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

[Andy Lutomirski]

On Tue, Mar 24, 2015 at 1:08 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Andy Lutomirski <luto@xxxxxxxxxx> 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?
>
> So the essence is that it wants to print this warning if we are
> holding a lock after a syscall:
>
>                 printk("[ BUG: lock held when returning to user space! ]\n");
>
> it manipulates no state and is not sensitive to whether it's called
> before or after return-work processing.
>
>> 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 */
>
> Should be safe to call it once again after user-work processing has
> been finished.
>
> I've picked up your fix for tip:x86/urgent.

FWIW, the tentative merge here:

https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?h=tmp.tmp&id=a77dd1607ad88a601259a74ba4d646fa68b7cd9a

looks funny.  Why aren't you jumping to int_ret_from_sys_call_irqs_off?

--Andy

>
> Thanks,
>
>         Ingo



-- 
Andy Lutomirski
AMA Capital Management, LLC
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