to rewrite
int_ret_from_sys_call in C.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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