| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <20141121225508.GV5050@linux.vnet.ibm.com> Date: Fri, 21 Nov 2014 14:55:08 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Andy Lutomirski <luto@...capital.net> Cc: Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, Oleg Nesterov <oleg@...hat.com>, Tony Luck <tony.luck@...el.com>, Andi Kleen <andi@...stfloor.org>, Josh Triplett <josh@...htriplett.org>, Frédéric Weisbecker <fweisbec@...il.com> Subject: Re: [PATCH v4 2/5] x86, traps: Track entry into and exit from IST context On Fri, Nov 21, 2014 at 02:19:17PM -0800, Andy Lutomirski wrote: > On Fri, Nov 21, 2014 at 2:07 PM, Paul E. McKenney > <paulmck@...ux.vnet.ibm.com> wrote: > > On Fri, Nov 21, 2014 at 01:32:50PM -0800, Andy Lutomirski wrote: > >> On Fri, Nov 21, 2014 at 1:26 PM, Andy Lutomirski <luto@...capital.net> wrote: > >> > We currently pretend that IST context is like standard exception > >> > context, but this is incorrect. IST entries from userspace are like > >> > standard exceptions except that they use per-cpu stacks, so they are > >> > atomic. IST entries from kernel space are like NMIs from RCU's > >> > perspective -- they are not quiescent states even if they > >> > interrupted the kernel during a quiescent state. > >> > > >> > Add and use ist_enter and ist_exit to track IST context. Even > >> > though x86_32 has no IST stacks, we track these interrupts the same > >> > way. > >> > >> I should add: > >> > >> I have no idea why RCU read-side critical sections are safe inside > >> __do_page_fault today. It's guarded by exception_enter(), but that > >> doesn't do anything if context tracking is off, and context tracking > >> is usually off. What am I missing here? > > > > Ah! There are three cases: > > > > 1. Context tracking is off on a non-idle CPU. In this case, RCU is > > still paying attention to CPUs running in both userspace and in > > the kernel. So if a page fault happens, RCU will be set up to > > notice any RCU read-side critical sections. > > > > 2. Context tracking is on on a non-idle CPU. In this case, RCU > > might well be ignoring userspace execution: NO_HZ_FULL and > > all that. However, as you pointed out, in this case the > > context-tracking code lets RCU know that we have entered the > > kernel, which means that RCU will again be paying attention to > > RCU read-side critical sections. > > > > 3. The CPU is idle. In this case, RCU is ignoring the CPU, so > > if we take a page fault when context tracking is off, life > > will be hard. But the kernel is not supposed to take page > > faults in the idle loop, so this is not a problem. > > I guess so, as long as there are really no page faults in the idle loop. As far as I know, there are not. If there are, someone needs to let me know! ;-) > There are, however, machine checks in the idle loop, and maybe kprobes > (haven't checked), so I think this patch might fix real bugs. If you can get ISTs from the idle loop, then the patch is needed. > > Just out of curiosity... Can an NMI occur in IST context? If it can, > > I need to make rcu_nmi_enter() and rcu_nmi_exit() deal properly with > > nested calls. > > Yes, and vice versa. That code looked like it handled nesting > correctly, but I wasn't entirely sure. It currently does not, please see below patch. Are you able to test nesting? It would be really cool if you could do so -- I have no way to test this patch. > Also, just to make sure: are we okay if rcu_nmi_enter() is called > before exception_enter if context tracking is on and we came directly > from userspace? If I understand correctly, this will result in context tracking invoking rcu_user_enter(), which will result in the rcu_dynticks counter having an odd value. In that case, rcu_nmi_enter() will notice that RCU is already paying attention to this CPU via its check of atomic_read(&rdtp->dynticks) & 0x1), and will thus just return. The matching rcu_nmi_exit() will notice that the nesting count is zero, and will also just return. Thus, everything works in that case. In contrast, if rcu_nmi_enter() was invoked from the idle loop, it would see that RCU is not paying attention to this CPU and that the NMI nesting depth (which rcu_nmi_enter() increments) used to be zero. It would then atomically increment rtdp->dynticks, forcing RCU to start paying attention to this CPU. The matching rcu_nmi_exit() will see that the nesting count was non-zero, but became zero when decremented. This will cause rcu_nmi_exit() to atomically increment rtdp->dynticks, which will tell RCU to stop paying attention to this CPU. Thanx, Paul ------------------------------------------------------------------------ rcu: Make rcu_nmi_enter() handle nesting Andy Lutomirski is introducing ISTs into x86, which from RCU's viewpoint are NMIs. Because ISTs and NMIs can nest, rcu_nmi_enter() and rcu_nmi_exit() must now correctly handle nesting. As luck would have it, rcu_nmi_exit() handles nesting but rcu_nmi_enter() does not. This patch therefore makes rcu_nmi_enter() handle nesting. Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 8749f43f3f05..875421aff6e3 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -770,7 +770,8 @@ void rcu_nmi_enter(void) if (rdtp->dynticks_nmi_nesting == 0 && (atomic_read(&rdtp->dynticks) & 0x1)) return; - rdtp->dynticks_nmi_nesting++; + if (rdtp->dynticks_nmi_nesting++ != 0) + return; /* Nested NMI/IST/whatever. */ smp_mb__before_atomic(); /* Force delay from prior write. */ atomic_inc(&rdtp->dynticks); /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists