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

[Ingo Molnar]

* 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.

Thanks,

	Ingo
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