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SubjectRe: [PATCH] x86, entry: Check for syscall exit work with IRQs disabled
On Mon, Mar 23, 2015 at 12:27 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Mon, Mar 23, 2015 at 12:21 PM, Denys Vlasenko <dvlasenk@redhat.com> 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@googlemail.com>
>>> Reported-by: Takashi Iwai <tiwai@suse.de>
>>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>>> ---
>>>
>>> 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.
>

To say that better: I don't want to further spread the %edi garbage
around entry_64.S. Saving a single load on the slow path isn't worth
any of this complexity, and, if we're going to rewrite it in C anyway,
then maybe we could consider microoptimizations like that later on.

--Andy


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