In use after free situations, it is possible for one thread to write to
memory that has just been reallocated to a new user. This could open up
potential security issues.
Close off those potential security holes by terminating the current
thread when kref encounters such a race condition or underflow.
Signed-off-by: Mateusz Guzik <[email protected]>
Cc: Rik van Riel <[email protected]>
---
include/linux/kref.h | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/include/linux/kref.h b/include/linux/kref.h
index 484604d..c3f8a0a 100644
--- a/include/linux/kref.h
+++ b/include/linux/kref.h
@@ -43,8 +43,10 @@ static inline void kref_get(struct kref *kref)
/* If refcount was 0 before incrementing then we have a race
* condition when this kref is freeing by some other thread right now.
* In this case one should use kref_get_unless_zero()
+ *
+ * Terminate the current thread to stop potential security exploits.
*/
- WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
+ BUG_ON(atomic_inc_return(&kref->refcount) < 2);
}
/**
--
1.8.3.1
On Thu, Feb 20, 2014 at 06:44:59PM +0100, Mateusz Guzik wrote:
> In use after free situations, it is possible for one thread to write to
> memory that has just been reallocated to a new user. This could open up
> potential security issues.
>
> diff --git a/include/linux/kref.h b/include/linux/kref.h
> index 484604d..c3f8a0a 100644
> --- a/include/linux/kref.h
> +++ b/include/linux/kref.h
> @@ -43,8 +43,10 @@ static inline void kref_get(struct kref *kref)
> /* If refcount was 0 before incrementing then we have a race
> * condition when this kref is freeing by some other thread right now.
> * In this case one should use kref_get_unless_zero()
> + *
> + * Terminate the current thread to stop potential security exploits.
> */
> - WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
> + BUG_ON(atomic_inc_return(&kref->refcount) < 2);
This isn't "terminating the thread", this is "lock up the box".
Dave
On 02/20/2014 01:14 PM, Dave Jones wrote:
> On Thu, Feb 20, 2014 at 06:44:59PM +0100, Mateusz Guzik wrote:
> > In use after free situations, it is possible for one thread to write to
> > memory that has just been reallocated to a new user. This could open up
> > potential security issues.
> >
> > diff --git a/include/linux/kref.h b/include/linux/kref.h
> > index 484604d..c3f8a0a 100644
> > --- a/include/linux/kref.h
> > +++ b/include/linux/kref.h
> > @@ -43,8 +43,10 @@ static inline void kref_get(struct kref *kref)
> > /* If refcount was 0 before incrementing then we have a race
> > * condition when this kref is freeing by some other thread right now.
> > * In this case one should use kref_get_unless_zero()
> > + *
> > + * Terminate the current thread to stop potential security exploits.
> > */
> > - WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
> > + BUG_ON(atomic_inc_return(&kref->refcount) < 2);
>
> This isn't "terminating the thread", this is "lock up the box".
Only if kref_get holds a lock while encountering a refcount
underflow, right?
--
All rights reversed
On Thu, Feb 20, 2014 at 01:14:40PM -0500, Dave Jones wrote:
> On Thu, Feb 20, 2014 at 06:44:59PM +0100, Mateusz Guzik wrote:
> > In use after free situations, it is possible for one thread to write to
> > memory that has just been reallocated to a new user. This could open up
> > potential security issues.
> >
> > diff --git a/include/linux/kref.h b/include/linux/kref.h
> > index 484604d..c3f8a0a 100644
> > --- a/include/linux/kref.h
> > +++ b/include/linux/kref.h
> > @@ -43,8 +43,10 @@ static inline void kref_get(struct kref *kref)
> > /* If refcount was 0 before incrementing then we have a race
> > * condition when this kref is freeing by some other thread right now.
> > * In this case one should use kref_get_unless_zero()
> > + *
> > + * Terminate the current thread to stop potential security exploits.
> > */
> > - WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
> > + BUG_ON(atomic_inc_return(&kref->refcount) < 2);
>
> This isn't "terminating the thread", this is "lock up the box".
>
Well, extent of damage caused by non-panicking BUG_ON (if any) depends
on the state when kref_get was executed.
However, since this condition is already a sign of big trouble (and a
potential exploitation attempt), I think a WARN_ON_ONCE is not
sufficient.
That said, can you elaborate on your concers? You just don't like that
comment, don't want that BUG_ON (want a panic instead) or maybe you
don't like the change at all (or something else)?
Thanks,
--
Mateusz Guzik
On Thu, Feb 20, 2014 at 01:17:44PM -0500, Rik van Riel wrote:
> On 02/20/2014 01:14 PM, Dave Jones wrote:
> > On Thu, Feb 20, 2014 at 06:44:59PM +0100, Mateusz Guzik wrote:
> > > In use after free situations, it is possible for one thread to write to
> > > memory that has just been reallocated to a new user. This could open up
> > > potential security issues.
> > >
> > > diff --git a/include/linux/kref.h b/include/linux/kref.h
> > > index 484604d..c3f8a0a 100644
> > > --- a/include/linux/kref.h
> > > +++ b/include/linux/kref.h
> > > @@ -43,8 +43,10 @@ static inline void kref_get(struct kref *kref)
> > > /* If refcount was 0 before incrementing then we have a race
> > > * condition when this kref is freeing by some other thread right now.
> > > * In this case one should use kref_get_unless_zero()
> > > + *
> > > + * Terminate the current thread to stop potential security exploits.
> > > */
> > > - WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2);
> > > + BUG_ON(atomic_inc_return(&kref->refcount) < 2);
> >
> > This isn't "terminating the thread", this is "lock up the box".
>
> Only if kref_get holds a lock while encountering a refcount
> underflow, right?
>
Yes, and in a quick glance through the tree it seems we have several
codesites where we can find such condition likely to happen,
unfortunately.