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.seyfr...@googlemail.com>
> Reported-by: Takashi Iwai <ti...@suse.de>
> Signed-off-by: Andy Lutomirski <l...@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
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