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

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Tue, Mar 24, 2015 at 1:08 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Andy Lutomirski <luto@kernel.org> 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?
> >
> > 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?

Indeed - the orphaned label should have told me that. The mismerge is 
functionally harmless (causes extra overhead in the slowpath), that's 
why it passed testing.

Does:

  06ab9c1ba6a1 Merge branch 'x86/urgent' into x86/asm, to resolve conflict

look better to you?

Thanks,

	Ingo