[PATCH 0/3] v2 ipc subsystem refcount coversions

[PATCH 0/3] v2 ipc subsystem refcount coversions

[Elena Reshetova]

Changes in v2:
 * add a new patch on kern_ipc_perm.refcount
 * drop the patch on ipc_rcu.refcount
 * rebase on top of linux-next/master
 * Now by default refcount_t = atomic_t (*) and uses all atomic
   standard operations unless CONFIG_REFCOUNT_FULL is enabled.
   This is a compromise for the systems that are critical on
   performance (such as net) and cannot accept even slight delay
   on the refcounter operations.

This series, for ipc subsystem components, replaces atomic_t reference
counters with the new refcount_t type and API (see include/linux/refcount.h).
By doing this we prevent intentional or accidental
underflows or overflows that can led to use-after-free vulnerabilities.

The patches are fully independent and can be cherry-picked separately.
If there are no objections to the patches, please merge them via respective trees.

* The respective change is currently merged into -next as
  "locking/refcount: Create unchecked atomic_t implementation".

Elena Reshetova (3):
  ipc: convert ipc_namespace.count from atomic_t to refcount_t
  ipc: convert sem_undo_list.refcnt from atomic_t to refcount_t
  ipc: convert kern_ipc_perm.refcount from atomic_t to refcount_t

 include/linux/ipc.h           | 3 ++-
 include/linux/ipc_namespace.h | 5 +++--
 ipc/msgutil.c                 | 2 +-
 ipc/namespace.c               | 4 ++--
 ipc/sem.c                     | 8 ++++----
 ipc/util.c                    | 6 +++---
 6 files changed, 15 insertions(+), 13 deletions(-)

-- 
2.7.4

[PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 include/linux/ipc_namespace.h | 5 +++--
 ipc/msgutil.c                 | 2 +-
 ipc/namespace.c               | 4 ++--
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index 65327ee..e81445c 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -7,6 +7,7 @@
 #include <linux/notifier.h>
 #include <linux/nsproxy.h>
 #include <linux/ns_common.h>
+#include <linux/refcount.h>
 
 struct user_namespace;
 
@@ -19,7 +20,7 @@ struct ipc_ids {
 };
 
 struct ipc_namespace {
-	atomic_t	count;
+	refcount_t	count;
 	struct ipc_ids	ids[3];
 
 	int		sem_ctls[4];
@@ -118,7 +119,7 @@ extern struct ipc_namespace *copy_ipcs(unsigned long flags,
 static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns)
 {
 	if (ns)
-		atomic_inc(&ns->count);
+		refcount_inc(&ns->count);
 	return ns;
 }
 
diff --git a/ipc/msgutil.c b/ipc/msgutil.c
index bf74eaa..8459802 100644
--- a/ipc/msgutil.c
+++ b/ipc/msgutil.c
@@ -29,7 +29,7 @@ DEFINE_SPINLOCK(mq_lock);
  * and not CONFIG_IPC_NS.
  */
 struct ipc_namespace init_ipc_ns = {
-	.count		= ATOMIC_INIT(1),
+	.count		= REFCOUNT_INIT(1),
 	.user_ns = &init_user_ns,
 	.ns.inum = PROC_IPC_INIT_INO,
 #ifdef CONFIG_IPC_NS
diff --git a/ipc/namespace.c b/ipc/namespace.c
index b4d80f9..7af6e6b 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c
@@ -50,7 +50,7 @@ static struct ipc_namespace *create_ipc_ns(struct user_namespace *user_ns,
 		goto fail_free;
 	ns->ns.ops = &ipcns_operations;
 
-	atomic_set(&ns->count, 1);
+	refcount_set(&ns->count, 1);
 	ns->user_ns = get_user_ns(user_ns);
 	ns->ucounts = ucounts;
 
@@ -144,7 +144,7 @@ static void free_ipc_ns(struct ipc_namespace *ns)
  */
 void put_ipc_ns(struct ipc_namespace *ns)
 {
-	if (atomic_dec_and_lock(&ns->count, &mq_lock)) {
+	if (refcount_dec_and_lock(&ns->count, &mq_lock)) {
 		mq_clear_sbinfo(ns);
 		spin_unlock(&mq_lock);
 		mq_put_mnt(ns);
-- 
2.7.4

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Eric W. Biederman]

Elena Reshetova <elena.reshetova@intel.com> writes:

> refcount_t type and corresponding API should be
> used instead of atomic_t when the variable is used as
> a reference counter. This allows to avoid accidental
> refcounter overflows that might lead to use-after-free
> situations.

In this patch you can see all of the uses of the count.
What accidental refcount overflows are possible?

Eric

> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: David Windsor <dwindsor@gmail.com>
> ---
>  include/linux/ipc_namespace.h | 5 +++--
>  ipc/msgutil.c                 | 2 +-
>  ipc/namespace.c               | 4 ++--
>  3 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
> index 65327ee..e81445c 100644
> --- a/include/linux/ipc_namespace.h
> +++ b/include/linux/ipc_namespace.h
> @@ -7,6 +7,7 @@
>  #include <linux/notifier.h>
>  #include <linux/nsproxy.h>
>  #include <linux/ns_common.h>
> +#include <linux/refcount.h>
>  
>  struct user_namespace;
>  
> @@ -19,7 +20,7 @@ struct ipc_ids {
>  };
>  
>  struct ipc_namespace {
> -	atomic_t	count;
> +	refcount_t	count;
>  	struct ipc_ids	ids[3];
>  
>  	int		sem_ctls[4];
> @@ -118,7 +119,7 @@ extern struct ipc_namespace *copy_ipcs(unsigned long flags,
>  static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns)
>  {
>  	if (ns)
> -		atomic_inc(&ns->count);
> +		refcount_inc(&ns->count);
>  	return ns;
>  }
>  
> diff --git a/ipc/msgutil.c b/ipc/msgutil.c
> index bf74eaa..8459802 100644
> --- a/ipc/msgutil.c
> +++ b/ipc/msgutil.c
> @@ -29,7 +29,7 @@ DEFINE_SPINLOCK(mq_lock);
>   * and not CONFIG_IPC_NS.
>   */
>  struct ipc_namespace init_ipc_ns = {
> -	.count		= ATOMIC_INIT(1),
> +	.count		= REFCOUNT_INIT(1),
>  	.user_ns = &init_user_ns,
>  	.ns.inum = PROC_IPC_INIT_INO,
>  #ifdef CONFIG_IPC_NS
> diff --git a/ipc/namespace.c b/ipc/namespace.c
> index b4d80f9..7af6e6b 100644
> --- a/ipc/namespace.c
> +++ b/ipc/namespace.c
> @@ -50,7 +50,7 @@ static struct ipc_namespace *create_ipc_ns(struct user_namespace *user_ns,
>  		goto fail_free;
>  	ns->ns.ops = &ipcns_operations;
>  
> -	atomic_set(&ns->count, 1);
> +	refcount_set(&ns->count, 1);
>  	ns->user_ns = get_user_ns(user_ns);
>  	ns->ucounts = ucounts;
>  
> @@ -144,7 +144,7 @@ static void free_ipc_ns(struct ipc_namespace *ns)
>   */
>  void put_ipc_ns(struct ipc_namespace *ns)
>  {
> -	if (atomic_dec_and_lock(&ns->count, &mq_lock)) {
> +	if (refcount_dec_and_lock(&ns->count, &mq_lock)) {
>  		mq_clear_sbinfo(ns);
>  		spin_unlock(&mq_lock);
>  		mq_put_mnt(ns);

RE: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Reshetova, Elena]

> Elena Reshetova <elena.reshetova@intel.com> writes:
> 
> > refcount_t type and corresponding API should be
> > used instead of atomic_t when the variable is used as
> > a reference counter. This allows to avoid accidental
> > refcounter overflows that might lead to use-after-free
> > situations.
> 
> In this patch you can see all of the uses of the count.
> What accidental refcount overflows are possible?

Even if one can guarantee and prove that in the current implementation
there are no overflows possible, we can't say that for
sure for any future implementation. Bugs might always happen
unfortunately, but if we convert the refcounter to a safer
type we can be sure that overflows are not possible. 

Does it make sense to you?

Best Regards,
Elena.

> 
> Eric
> 
> > Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> > Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > Signed-off-by: David Windsor <dwindsor@gmail.com>
> > ---
> >  include/linux/ipc_namespace.h | 5 +++--
> >  ipc/msgutil.c                 | 2 +-
> >  ipc/namespace.c               | 4 ++--
> >  3 files changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
> > index 65327ee..e81445c 100644
> > --- a/include/linux/ipc_namespace.h
> > +++ b/include/linux/ipc_namespace.h
> > @@ -7,6 +7,7 @@
> >  #include <linux/notifier.h>
> >  #include <linux/nsproxy.h>
> >  #include <linux/ns_common.h>
> > +#include <linux/refcount.h>
> >
> >  struct user_namespace;
> >
> > @@ -19,7 +20,7 @@ struct ipc_ids {
> >  };
> >
> >  struct ipc_namespace {
> > -	atomic_t	count;
> > +	refcount_t	count;
> >  	struct ipc_ids	ids[3];
> >
> >  	int		sem_ctls[4];
> > @@ -118,7 +119,7 @@ extern struct ipc_namespace *copy_ipcs(unsigned long
> flags,
> >  static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns)
> >  {
> >  	if (ns)
> > -		atomic_inc(&ns->count);
> > +		refcount_inc(&ns->count);
> >  	return ns;
> >  }
> >
> > diff --git a/ipc/msgutil.c b/ipc/msgutil.c
> > index bf74eaa..8459802 100644
> > --- a/ipc/msgutil.c
> > +++ b/ipc/msgutil.c
> > @@ -29,7 +29,7 @@ DEFINE_SPINLOCK(mq_lock);
> >   * and not CONFIG_IPC_NS.
> >   */
> >  struct ipc_namespace init_ipc_ns = {
> > -	.count		= ATOMIC_INIT(1),
> > +	.count		= REFCOUNT_INIT(1),
> >  	.user_ns = &init_user_ns,
> >  	.ns.inum = PROC_IPC_INIT_INO,
> >  #ifdef CONFIG_IPC_NS
> > diff --git a/ipc/namespace.c b/ipc/namespace.c
> > index b4d80f9..7af6e6b 100644
> > --- a/ipc/namespace.c
> > +++ b/ipc/namespace.c
> > @@ -50,7 +50,7 @@ static struct ipc_namespace *create_ipc_ns(struct
> user_namespace *user_ns,
> >  		goto fail_free;
> >  	ns->ns.ops = &ipcns_operations;
> >
> > -	atomic_set(&ns->count, 1);
> > +	refcount_set(&ns->count, 1);
> >  	ns->user_ns = get_user_ns(user_ns);
> >  	ns->ucounts = ucounts;
> >
> > @@ -144,7 +144,7 @@ static void free_ipc_ns(struct ipc_namespace *ns)
> >   */
> >  void put_ipc_ns(struct ipc_namespace *ns)
> >  {
> > -	if (atomic_dec_and_lock(&ns->count, &mq_lock)) {
> > +	if (refcount_dec_and_lock(&ns->count, &mq_lock)) {
> >  		mq_clear_sbinfo(ns);
> >  		spin_unlock(&mq_lock);
> >  		mq_put_mnt(ns);

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Eric W. Biederman]

"Reshetova, Elena" <elena.reshetova@intel.com> writes:

2>> Elena Reshetova <elena.reshetova@intel.com> writes:
>> 
>> > refcount_t type and corresponding API should be
>> > used instead of atomic_t when the variable is used as
>> > a reference counter. This allows to avoid accidental
>> > refcounter overflows that might lead to use-after-free
>> > situations.
>> 
>> In this patch you can see all of the uses of the count.
>> What accidental refcount overflows are possible?
>
> Even if one can guarantee and prove that in the current implementation
> there are no overflows possible, we can't say that for
> sure for any future implementation. Bugs might always happen
> unfortunately, but if we convert the refcounter to a safer
> type we can be sure that overflows are not possible. 
>
> Does it make sense to you?

Not for code that is likely to remain unchanged for a decade no.

This looks like a large set of unautomated changes without any real
thought put into it.  That almost always results in a typo somewhere
that breaks things.

So there is no benefit to the code, and a non-zero chance that there
will be a typo breaking the code.

All to harden the code for an unlikely future when the code is
updated with a full test cycle and people paying attention.

Introduce a bug now to avoid a bug in the future.  That seems like a
very poor engineering trade off.

Eric

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Alexey Dobriyan]

On Mon, Jul 10, 2017 at 11:37 AM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
> "Reshetova, Elena" <elena.reshetova@intel.com> writes:
>
> 2>> Elena Reshetova <elena.reshetova@intel.com> writes:
>>>
>>> > refcount_t type and corresponding API should be
>>> > used instead of atomic_t when the variable is used as
>>> > a reference counter. This allows to avoid accidental
>>> > refcounter overflows that might lead to use-after-free
>>> > situations.
>>>
>>> In this patch you can see all of the uses of the count.
>>> What accidental refcount overflows are possible?
>>
>> Even if one can guarantee and prove that in the current implementation
>> there are no overflows possible, we can't say that for
>> sure for any future implementation. Bugs might always happen
>> unfortunately, but if we convert the refcounter to a safer
>> type we can be sure that overflows are not possible.
>>
>> Does it make sense to you?
>
> Not for code that is likely to remain unchanged for a decade no.
>
> This looks like a large set of unautomated changes without any real
> thought put into it.  That almost always results in a typo somewhere
> that breaks things.

This is nonsense. The wrong code would simply emit a warning
which are caught very quickly.

> So there is no benefit to the code, and a non-zero chance that there
> will be a typo breaking the code.

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Eric W. Biederman]

Alexey Dobriyan <adobriyan@gmail.com> writes:

> On Mon, Jul 10, 2017 at 11:37 AM, Eric W. Biederman
> <ebiederm@xmission.com> wrote:
>> "Reshetova, Elena" <elena.reshetova@intel.com> writes:
>>
>> 2>> Elena Reshetova <elena.reshetova@intel.com> writes:
>>>>
>>>> > refcount_t type and corresponding API should be
>>>> > used instead of atomic_t when the variable is used as
>>>> > a reference counter. This allows to avoid accidental
>>>> > refcounter overflows that might lead to use-after-free
>>>> > situations.
>>>>
>>>> In this patch you can see all of the uses of the count.
>>>> What accidental refcount overflows are possible?
>>>
>>> Even if one can guarantee and prove that in the current implementation
>>> there are no overflows possible, we can't say that for
>>> sure for any future implementation. Bugs might always happen
>>> unfortunately, but if we convert the refcounter to a safer
>>> type we can be sure that overflows are not possible.
>>>
>>> Does it make sense to you?
>>
>> Not for code that is likely to remain unchanged for a decade no.
>>
>> This looks like a large set of unautomated changes without any real
>> thought put into it.  That almost always results in a typo somewhere
>> that breaks things.
>
> This is nonsense. The wrong code would simply emit a warning
> which are caught very quickly.

That depends on the typo.

Eric

RE: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Reshetova, Elena]

> "Reshetova, Elena" <elena.reshetova@intel.com> writes:
> 
> 2>> Elena Reshetova <elena.reshetova@intel.com> writes:
> >>
> >> > refcount_t type and corresponding API should be
> >> > used instead of atomic_t when the variable is used as
> >> > a reference counter. This allows to avoid accidental
> >> > refcounter overflows that might lead to use-after-free
> >> > situations.
> >>
> >> In this patch you can see all of the uses of the count.
> >> What accidental refcount overflows are possible?
> >
> > Even if one can guarantee and prove that in the current implementation
> > there are no overflows possible, we can't say that for
> > sure for any future implementation. Bugs might always happen
> > unfortunately, but if we convert the refcounter to a safer
> > type we can be sure that overflows are not possible.
> >
> > Does it make sense to you?
> 
> Not for code that is likely to remain unchanged for a decade no.

Can we really be sure for any kernel code about this? And does it make
sense to trust our security on a fact like this?

> 
> This looks like a large set of unautomated changes without any real
> thought put into it. 

We are soon into the end of the first year that we started to look into
refcounter overflow/underflow problem and coming up this far was 
not easy enough (just check all the millions of emails on kernel-hardening
mailing list). Each refcount_t conversion candidate was first found by Coccinelle
analysis and then manually checked and converted. The story of 
refcount_t API and all discussions go even further. 
So you can't really claim that there is no " thought put into it " :)


 That almost always results in a typo somewhere
> that breaks things.
> 
> So there is no benefit to the code, and a non-zero chance that there
> will be a typo breaking the code.

The code is very active on issuing WARNs when anything goes wrong. 
Using this feature we have not only found errors in conversions, but
sometimes errors in code itself. So, any bug would be actually much
faster visible than using old atomic_t interface. 

In addition by default refcount_t equals to atomic, which also gives a
possibility to make a softer transition and catch all related bugs in couple
of cycles when enabling CONFIG_REFCOUNT_FULL. 


Best Regards,
Elena.

 
> 
> All to harden the code for an unlikely future when the code is
> updated with a full test cycle and people paying attention.
> 
> Introduce a bug now to avoid a bug in the future.  That seems like a
> very poor engineering trade off.
> 
> Eric

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Eric W. Biederman]

"Reshetova, Elena" <elena.reshetova@intel.com> writes:

>> "Reshetova, Elena" <elena.reshetova@intel.com> writes:
>> 
>> 2>> Elena Reshetova <elena.reshetova@intel.com> writes:
>> >>
>> >> > refcount_t type and corresponding API should be
>> >> > used instead of atomic_t when the variable is used as
>> >> > a reference counter. This allows to avoid accidental
>> >> > refcounter overflows that might lead to use-after-free
>> >> > situations.
>> >>
>> >> In this patch you can see all of the uses of the count.
>> >> What accidental refcount overflows are possible?
>> >
>> > Even if one can guarantee and prove that in the current implementation
>> > there are no overflows possible, we can't say that for
>> > sure for any future implementation. Bugs might always happen
>> > unfortunately, but if we convert the refcounter to a safer
>> > type we can be sure that overflows are not possible.
>> >
>> > Does it make sense to you?
>> 
>> Not for code that is likely to remain unchanged for a decade no.
>
> Can we really be sure for any kernel code about this? And does it make
> sense to trust our security on a fact like this?

But refcount_t doesn't fix anything.  At best it changes a bad bug to a
less bad bug.  So now my machine OOMS instead of allows a memory
overwrite.   It still doesn't work.

Plus refcount_t does not provide any safety on the architectures where
it is a noop.

>> This looks like a large set of unautomated changes without any real
>> thought put into it. 
>
> We are soon into the end of the first year that we started to look into
> refcounter overflow/underflow problem and coming up this far was 
> not easy enough (just check all the millions of emails on kernel-hardening
> mailing list). Each refcount_t conversion candidate was first found by Coccinelle
> analysis and then manually checked and converted. The story of 
> refcount_t API and all discussions go even further. 
> So you can't really claim that there is no " thought put into it " :)

But the conversion of the instance happens without thought and manually.
Which is a good recipe for typos.  Which is what I am saying.

There have been lots of conversions like that in the kernel and
practically every one has introduced at least one typo.

So from an engineering standpoint it is a very valid question to ask
about.  And I find the apparent insistence that you don't make typos
very disturbing.

>  That almost always results in a typo somewhere
>> that breaks things.
>> 
>> So there is no benefit to the code, and a non-zero chance that there
>> will be a typo breaking the code.
>
> The code is very active on issuing WARNs when anything goes wrong. 
> Using this feature we have not only found errors in conversions, but
> sometimes errors in code itself. So, any bug would be actually much
> faster visible than using old atomic_t interface. 
>
> In addition by default refcount_t equals to atomic, which also gives a
> possibility to make a softer transition and catch all related bugs in couple
> of cycles when enabling CONFIG_REFCOUNT_FULL.

But if you make a typo and change one operation for another I don't see
how any of that applies.

And that is what it looks like I we are looking at here.

Eric

RE: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Reshetova, Elena]

> "Reshetova, Elena" <elena.reshetova@intel.com> writes:
> 
> >> "Reshetova, Elena" <elena.reshetova@intel.com> writes:
> >>
> >> 2>> Elena Reshetova <elena.reshetova@intel.com> writes:
> >> >>
> >> >> > refcount_t type and corresponding API should be
> >> >> > used instead of atomic_t when the variable is used as
> >> >> > a reference counter. This allows to avoid accidental
> >> >> > refcounter overflows that might lead to use-after-free
> >> >> > situations.
> >> >>
> >> >> In this patch you can see all of the uses of the count.
> >> >> What accidental refcount overflows are possible?
> >> >
> >> > Even if one can guarantee and prove that in the current implementation
> >> > there are no overflows possible, we can't say that for
> >> > sure for any future implementation. Bugs might always happen
> >> > unfortunately, but if we convert the refcounter to a safer
> >> > type we can be sure that overflows are not possible.
> >> >
> >> > Does it make sense to you?
> >>
> >> Not for code that is likely to remain unchanged for a decade no.
> >
> > Can we really be sure for any kernel code about this? And does it make
> > sense to trust our security on a fact like this?
> 
> But refcount_t doesn't fix anything.  At best it changes a bad bug to a
> less bad bug.  So now my machine OOMS instead of allows a memory
> overwrite.   It still doesn't work.

Well, it is a step forward from security standpoint. OOMS is really hard
to exploit vs. memory overwrites. Pretty much all exploits need either
memory write or memory read, out of memory is really much harder to
exploit. 

> 
> Plus refcount_t does not provide any safety on the architectures where
> it is a noop.

Not sure I understood this. What do you mean by "noop"? 
refcount_t is currently architecture independent. 


> 
> >> This looks like a large set of unautomated changes without any real
> >> thought put into it.
> >
> > We are soon into the end of the first year that we started to look into
> > refcounter overflow/underflow problem and coming up this far was
> > not easy enough (just check all the millions of emails on kernel-hardening
> > mailing list). Each refcount_t conversion candidate was first found by Coccinelle
> > analysis and then manually checked and converted. The story of
> > refcount_t API and all discussions go even further.
> > So you can't really claim that there is no " thought put into it " :)
> 
> But the conversion of the instance happens without thought and manually.
> Which is a good recipe for typos.  Which is what I am saying.
> 
> There have been lots of conversions like that in the kernel and
> practically every one has introduced at least one typo.

What do you exactly mean by "typo"? Typos should be detected at these stages:

1) typos like wrong function name etc. can be found at compile time
    (trust me I have found a number of these on the very first iteration with patches)
2) "typos" (not sure if it is correct to call them typos) like usage of wrong refcount_t
      API vs. original atomic_t API can be found during internal reviews or reviews by maintainers
3) much bigger problem is actually not any typos, but hidden issues that show up only
    in run-time that detect underflows/overflows or inability to increment from zero. 
 These only are nasty, but given that refcount_t WARNs left and right about them,
  we can detect them fast. 

I don't know what is a better recipe for doing API changes like this?
Do you have any suggestions? 

> 
> So from an engineering standpoint it is a very valid question to ask
> about.  And I find the apparent insistence that you don't make typos
> very disturbing.
> 
> >  That almost always results in a typo somewhere
> >> that breaks things.
> >>
> >> So there is no benefit to the code, and a non-zero chance that there
> >> will be a typo breaking the code.
> >
> > The code is very active on issuing WARNs when anything goes wrong.
> > Using this feature we have not only found errors in conversions, but
> > sometimes errors in code itself. So, any bug would be actually much
> > faster visible than using old atomic_t interface.
> >
> > In addition by default refcount_t equals to atomic, which also gives a
> > possibility to make a softer transition and catch all related bugs in couple
> > of cycles when enabling CONFIG_REFCOUNT_FULL.
> 
> But if you make a typo and change one operation for another I don't see
> how any of that applies.

It is hard to make a "typo" to change one operation to another. It is not a
one-two char mismatch error. When doing these patches we followed the
logic of "less code changes - better" (since less chances of making mistake),
so if in some cases functions are changed (like from atomic_sub to
refcount_sub_and_test(), or from atomic_inc_not_zero() to atomic_inc() etc.)
there was a reason for making it and the change wasn't automatic and without
thinking at all. Again, we do have our maintainers also to catch if a change that
we did doesn't actually work for them right?

Best Regards,
Elena. 
> 
> And that is what it looks like I we are looking at here.
> 
> Eric

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Eric W. Biederman]

"Reshetova, Elena" <elena.reshetova@intel.com> writes:

>> "Reshetova, Elena" <elena.reshetova@intel.com> writes:
>> 
>> >> "Reshetova, Elena" <elena.reshetova@intel.com> writes:
>> >>
>> >> 2>> Elena Reshetova <elena.reshetova@intel.com> writes:
>> >> >>
>> >> >> > refcount_t type and corresponding API should be
>> >> >> > used instead of atomic_t when the variable is used as
>> >> >> > a reference counter. This allows to avoid accidental
>> >> >> > refcounter overflows that might lead to use-after-free
>> >> >> > situations.
>> >> >>
>> >> >> In this patch you can see all of the uses of the count.
>> >> >> What accidental refcount overflows are possible?
>> >> >
>> >> > Even if one can guarantee and prove that in the current implementation
>> >> > there are no overflows possible, we can't say that for
>> >> > sure for any future implementation. Bugs might always happen
>> >> > unfortunately, but if we convert the refcounter to a safer
>> >> > type we can be sure that overflows are not possible.
>> >> >
>> >> > Does it make sense to you?
>> >>
>> >> Not for code that is likely to remain unchanged for a decade no.
>> >
>> > Can we really be sure for any kernel code about this? And does it make
>> > sense to trust our security on a fact like this?
>> 
>> But refcount_t doesn't fix anything.  At best it changes a bad bug to a
>> less bad bug.  So now my machine OOMS instead of allows a memory
>> overwrite.   It still doesn't work.
>
> Well, it is a step forward from security standpoint. OOMS is really hard
> to exploit vs. memory overwrites. Pretty much all exploits need either
> memory write or memory read, out of memory is really much harder to
> exploit.

OOM in production is a denial of service attack.  From a serverity point
of view an OOM can be considered equivalent to a kernel panic and
something that requires a box to reboot.

>From a long term perspective I expect we will need to change all
reference counters to a type where that is not saturating but instead
fails to increment and returns an error.  If we want to keep a system
functioning in the face of maxing out a reference count that is the only
way it can truly be done.

>> Plus refcount_t does not provide any safety on the architectures where
>> it is a noop.
>
> Not sure I understood this. What do you mean by "noop"? 
> refcount_t is currently architecture independent.

noop being short for no operation.

I believe there were some/most archicture implementations that define
refcount_t to atomic_t out of performance concerns.  I know I saw
patches fly by to that effect.  On an architecture where refcount_t is
equivalent to atomic_t the change in these patches is a noop.

>> >> This looks like a large set of unautomated changes without any real
>> >> thought put into it.
>> >
>> > We are soon into the end of the first year that we started to look into
>> > refcounter overflow/underflow problem and coming up this far was
>> > not easy enough (just check all the millions of emails on kernel-hardening
>> > mailing list). Each refcount_t conversion candidate was first found by Coccinelle
>> > analysis and then manually checked and converted. The story of
>> > refcount_t API and all discussions go even further.
>> > So you can't really claim that there is no " thought put into it " :)
>> 
>> But the conversion of the instance happens without thought and manually.
>> Which is a good recipe for typos.  Which is what I am saying.
>> 
>> There have been lots of conversions like that in the kernel and
>> practically every one has introduced at least one typo.
>
> What do you exactly mean by "typo"? Typos should be detected at these
> stages:

An unintentional mistake.  The term "thinko" might be more what I am
thinking of.   Many human mistakes are not slips of the fingers but
accidentally giving the wrong command at some level.  

> 1) typos like wrong function name etc. can be found at compile time
>     (trust me I have found a number of these on the very first iteration with patches)
> 2) "typos" (not sure if it is correct to call them typos) like usage of wrong refcount_t
>       API vs. original atomic_t API can be found during internal reviews or reviews by maintainers

This I worry about most as the mental distance from xxx_inc to xxx_dec
can be very short.

> 3) much bigger problem is actually not any typos, but hidden issues that show up only
>     in run-time that detect underflows/overflows or inability to increment from zero. 
>  These only are nasty, but given that refcount_t WARNs left and right about them,
>   we can detect them fast.

Which means I worry about those less.

> I don't know what is a better recipe for doing API changes like this?
> Do you have any suggestions?

I would think a semantic patch targeting a specific lock would be less
error prone.  I would think that the same semantic patch could be used
from lock to lock with just a change of the lock that is being targeted.

I strongly suspect that would reduce the chance of accident when dealing
with a particular API and being scripted would increase the confidence
in the changes.

>> So from an engineering standpoint it is a very valid question to ask
>> about.  And I find the apparent insistence that you don't make typos
>> very disturbing.
>> 
>> >  That almost always results in a typo somewhere
>> >> that breaks things.
>> >>
>> >> So there is no benefit to the code, and a non-zero chance that there
>> >> will be a typo breaking the code.
>> >
>> > The code is very active on issuing WARNs when anything goes wrong.
>> > Using this feature we have not only found errors in conversions, but
>> > sometimes errors in code itself. So, any bug would be actually much
>> > faster visible than using old atomic_t interface.
>> >
>> > In addition by default refcount_t equals to atomic, which also gives a
>> > possibility to make a softer transition and catch all related bugs in couple
>> > of cycles when enabling CONFIG_REFCOUNT_FULL.
>> 
>> But if you make a typo and change one operation for another I don't see
>> how any of that applies.
>
> It is hard to make a "typo" to change one operation to another. It is not a
> one-two char mismatch error.

Those might be more properly "thinkos" but they do happen.

> When doing these patches we followed the
> logic of "less code changes - better" (since less chances of making mistake),
> so if in some cases functions are changed (like from atomic_sub to
> refcount_sub_and_test(), or from atomic_inc_not_zero() to atomic_inc() etc.)
> there was a reason for making it and the change wasn't automatic and without
> thinking at all. Again, we do have our maintainers also to catch if a change that
> we did doesn't actually work for them right?

In general a maintainers job is make certain the appropriate code review
and due dilligence has happened not nececessarily to perform that code
review themselves.

Changing from atomoic_inc_not_zero to a simple refcount_inc really
troubles me because it makes it harder to switch to something fully
correct.

Eric

RE: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Reshetova, Elena]

 
> "Reshetova, Elena" <elena.reshetova@intel.com> writes:
> 
> >> "Reshetova, Elena" <elena.reshetova@intel.com> writes:
> >>
> >> >> "Reshetova, Elena" <elena.reshetova@intel.com> writes:
> >> >>
> >> >> 2>> Elena Reshetova <elena.reshetova@intel.com> writes:
> >> >> >>
> >> >> >> > refcount_t type and corresponding API should be
> >> >> >> > used instead of atomic_t when the variable is used as
> >> >> >> > a reference counter. This allows to avoid accidental
> >> >> >> > refcounter overflows that might lead to use-after-free
> >> >> >> > situations.
> >> >> >>
> >> >> >> In this patch you can see all of the uses of the count.
> >> >> >> What accidental refcount overflows are possible?
> >> >> >
> >> >> > Even if one can guarantee and prove that in the current implementation
> >> >> > there are no overflows possible, we can't say that for
> >> >> > sure for any future implementation. Bugs might always happen
> >> >> > unfortunately, but if we convert the refcounter to a safer
> >> >> > type we can be sure that overflows are not possible.
> >> >> >
> >> >> > Does it make sense to you?
> >> >>
> >> >> Not for code that is likely to remain unchanged for a decade no.
> >> >
> >> > Can we really be sure for any kernel code about this? And does it make
> >> > sense to trust our security on a fact like this?
> >>
> >> But refcount_t doesn't fix anything.  At best it changes a bad bug to a
> >> less bad bug.  So now my machine OOMS instead of allows a memory
> >> overwrite.   It still doesn't work.
> >
> > Well, it is a step forward from security standpoint. OOMS is really hard
> > to exploit vs. memory overwrites. Pretty much all exploits need either
> > memory write or memory read, out of memory is really much harder to
> > exploit.
> 
> OOM in production is a denial of service attack.  From a serverity point
> of view an OOM can be considered equivalent to a kernel panic and
> something that requires a box to reboot.

Denial of service is usually (unless the system is designed poorly) not the
highest security concern. It is one thing to get your system down for a
certain period of time vs. telling your customers that all your database
of their data and credit cards info is now public in the Internet. 
So, I would still stand by the statement that it makes the situation better.

> 
> From a long term perspective I expect we will need to change all
> reference counters to a type where that is not saturating but instead
> fails to increment and returns an error.  If we want to keep a system
> functioning in the face of maxing out a reference count that is the only
> way it can truly be done.

I think the work on refcount_t internals will continue. We have a number
of things that need to be improved further. Kees has first set of patches
in work now, and no one expects it to be the last. However, we also do
need to make conversions in the kernel, otherwise we are not moving
anywhere and it is also easier to test implications of changes inside
refcount_t  on as many converted cases as possible when they are merged.

> 
> >> Plus refcount_t does not provide any safety on the architectures where
> >> it is a noop.
> >
> > Not sure I understood this. What do you mean by "noop"?
> > refcount_t is currently architecture independent.
> 
> noop being short for no operation.
> 
> I believe there were some/most archicture implementations that define
> refcount_t to atomic_t out of performance concerns.  I know I saw
> patches fly by to that effect.  On an architecture where refcount_t is
> equivalent to atomic_t the change in these patches is a noop.

This choice isn't architecture-dependent now. It is defaults to atomic_t for all
architectures unless CONFIG_REFCOUNT_FULL is enabled
(then it is arch. independent refcount_t implementation). Next set of
patches (still under review) provides x86 arch. dependent fast
refcount_t implementation. 
> 
> >> >> This looks like a large set of unautomated changes without any real
> >> >> thought put into it.
> >> >
> >> > We are soon into the end of the first year that we started to look into
> >> > refcounter overflow/underflow problem and coming up this far was
> >> > not easy enough (just check all the millions of emails on kernel-hardening
> >> > mailing list). Each refcount_t conversion candidate was first found by
> Coccinelle
> >> > analysis and then manually checked and converted. The story of
> >> > refcount_t API and all discussions go even further.
> >> > So you can't really claim that there is no " thought put into it " :)
> >>
> >> But the conversion of the instance happens without thought and manually.
> >> Which is a good recipe for typos.  Which is what I am saying.
> >>
> >> There have been lots of conversions like that in the kernel and
> >> practically every one has introduced at least one typo.
> >
> > What do you exactly mean by "typo"? Typos should be detected at these
> > stages:
> 
> An unintentional mistake.  The term "thinko" might be more what I am
> thinking of.   Many human mistakes are not slips of the fingers but
> accidentally giving the wrong command at some level.
> 
> > 1) typos like wrong function name etc. can be found at compile time
> >     (trust me I have found a number of these on the very first iteration with
> patches)
> > 2) "typos" (not sure if it is correct to call them typos) like usage of wrong
> refcount_t
> >       API vs. original atomic_t API can be found during internal reviews or reviews
> by maintainers
> 
> This I worry about most as the mental distance from xxx_inc to xxx_dec
> can be very short.

I would say the distance is huge :) Can't think of possible way one would
make such a mistake. 

> 
> > 3) much bigger problem is actually not any typos, but hidden issues that show
> up only
> >     in run-time that detect underflows/overflows or inability to increment from
> zero.
> >  These only are nasty, but given that refcount_t WARNs left and right about
> them,
> >   we can detect them fast.
> 
> Which means I worry about those less.
> 
> > I don't know what is a better recipe for doing API changes like this?
> > Do you have any suggestions?
> 
> I would think a semantic patch targeting a specific lock would be less
> error prone.  I would think that the same semantic patch could be used
> from lock to lock with just a change of the lock that is being targeted.

I was considering in the beginning of writing a full cocci semantic patch, 
but based on how spread the code for refcounters might be (take smth like
socket's refcounter), how many different edge cases we had,
the semantic patch would be so complex (and I
would even not really trust the output personally), needs manual verification
anyway etc. So, at the end we end up with just using .cocci file to find
occurrences, but do conversions manually. And when I say manually, it
didn't mean that I manually typed each function name, the process was
roughly like this:

1. take a look on the reported occurrence. Determine if it should be
converted at the first place
2. If needs conversion, run substitute atomic_* to refcount_* (not this
only changes prefixes)
3. resolve cases that need manual addressing (i.e refcount_* analog
function doesn't simply exists)
4. check the conversion overall if everything makes sense together, 
fix other issues (remove unneeded WARNs, change inc to set for
initializing refcounting etc.)

Note the 4th step has many of these angles.
> 
> I strongly suspect that would reduce the chance of accident when dealing
> with a particular API and being scripted would increase the confidence
> in the changes.

Even if I made a wrong decision with it, I guess now it doesn't matter so much
because we have our conversions now and I am very confident that if there
are mistakes there, they could not be caught by semantic patch. Unless we
write now a script to check the validity of each patch, which is probably
doable, but I don't think I have energy for this left. Any volunteers? :) 
> 
> >> So from an engineering standpoint it is a very valid question to ask
> >> about.  And I find the apparent insistence that you don't make typos
> >> very disturbing.
> >>
> >> >  That almost always results in a typo somewhere
> >> >> that breaks things.
> >> >>
> >> >> So there is no benefit to the code, and a non-zero chance that there
> >> >> will be a typo breaking the code.
> >> >
> >> > The code is very active on issuing WARNs when anything goes wrong.
> >> > Using this feature we have not only found errors in conversions, but
> >> > sometimes errors in code itself. So, any bug would be actually much
> >> > faster visible than using old atomic_t interface.
> >> >
> >> > In addition by default refcount_t equals to atomic, which also gives a
> >> > possibility to make a softer transition and catch all related bugs in couple
> >> > of cycles when enabling CONFIG_REFCOUNT_FULL.
> >>
> >> But if you make a typo and change one operation for another I don't see
> >> how any of that applies.
> >
> > It is hard to make a "typo" to change one operation to another. It is not a
> > one-two char mismatch error.
> 
> Those might be more properly "thinkos" but they do happen.
> 
> > When doing these patches we followed the
> > logic of "less code changes - better" (since less chances of making mistake),
> > so if in some cases functions are changed (like from atomic_sub to
> > refcount_sub_and_test(), or from atomic_inc_not_zero() to atomic_inc() etc.)
> > there was a reason for making it and the change wasn't automatic and without
> > thinking at all. Again, we do have our maintainers also to catch if a change that
> > we did doesn't actually work for them right?
> 
> In general a maintainers job is make certain the appropriate code review
> and due dilligence has happened not nececessarily to perform that code
> review themselves.
> 
> Changing from atomoic_inc_not_zero to a simple refcount_inc really
> troubles me because it makes it harder to switch to something fully
> correct.

I actually meant case like this (just came up recently, so stayed in my head):

static void get_pi_state(struct futex_pi_state *pi_state)
 {
-	WARN_ON_ONCE(!atomic_inc_not_zero(&pi_state->refcount));
+	refcount_inc(&pi_state->refcount);

Just to avoid extra layer of unneeded warning. But even with cases like this
as I said we were super conservative and changing as little as possible unless
It is 100% clear case or people suggest the change during patch review. 

Best Regards,
Elena.


> 
> Eric

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Andrew Morton]

On Sun, 09 Jul 2017 16:59:55 -0500 ebiederm@xmission.com (Eric W. Biederman) wrote:

> Elena Reshetova <elena.reshetova@intel.com> writes:
> 
> > refcount_t type and corresponding API should be
> > used instead of atomic_t when the variable is used as
> > a reference counter. This allows to avoid accidental
> > refcounter overflows that might lead to use-after-free
> > situations.
> 
> In this patch you can see all of the uses of the count.
> What accidental refcount overflows are possible?

I do rather dislike these conversions from the point of view of
performance overhead and general code bloat.  But I seem to have lost
that struggle and I don't think any of these are fastpath(?).

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Davidlohr Bueso]

On Wed, 19 Jul 2017, Andrew Morton wrote:

>I do rather dislike these conversions from the point of view of
>performance overhead and general code bloat.  But I seem to have lost
>that struggle and I don't think any of these are fastpath(?).

Well, since we now have fd25d19 (locking/refcount: Create unchecked atomic_t
implementation), performance is supposed to be ok. It would be lovely to have
some actual numbers nonetheless.

Thanks,
Davidlohr

>

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Andrew Morton]

On Wed, 19 Jul 2017 15:54:27 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote:

> On Wed, 19 Jul 2017, Andrew Morton wrote:
> 
> >I do rather dislike these conversions from the point of view of
> >performance overhead and general code bloat.  But I seem to have lost
> >that struggle and I don't think any of these are fastpath(?).
> 
> Well, since we now have fd25d19 (locking/refcount: Create unchecked atomic_t
> implementation), performance is supposed to be ok.

Sure, things are OK for people who disable the feature.

But for people who want to enable the feature we really should minimize
the cost by avoiding blindly converting sites which simply don't need
it: simple, safe, old, well-tested code.  Why go and slow down such
code?  Need to apply some common sense here...

> It would be lovely to have
> some actual numbers nonetheless.

Very much so.

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Davidlohr Bueso]

On Wed, 19 Jul 2017, Andrew Morton wrote:

>On Wed, 19 Jul 2017 15:54:27 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote:
>
>> On Wed, 19 Jul 2017, Andrew Morton wrote:
>>
>> >I do rather dislike these conversions from the point of view of
>> >performance overhead and general code bloat.  But I seem to have lost
>> >that struggle and I don't think any of these are fastpath(?).
>>
>> Well, since we now have fd25d19 (locking/refcount: Create unchecked atomic_t
>> implementation), performance is supposed to be ok.
>
>Sure, things are OK for people who disable the feature.
>
>But for people who want to enable the feature we really should minimize
>the cost by avoiding blindly converting sites which simply don't need
>it: simple, safe, old, well-tested code.  Why go and slow down such
>code?  Need to apply some common sense here...

Fair points.

>
>> It would be lovely to have
>> some actual numbers nonetheless.
>
>Very much so.

May I suggest using mmtests with the following config file:

https://github.com/gormanm/mmtests/blob/7e070a810bc0af92e592e5121d0ea75fada51aeb/configs/config-global-dhp__workload-ipc-scale-short

It will run two of Manfred's ipcscale sem benchmarks.

Thanks,
Davidlohr

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Kees Cook]

On Wed, Jul 19, 2017 at 4:11 PM, Davidlohr Bueso <dave@stgolabs.net> wrote:
> On Wed, 19 Jul 2017, Andrew Morton wrote:
>
>> On Wed, 19 Jul 2017 15:54:27 -0700 Davidlohr Bueso <dave@stgolabs.net>
>> wrote:
>>
>>> On Wed, 19 Jul 2017, Andrew Morton wrote:
>>>
>>> >I do rather dislike these conversions from the point of view of
>>> >performance overhead and general code bloat.  But I seem to have lost
>>> >that struggle and I don't think any of these are fastpath(?).
>>>
>>> Well, since we now have fd25d19 (locking/refcount: Create unchecked
>>> atomic_t
>>> implementation), performance is supposed to be ok.
>>
>>
>> Sure, things are OK for people who disable the feature.

FWIW, it's off by default.

>>
>> But for people who want to enable the feature we really should minimize
>> the cost by avoiding blindly converting sites which simply don't need
>> it: simple, safe, old, well-tested code.  Why go and slow down such
>> code?  Need to apply some common sense here...

These are the very code paths we'd want to make sure are well
protected since people may never expect them to misbehave when some
"small change" goes in.

> Fair points.
>
>>> It would be lovely to have
>>> some actual numbers nonetheless.
>>
>> Very much so.
>
> May I suggest using mmtests with the following config file:
>
> https://github.com/gormanm/mmtests/blob/7e070a810bc0af92e592e5121d0ea75fada51aeb/configs/config-global-dhp__workload-ipc-scale-short
>
> It will run two of Manfred's ipcscale sem benchmarks.

I'll see if I can figure out how to use this for testing the fast
refcount protection:
https://lkml.org/lkml/2017/7/18/1223

Then we could see:

before conversion
after conversion
with CONFIG_REFCOUNT_FULL
with fast refcount protection

-Kees

-- 
Kees Cook
Pixel Security

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Kees Cook]

On Wed, Jul 19, 2017 at 4:20 PM, Kees Cook <keescook@chromium.org> wrote:
> On Wed, Jul 19, 2017 at 4:11 PM, Davidlohr Bueso <dave@stgolabs.net> wrote:
>> May I suggest using mmtests with the following config file:
>>
>> https://github.com/gormanm/mmtests/blob/7e070a810bc0af92e592e5121d0ea75fada51aeb/configs/config-global-dhp__workload-ipc-scale-short
>>
>> It will run two of Manfred's ipcscale sem benchmarks.
>
> I'll see if I can figure out how to use this for testing the fast
> refcount protection:
> https://lkml.org/lkml/2017/7/18/1223
>
> Then we could see:
>
> before conversion
> after conversion
> with CONFIG_REFCOUNT_FULL
> with fast refcount protection

I have no idea how to read this report. It seems to be mostly noise
(multiple baseline runs seem to show greater variability than compared
against the other possible results).

Test runs were atomic_t, atomic_t-2, refcount_t, refcount-full, and
refcount-fast. (Two baselines, refcount_t conversion, with FULL, and
with the fast implementation.) Output here:
http://pastebin.ubuntu.com/25129382/

-Kees

-- 
Kees Cook
Pixel Security

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Ingo Molnar]

* Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed, 19 Jul 2017 15:54:27 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote:
> 
> > On Wed, 19 Jul 2017, Andrew Morton wrote:
> > 
> > >I do rather dislike these conversions from the point of view of
> > >performance overhead and general code bloat.  But I seem to have lost
> > >that struggle and I don't think any of these are fastpath(?).
> > 
> > Well, since we now have fd25d19 (locking/refcount: Create unchecked atomic_t
> > implementation), performance is supposed to be ok.
> 
> Sure, things are OK for people who disable the feature.

So with the WIP fast-refcount series from Kees:

	[PATCH v6 0/2] x86: Implement fast refcount overflow protection

I believe the robustness difference between optimized-refcount_t and 
full-refcount_t will be marginal.

I.e. we'll be able to have both higher API safety _and_ performance.

> But for people who want to enable the feature we really should minimize the cost 
> by avoiding blindly converting sites which simply don't need it: simple, safe, 
> old, well-tested code.  Why go and slow down such code?  Need to apply some 
> common sense here...

It's old, well-tested code _for existing, sane parameters_, until someone finds a 
decade old bug in one of these with an insane parameters no-one stumbled upon so 
far, and builds an exploit on top of it.

Only by touching all these places do we have a chance to improve things measurably 
in terms of reducing the probability of bugs.

Thanks,

	Ingo

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Eric W. Biederman]

Ingo Molnar <mingo@kernel.org> writes:

> * Andrew Morton <akpm@linux-foundation.org> wrote:
>
>> On Wed, 19 Jul 2017 15:54:27 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote:
>> 
>> > On Wed, 19 Jul 2017, Andrew Morton wrote:
>> > 
>> > >I do rather dislike these conversions from the point of view of
>> > >performance overhead and general code bloat.  But I seem to have lost
>> > >that struggle and I don't think any of these are fastpath(?).
>> > 
>> > Well, since we now have fd25d19 (locking/refcount: Create unchecked atomic_t
>> > implementation), performance is supposed to be ok.
>> 
>> Sure, things are OK for people who disable the feature.
>
> So with the WIP fast-refcount series from Kees:
>
> 	[PATCH v6 0/2] x86: Implement fast refcount overflow protection
>
> I believe the robustness difference between optimized-refcount_t and 
> full-refcount_t will be marginal.
>
> I.e. we'll be able to have both higher API safety _and_ performance.
>
>> But for people who want to enable the feature we really should minimize the cost 
>> by avoiding blindly converting sites which simply don't need it: simple, safe, 
>> old, well-tested code.  Why go and slow down such code?  Need to apply some 
>> common sense here...
>
> It's old, well-tested code _for existing, sane parameters_, until someone finds a 
> decade old bug in one of these with an insane parameters no-one stumbled upon so 
> far, and builds an exploit on top of it.
>
> Only by touching all these places do we have a chance to improve things measurably 
> in terms of reducing the probability of bugs.

The more I hear people pushing the upsides of refcount_t without
considering the downsides the more I dislike it.

- refcount_t is really the wrong thing because it uses saturation
  semantics.  So by definition it includes a bug.

- refcount_t will only really prevent something if there is an extra
  increment.  That is not the kind of bug people are likely to make.

- refcount_t won't help if you have an extra decrement.  The bad
  use-after-free will still happen.

- refcount_t won't help if there is a memory stomp.  As with an extra
  decrement the bad use-after-free will still happen.
  
So all I see is a huge amount of code churn to implement a buggy (by
definition) refcounting API, that risks adding new bugs and only truly
helps with bugs that are unlikely in the first place.

I really don't think this is an obvious slam dunk.

Eric





  
 

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Kees Cook]

On Thu, Jul 20, 2017 at 5:34 AM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
> Ingo Molnar <mingo@kernel.org> writes:
>
>> * Andrew Morton <akpm@linux-foundation.org> wrote:
>>
>>> On Wed, 19 Jul 2017 15:54:27 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote:
>>>
>>> > On Wed, 19 Jul 2017, Andrew Morton wrote:
>>> >
>>> > >I do rather dislike these conversions from the point of view of
>>> > >performance overhead and general code bloat.  But I seem to have lost
>>> > >that struggle and I don't think any of these are fastpath(?).
>>> >
>>> > Well, since we now have fd25d19 (locking/refcount: Create unchecked atomic_t
>>> > implementation), performance is supposed to be ok.
>>>
>>> Sure, things are OK for people who disable the feature.
>>
>> So with the WIP fast-refcount series from Kees:
>>
>>       [PATCH v6 0/2] x86: Implement fast refcount overflow protection
>>
>> I believe the robustness difference between optimized-refcount_t and
>> full-refcount_t will be marginal.
>>
>> I.e. we'll be able to have both higher API safety _and_ performance.
>>
>>> But for people who want to enable the feature we really should minimize the cost
>>> by avoiding blindly converting sites which simply don't need it: simple, safe,
>>> old, well-tested code.  Why go and slow down such code?  Need to apply some
>>> common sense here...
>>
>> It's old, well-tested code _for existing, sane parameters_, until someone finds a
>> decade old bug in one of these with an insane parameters no-one stumbled upon so
>> far, and builds an exploit on top of it.
>>
>> Only by touching all these places do we have a chance to improve things measurably
>> in terms of reducing the probability of bugs.
>
> The more I hear people pushing the upsides of refcount_t without
> considering the downsides the more I dislike it.
>
> - refcount_t is really the wrong thing because it uses saturation
>   semantics.  So by definition it includes a bug.

This is a feature, not a bug. :) If the kernel has a refcount overflow
flaw (which, in the pantheon of exploitable kernel bugs, is
_common_[1], as I've referenced earlier), then we're downgrading an
exploitable use-after-free to a harmless memory allocation leak. Even
if you don't include malicious attackers in the consideration, this
changes a memory corruption of unknown results into a memory leak.
That's actually an _improvement_ to availability and integrity.

> - refcount_t will only really prevent something if there is an extra
>   increment.  That is not the kind of bug people are likely to make.

Like I've said, this is common. This is usually a mistake in error
handling which forgets (or misplaces) a "put".

> - refcount_t won't help if you have an extra decrement.  The bad
>   use-after-free will still happen.

Yes, and not having a protected refcount_t will also allow a
use-after-free. There is no change here, so it's not a "downside" of
refcount_t. In fact, having gained the implicit annotation of
refcount_t being a refcounter (rather than a simple atomic_t) means
that auditing users is easier and more focused. This could reduce the
chance people make mistakes in the first place, especially since the
API is more constrained than atomic_t.

> - refcount_t won't help if there is a memory stomp.  As with an extra
>   decrement the bad use-after-free will still happen.

A stomp of the refcount_t value itself? Sure, and this remains as
vulnerable as atomic_t. This isn't a downside to refcount_t. And
again, since there _is_ checking of the value in places, it's possible
an actionable warning will be produced (though, yes, after the
use-after-free has been exposed), which is a benefit over simple
atomic_t. I mention this in the commit log ("better to maybe produce
the warning than be universally silent").

> So all I see is a huge amount of code churn to implement a buggy (by
> definition) refcounting API, that risks adding new bugs and only truly
> helps with bugs that are unlikely in the first place.

Given that the conversions alone have been uncovering refcount bugs
and that the implementation isn't "buggy" (it provides a specific set
of protections), I strongly disagree with your assessment.

> I really don't think this is an obvious slam dunk.

It entirely blocks a commonly exploitable flaw in the kernel. This
isn't a probabilistic mitigation, either. While I'm not sure I'd ever
describe a security protection as a slam dunk, I think this is up
there. :)

-Kees

[1] When I say "common", I'm speaking from the perspective of security
flaw frequency. The kernel sees about 1-2 high severity security flaws
a year (with an average lifetime of 5 years), and the
refcount-overflow use-after-free class of flaw is normally reliable
for attackers (and I'd classify as high severity). With 2016 seeing
two known separate refcount-overflow use-after-free flaws, this could
be better described as an epidemic, but I'll try to be less
inflammatory and just say "common".

-- 
Kees Cook
Pixel Security

[PATCH 2/3] ipc: convert sem_undo_list.refcnt from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 ipc/sem.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index 9e70cd7..d3ab140 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -122,7 +122,7 @@ struct sem_undo {
  * that may be shared among all a CLONE_SYSVSEM task group.
  */
 struct sem_undo_list {
-	atomic_t		refcnt;
+	refcount_t		refcnt;
 	spinlock_t		lock;
 	struct list_head	list_proc;
 };
@@ -1642,7 +1642,7 @@ static inline int get_undo_list(struct sem_undo_list **undo_listp)
 		if (undo_list == NULL)
 			return -ENOMEM;
 		spin_lock_init(&undo_list->lock);
-		atomic_set(&undo_list->refcnt, 1);
+		refcount_set(&undo_list->refcnt, 1);
 		INIT_LIST_HEAD(&undo_list->list_proc);
 
 		current->sysvsem.undo_list = undo_list;
@@ -2041,7 +2041,7 @@ int copy_semundo(unsigned long clone_flags, struct task_struct *tsk)
 		error = get_undo_list(&undo_list);
 		if (error)
 			return error;
-		atomic_inc(&undo_list->refcnt);
+		refcount_inc(&undo_list->refcnt);
 		tsk->sysvsem.undo_list = undo_list;
 	} else
 		tsk->sysvsem.undo_list = NULL;
@@ -2070,7 +2070,7 @@ void exit_sem(struct task_struct *tsk)
 		return;
 	tsk->sysvsem.undo_list = NULL;
 
-	if (!atomic_dec_and_test(&ulp->refcnt))
+	if (!refcount_dec_and_test(&ulp->refcnt))
 		return;
 
 	for (;;) {
-- 
2.7.4

[PATCH 3/3] ipc: convert kern_ipc_perm.refcount from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 include/linux/ipc.h | 3 ++-
 ipc/util.c          | 6 +++---
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/include/linux/ipc.h b/include/linux/ipc.h
index fadd579..ae68980 100644
--- a/include/linux/ipc.h
+++ b/include/linux/ipc.h
@@ -4,6 +4,7 @@
 #include <linux/spinlock.h>
 #include <linux/uidgid.h>
 #include <uapi/linux/ipc.h>
+#include <linux/refcount.h>
 
 #define IPCMNI 32768  /* <= MAX_INT limit for ipc arrays (including sysctl changes) */
 
@@ -22,7 +23,7 @@ struct kern_ipc_perm {
 	void		*security;
 
 	struct rcu_head rcu;
-	atomic_t refcount;
+	refcount_t refcount;
 } ____cacheline_aligned_in_smp __randomize_layout;
 
 #endif /* _LINUX_IPC_H */
diff --git a/ipc/util.c b/ipc/util.c
index 1a2cb02..069bb22 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -232,7 +232,7 @@ int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int size)
 
 	idr_preload(GFP_KERNEL);
 
-	atomic_set(&new->refcount, 1);
+	refcount_set(&new->refcount, 1);
 	spin_lock_init(&new->lock);
 	new->deleted = false;
 	rcu_read_lock();
@@ -397,13 +397,13 @@ void ipc_rmid(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
 
 int ipc_rcu_getref(struct kern_ipc_perm *ptr)
 {
-	return atomic_inc_not_zero(&ptr->refcount);
+	return refcount_inc_not_zero(&ptr->refcount);
 }
 
 void ipc_rcu_putref(struct kern_ipc_perm *ptr,
 			void (*func)(struct rcu_head *head))
 {
-	if (!atomic_dec_and_test(&ptr->refcount))
+	if (!refcount_dec_and_test(&ptr->refcount))
 		return;
 
 	call_rcu(&ptr->rcu, func);
-- 
2.7.4

[PATCH 0/3] ipc subsystem refcounter conversions

[Elena Reshetova]

Now when new refcount_t type and API are finally merged
(see include/linux/refcount.h), the following
patches convert various refcounters in the ipc susystem from atomic_t
to refcount_t. By doing this we prevent intentional or accidental
underflows or overflows that can led to use-after-free vulnerabilities.

The below patches are fully independent and can be cherry-picked separately.
Since we convert all kernel subsystems in the same fashion, resulting
in about 300 patches, we have to group them for sending at least in some
fashion to be manageable. Please excuse the long cc list.

Elena Reshetova (3):
  ipc: convert ipc_namespace.count from atomic_t to refcount_t
  ipc: convert sem_undo_list.refcnt from atomic_t to refcount_t
  ipc: convert ipc_rcu.refcount from atomic_t to refcount_t

 include/linux/ipc_namespace.h | 5 +++--
 ipc/msgutil.c                 | 2 +-
 ipc/namespace.c               | 4 ++--
 ipc/sem.c                     | 8 ++++----
 ipc/util.c                    | 6 +++---
 ipc/util.h                    | 3 ++-
 6 files changed, 15 insertions(+), 13 deletions(-)

-- 
2.7.4

[PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Elena Reshetova]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
---
 include/linux/ipc_namespace.h | 5 +++--
 ipc/msgutil.c                 | 2 +-
 ipc/namespace.c               | 4 ++--
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index 848e579..7230638 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -7,6 +7,7 @@
 #include <linux/notifier.h>
 #include <linux/nsproxy.h>
 #include <linux/ns_common.h>
+#include <linux/refcount.h>
 
 struct user_namespace;
 
@@ -19,7 +20,7 @@ struct ipc_ids {
 };
 
 struct ipc_namespace {
-	atomic_t	count;
+	refcount_t	count;
 	struct ipc_ids	ids[3];
 
 	int		sem_ctls[4];
@@ -118,7 +119,7 @@ extern struct ipc_namespace *copy_ipcs(unsigned long flags,
 static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns)
 {
 	if (ns)
-		atomic_inc(&ns->count);
+		refcount_inc(&ns->count);
 	return ns;
 }
 
diff --git a/ipc/msgutil.c b/ipc/msgutil.c
index bf74eaa..8459802 100644
--- a/ipc/msgutil.c
+++ b/ipc/msgutil.c
@@ -29,7 +29,7 @@ DEFINE_SPINLOCK(mq_lock);
  * and not CONFIG_IPC_NS.
  */
 struct ipc_namespace init_ipc_ns = {
-	.count		= ATOMIC_INIT(1),
+	.count		= REFCOUNT_INIT(1),
 	.user_ns = &init_user_ns,
 	.ns.inum = PROC_IPC_INIT_INO,
 #ifdef CONFIG_IPC_NS
diff --git a/ipc/namespace.c b/ipc/namespace.c
index 0abdea4..ed10bbc 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c
@@ -48,7 +48,7 @@ static struct ipc_namespace *create_ipc_ns(struct user_namespace *user_ns,
 		goto fail_free;
 	ns->ns.ops = &ipcns_operations;
 
-	atomic_set(&ns->count, 1);
+	refcount_set(&ns->count, 1);
 	ns->user_ns = get_user_ns(user_ns);
 	ns->ucounts = ucounts;
 
@@ -142,7 +142,7 @@ static void free_ipc_ns(struct ipc_namespace *ns)
  */
 void put_ipc_ns(struct ipc_namespace *ns)
 {
-	if (atomic_dec_and_lock(&ns->count, &mq_lock)) {
+	if (refcount_dec_and_lock(&ns->count, &mq_lock)) {
 		mq_clear_sbinfo(ns);
 		spin_unlock(&mq_lock);
 		mq_put_mnt(ns);
-- 
2.7.4

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Kees Cook]

On Mon, Feb 20, 2017 at 3:29 AM, Elena Reshetova
<elena.reshetova@intel.com> wrote:
> refcount_t type and corresponding API should be
> used instead of atomic_t when the variable is used as
> a reference counter. This allows to avoid accidental
> refcounter overflows that might lead to use-after-free
> situations.
>
> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: David Windsor <dwindsor@gmail.com>

Manfred, is this okay by you? I think this should go via -mm...

-Kees

> ---
>  include/linux/ipc_namespace.h | 5 +++--
>  ipc/msgutil.c                 | 2 +-
>  ipc/namespace.c               | 4 ++--
>  3 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
> index 848e579..7230638 100644
> --- a/include/linux/ipc_namespace.h
> +++ b/include/linux/ipc_namespace.h
> @@ -7,6 +7,7 @@
>  #include <linux/notifier.h>
>  #include <linux/nsproxy.h>
>  #include <linux/ns_common.h>
> +#include <linux/refcount.h>
>
>  struct user_namespace;
>
> @@ -19,7 +20,7 @@ struct ipc_ids {
>  };
>
>  struct ipc_namespace {
> -       atomic_t        count;
> +       refcount_t      count;
>         struct ipc_ids  ids[3];
>
>         int             sem_ctls[4];
> @@ -118,7 +119,7 @@ extern struct ipc_namespace *copy_ipcs(unsigned long flags,
>  static inline struct ipc_namespace *get_ipc_ns(struct ipc_namespace *ns)
>  {
>         if (ns)
> -               atomic_inc(&ns->count);
> +               refcount_inc(&ns->count);
>         return ns;
>  }
>
> diff --git a/ipc/msgutil.c b/ipc/msgutil.c
> index bf74eaa..8459802 100644
> --- a/ipc/msgutil.c
> +++ b/ipc/msgutil.c
> @@ -29,7 +29,7 @@ DEFINE_SPINLOCK(mq_lock);
>   * and not CONFIG_IPC_NS.
>   */
>  struct ipc_namespace init_ipc_ns = {
> -       .count          = ATOMIC_INIT(1),
> +       .count          = REFCOUNT_INIT(1),
>         .user_ns = &init_user_ns,
>         .ns.inum = PROC_IPC_INIT_INO,
>  #ifdef CONFIG_IPC_NS
> diff --git a/ipc/namespace.c b/ipc/namespace.c
> index 0abdea4..ed10bbc 100644
> --- a/ipc/namespace.c
> +++ b/ipc/namespace.c
> @@ -48,7 +48,7 @@ static struct ipc_namespace *create_ipc_ns(struct user_namespace *user_ns,
>                 goto fail_free;
>         ns->ns.ops = &ipcns_operations;
>
> -       atomic_set(&ns->count, 1);
> +       refcount_set(&ns->count, 1);
>         ns->user_ns = get_user_ns(user_ns);
>         ns->ucounts = ucounts;
>
> @@ -142,7 +142,7 @@ static void free_ipc_ns(struct ipc_namespace *ns)
>   */
>  void put_ipc_ns(struct ipc_namespace *ns)
>  {
> -       if (atomic_dec_and_lock(&ns->count, &mq_lock)) {
> +       if (refcount_dec_and_lock(&ns->count, &mq_lock)) {
>                 mq_clear_sbinfo(ns);
>                 spin_unlock(&mq_lock);
>                 mq_put_mnt(ns);
> --
> 2.7.4
>



-- 
Kees Cook
Pixel Security

Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount_t

[Manfred Spraul]

On 05/27/2017 09:41 PM, Kees Cook wrote:
> On Mon, Feb 20, 2017 at 3:29 AM, Elena Reshetova
> <elena.reshetova@intel.com> wrote:
>> refcount_t type and corresponding API should be
>> used instead of atomic_t when the variable is used as
>> a reference counter. This allows to avoid accidental
>> refcounter overflows that might lead to use-after-free
>> situations.
>>
>> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
>> Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> Signed-off-by: David Windsor <dwindsor@gmail.com>
> Manfred, is this okay by you? I think this should go via -mm...
The patch is ok, the refcounters are not in the critical path.
I'll try to test the patches and then I would send an 'Acked-by', but 
this will take some days.

--
     Manfred