Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 24 Mar 2015 17:35:50 -0700 | Subject | Re: [PATCH] x86, entry: Check for syscall exit work with IRQs disabled |
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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?
--Andy
> > Thanks, > > Ingo
-- Andy Lutomirski AMA Capital Management, LLC
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