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SubjectRe: tip -ENOBOOT - bisected to locking/refcounts, x86/asm: Implement fast refcount overflow protection
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On Fri, 2017-09-01 at 08:57 +0200, Mike Galbraith wrote:
> On Thu, 2017-08-31 at 11:45 -0700, Kees Cook wrote:
> > On Thu, Aug 31, 2017 at 10:19 AM, Mike Galbraith <efault@gmx.de> wrote:
> > > On Thu, 2017-08-31 at 10:00 -0700, Kees Cook wrote:
> > >>
> > >> Oh! So it's gcc-version sensitive? That's alarming. Is this mapping correct:
> > >>
> > >> 4.8.5: WARN, eventual kernel hang
> > >> 6.3.1, 7.0.1: WARN, but continues working
> > >
> > > Yeah, that's correct. I find that troubling, simply because this gcc
> > > version has been through one hell of a lot of kernels with me. Yeah, I
> > > know, that doesn't exempt it from having bugs, but color me suspicious.
> >
> > I still can't hit this with a 4.8.5 build. :(
> >
> > With _RATELIMIT removed, this should, in theory, report whatever goes
> > negative first...
>
> I applied the other patch you posted, and built with gcc-6.3.1 to
> remove the gcc-4.8.5 aspect.  Look below the resulting splat.

Grr, that one has a in6_dev_getx() line missing for the first
increment, where things go pear shaped.

With that added, looking at counter both before, and after incl, with a
trace_printk() in the exception handler showing it doing its saturate
thing, irqs disabled across the whole damn refcount_inc(), and even
booting box nr_cpus=1 for extra credit...

HTH can that first refcount_inc() get there?

# tracer: nop
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
systemd-1 [000] d..1 1.937284: in6_dev_getx: PRE refs.counter:3
systemd-1 [000] d..1 1.937295: ex_handler_refcount: *(int *)regs->cx = -1073741824
systemd-1 [000] d..1 1.937296: in6_dev_getx: POST refs.counter:-1073741824
systemd-1 [000] d..1 1.937296: in6_dev_getx: PRE refs.counter:-1073741824
systemd-1 [000] d..1 1.937297: ex_handler_refcount: *(int *)regs->cx = -1073741824
systemd-1 [000] d..1 1.937297: in6_dev_getx: POST refs.counter:-1073741824
systemd-1 [000] d..1 1.937297: in6_dev_getx: PRE refs.counter:-1073741824
systemd-1 [000] d..1 1.937298: ex_handler_refcount: *(int *)regs->cx = -1073741824
systemd-1 [000] d..1 1.937299: in6_dev_getx: POST refs.counter:-1073741824

---
arch/x86/include/asm/refcount.h | 14 ++++++++++++++
arch/x86/mm/extable.c | 1 +
include/net/addrconf.h | 12 ++++++++++++
net/ipv6/route.c | 6 +++---
4 files changed, 30 insertions(+), 3 deletions(-)

--- a/arch/x86/include/asm/refcount.h
+++ b/arch/x86/include/asm/refcount.h
@@ -55,6 +55,20 @@ static __always_inline void refcount_inc
: : "cc", "cx");
}

+static __always_inline void refcount_inc_x(refcount_t *r)
+{
+ unsigned long flags;
+
+ local_irq_save(flags);
+ trace_printk("PRE refs.counter:%d\n", r->refs.counter);
+ asm volatile(LOCK_PREFIX "incl %0\n\t"
+ REFCOUNT_CHECK_LT_ZERO
+ : [counter] "+m" (r->refs.counter)
+ : : "cc", "cx");
+ trace_printk("POST refs.counter:%d\n", r->refs.counter);
+ local_irq_restore(flags);
+}
+
static __always_inline void refcount_dec(refcount_t *r)
{
asm volatile(LOCK_PREFIX "decl %0\n\t"
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -45,6 +45,7 @@ bool ex_handler_refcount(const struct ex
{
/* First unconditionally saturate the refcount. */
*(int *)regs->cx = INT_MIN / 2;
+ trace_printk("*(int *)regs->cx = %d\n", *(int *)regs->cx);

/*
* Strictly speaking, this reports the fixup destination, not
--- a/include/net/addrconf.h
+++ b/include/net/addrconf.h
@@ -321,6 +321,18 @@ static inline struct inet6_dev *in6_dev_
return idev;
}

+static inline struct inet6_dev *in6_dev_getx(const struct net_device *dev)
+{
+ struct inet6_dev *idev;
+
+ rcu_read_lock();
+ idev = rcu_dereference(dev->ip6_ptr);
+ if (idev)
+ refcount_inc_x(&idev->refcnt);
+ rcu_read_unlock();
+ return idev;
+}
+
static inline struct neigh_parms *__in6_dev_nd_parms_get_rcu(const struct net_device *dev)
{
struct inet6_dev *idev = __in6_dev_get(dev);
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -4041,12 +4041,12 @@ void __init ip6_route_init_special_entri
* the loopback reference in rt6_info will not be taken, do it
* manually for init_net */
init_net.ipv6.ip6_null_entry->dst.dev = init_net.loopback_dev;
- init_net.ipv6.ip6_null_entry->rt6i_idev = in6_dev_get(init_net.loopback_dev);
+ init_net.ipv6.ip6_null_entry->rt6i_idev = in6_dev_getx(init_net.loopback_dev);
#ifdef CONFIG_IPV6_MULTIPLE_TABLES
init_net.ipv6.ip6_prohibit_entry->dst.dev = init_net.loopback_dev;
- init_net.ipv6.ip6_prohibit_entry->rt6i_idev = in6_dev_get(init_net.loopback_dev);
+ init_net.ipv6.ip6_prohibit_entry->rt6i_idev = in6_dev_getx(init_net.loopback_dev);
init_net.ipv6.ip6_blk_hole_entry->dst.dev = init_net.loopback_dev;
- init_net.ipv6.ip6_blk_hole_entry->rt6i_idev = in6_dev_get(init_net.loopback_dev);
+ init_net.ipv6.ip6_blk_hole_entry->rt6i_idev = in6_dev_getx(init_net.loopback_dev);
#endif
}

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