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

[Denys Vlasenko <dvlasenk@redhat.com>]

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/


> +
>  	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