[v2] Convert struct pid count to refcount_t
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Message ID 20190628193442.94745-1-joel@joelfernandes.org
State New
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Series
  • [v2] Convert struct pid count to refcount_t
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Commit Message

Joel Fernandes June 28, 2019, 7:34 p.m. UTC
struct pid's count is an atomic_t field used as a refcount. Use
refcount_t for it which is basically atomic_t but does additional
checking to prevent use-after-free bugs.

For memory ordering, the only change is with the following:
 -	if ((atomic_read(&pid->count) == 1) ||
 -	     atomic_dec_and_test(&pid->count)) {
 +	if (refcount_dec_and_test(&pid->count)) {
 		kmem_cache_free(ns->pid_cachep, pid);

Here the change is from:
Fully ordered --> RELEASE + ACQUIRE (as per refcount-vs-atomic.rst)
This ACQUIRE should take care of making sure the free happens after the
refcount_dec_and_test().

The above hunk also removes atomic_read() since it is not needed for the
code to work and it is unclear how beneficial it is. The removal lets
refcount_dec_and_test() check for cases where get_pid() happened before
the object was freed.

Cc: mathieu.desnoyers@efficios.com
Cc: willy@infradead.org
Cc: peterz@infradead.org
Cc: will.deacon@arm.com
Cc: paulmck@linux.vnet.ibm.com
Cc: elena.reshetova@intel.com
Cc: keescook@chromium.org
Cc: kernel-team@android.com
Cc: kernel-hardening@lists.openwall.com
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

---
Only change from v1->v2 is to get rid of the atomic_read().

 include/linux/pid.h | 5 +++--
 kernel/pid.c        | 7 +++----
 2 files changed, 6 insertions(+), 6 deletions(-)

Comments

Kees Cook June 28, 2019, 7:48 p.m. UTC | #1
On Fri, Jun 28, 2019 at 03:34:42PM -0400, Joel Fernandes (Google) wrote:
> struct pid's count is an atomic_t field used as a refcount. Use
> refcount_t for it which is basically atomic_t but does additional
> checking to prevent use-after-free bugs.
> 
> For memory ordering, the only change is with the following:
>  -	if ((atomic_read(&pid->count) == 1) ||
>  -	     atomic_dec_and_test(&pid->count)) {
>  +	if (refcount_dec_and_test(&pid->count)) {
>  		kmem_cache_free(ns->pid_cachep, pid);
> 
> Here the change is from:
> Fully ordered --> RELEASE + ACQUIRE (as per refcount-vs-atomic.rst)
> This ACQUIRE should take care of making sure the free happens after the
> refcount_dec_and_test().
> 
> The above hunk also removes atomic_read() since it is not needed for the
> code to work and it is unclear how beneficial it is. The removal lets
> refcount_dec_and_test() check for cases where get_pid() happened before
> the object was freed.
> 
> Cc: mathieu.desnoyers@efficios.com
> Cc: willy@infradead.org
> Cc: peterz@infradead.org
> Cc: will.deacon@arm.com
> Cc: paulmck@linux.vnet.ibm.com
> Cc: elena.reshetova@intel.com
> Cc: keescook@chromium.org
> Cc: kernel-team@android.com
> Cc: kernel-hardening@lists.openwall.com
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> 
> ---
> Only change from v1->v2 is to get rid of the atomic_read().
> 
>  include/linux/pid.h | 5 +++--
>  kernel/pid.c        | 7 +++----
>  2 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 14a9a39da9c7..8cb86d377ff5 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -3,6 +3,7 @@
>  #define _LINUX_PID_H
>  
>  #include <linux/rculist.h>
> +#include <linux/refcount.h>
>  
>  enum pid_type
>  {
> @@ -56,7 +57,7 @@ struct upid {
>  
>  struct pid
>  {
> -	atomic_t count;
> +	refcount_t count;
>  	unsigned int level;
>  	/* lists of tasks that use this pid */
>  	struct hlist_head tasks[PIDTYPE_MAX];
> @@ -69,7 +70,7 @@ extern struct pid init_struct_pid;
>  static inline struct pid *get_pid(struct pid *pid)
>  {
>  	if (pid)
> -		atomic_inc(&pid->count);
> +		refcount_inc(&pid->count);
>  	return pid;
>  }
>  
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 20881598bdfa..89c4849fab5d 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -37,7 +37,7 @@
>  #include <linux/init_task.h>
>  #include <linux/syscalls.h>
>  #include <linux/proc_ns.h>
> -#include <linux/proc_fs.h>
> +#include <linux/refcount.h>
>  #include <linux/sched/task.h>
>  #include <linux/idr.h>
>  
> @@ -106,8 +106,7 @@ void put_pid(struct pid *pid)
>  		return;
>  
>  	ns = pid->numbers[pid->level].ns;
> -	if ((atomic_read(&pid->count) == 1) ||
> -	     atomic_dec_and_test(&pid->count)) {
> +	if (refcount_dec_and_test(&pid->count)) {
>  		kmem_cache_free(ns->pid_cachep, pid);
>  		put_pid_ns(ns);
>  	}
> @@ -210,7 +209,7 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>  	}
>  
>  	get_pid_ns(ns);
> -	atomic_set(&pid->count, 1);
> +	refcount_set(&pid->count, 1);
>  	for (type = 0; type < PIDTYPE_MAX; ++type)
>  		INIT_HLIST_HEAD(&pid->tasks[type]);
>  
> -- 
> 2.22.0.410.gd8fdbe21b5-goog
Jann Horn July 1, 2019, 5:48 p.m. UTC | #2
On Fri, Jun 28, 2019 at 9:35 PM Joel Fernandes (Google)
<joel@joelfernandes.org> wrote:
> struct pid's count is an atomic_t field used as a refcount. Use
> refcount_t for it which is basically atomic_t but does additional
> checking to prevent use-after-free bugs.
[...]
>  struct pid
>  {
> -       atomic_t count;
> +       refcount_t count;
[...]
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 20881598bdfa..89c4849fab5d 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -37,7 +37,7 @@
>  #include <linux/init_task.h>
>  #include <linux/syscalls.h>
>  #include <linux/proc_ns.h>
> -#include <linux/proc_fs.h>
> +#include <linux/refcount.h>
>  #include <linux/sched/task.h>
>  #include <linux/idr.h>
>
> @@ -106,8 +106,7 @@ void put_pid(struct pid *pid)

init_struct_pid is defined as follows:

struct pid init_struct_pid = {
        .count          = ATOMIC_INIT(1),
[...]
};

This should be changed to REFCOUNT_INIT(1).

You should have received a compiler warning about this; I get the
following when trying to build with your patch applied:

jannh@jannh2:~/git/foreign/linux$ make kernel/pid.o
  CALL    scripts/checksyscalls.sh
  CALL    scripts/atomic/check-atomics.sh
  DESCEND  objtool
  CC      kernel/pid.o
kernel/pid.c:44:30: warning: missing braces around initializer
[-Wmissing-braces]
 struct pid init_struct_pid = {
                              ^
kernel/pid.c:44:30: warning: missing braces around initializer
[-Wmissing-braces]
kernel/pid.c:44:30: warning: missing braces around initializer
[-Wmissing-braces]
kernel/pid.c:44:30: warning: missing braces around initializer
[-Wmissing-braces]
kernel/pid.c:44:30: warning: missing braces around initializer
[-Wmissing-braces]
Joel Fernandes July 1, 2019, 6:25 p.m. UTC | #3
On Mon, Jul 01, 2019 at 07:48:26PM +0200, Jann Horn wrote:
> On Fri, Jun 28, 2019 at 9:35 PM Joel Fernandes (Google)
> <joel@joelfernandes.org> wrote:
> > struct pid's count is an atomic_t field used as a refcount. Use
> > refcount_t for it which is basically atomic_t but does additional
> > checking to prevent use-after-free bugs.
> [...]
> >  struct pid
> >  {
> > -       atomic_t count;
> > +       refcount_t count;
> [...]
> > diff --git a/kernel/pid.c b/kernel/pid.c
> > index 20881598bdfa..89c4849fab5d 100644
> > --- a/kernel/pid.c
> > +++ b/kernel/pid.c
> > @@ -37,7 +37,7 @@
> >  #include <linux/init_task.h>
> >  #include <linux/syscalls.h>
> >  #include <linux/proc_ns.h>
> > -#include <linux/proc_fs.h>
> > +#include <linux/refcount.h>
> >  #include <linux/sched/task.h>
> >  #include <linux/idr.h>
> >
> > @@ -106,8 +106,7 @@ void put_pid(struct pid *pid)
> 
> init_struct_pid is defined as follows:
> 
> struct pid init_struct_pid = {
>         .count          = ATOMIC_INIT(1),
> [...]
> };
> 
> This should be changed to REFCOUNT_INIT(1).
> 
> You should have received a compiler warning about this; I get the
> following when trying to build with your patch applied:

Thanks. Andrew had fixed this in patch v1 but Linus dropped it for other
reasons. Anyway, I should have fixed this in my resubmit.

Sorry, I'll fix and resend!

 - Joel
Joel Fernandes July 1, 2019, 6:48 p.m. UTC | #4
On Mon, Jul 1, 2019 at 2:47 PM kbuild test robot <lkp@intel.com> wrote:
>
> Hi "Joel,
>
> Thank you for the patch! Perhaps something to improve:
>
> [auto build test WARNING on linus/master]
> [also build test WARNING on v5.2-rc6]
> [cannot apply to next-20190625]
> [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

Jann H was faster than the robot in catching this, so humanity still has hope!

Its fixed in v3.

Patch
diff mbox series

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 14a9a39da9c7..8cb86d377ff5 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -3,6 +3,7 @@ 
 #define _LINUX_PID_H
 
 #include <linux/rculist.h>
+#include <linux/refcount.h>
 
 enum pid_type
 {
@@ -56,7 +57,7 @@  struct upid {
 
 struct pid
 {
-	atomic_t count;
+	refcount_t count;
 	unsigned int level;
 	/* lists of tasks that use this pid */
 	struct hlist_head tasks[PIDTYPE_MAX];
@@ -69,7 +70,7 @@  extern struct pid init_struct_pid;
 static inline struct pid *get_pid(struct pid *pid)
 {
 	if (pid)
-		atomic_inc(&pid->count);
+		refcount_inc(&pid->count);
 	return pid;
 }
 
diff --git a/kernel/pid.c b/kernel/pid.c
index 20881598bdfa..89c4849fab5d 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -37,7 +37,7 @@ 
 #include <linux/init_task.h>
 #include <linux/syscalls.h>
 #include <linux/proc_ns.h>
-#include <linux/proc_fs.h>
+#include <linux/refcount.h>
 #include <linux/sched/task.h>
 #include <linux/idr.h>
 
@@ -106,8 +106,7 @@  void put_pid(struct pid *pid)
 		return;
 
 	ns = pid->numbers[pid->level].ns;
-	if ((atomic_read(&pid->count) == 1) ||
-	     atomic_dec_and_test(&pid->count)) {
+	if (refcount_dec_and_test(&pid->count)) {
 		kmem_cache_free(ns->pid_cachep, pid);
 		put_pid_ns(ns);
 	}
@@ -210,7 +209,7 @@  struct pid *alloc_pid(struct pid_namespace *ns)
 	}
 
 	get_pid_ns(ns);
-	atomic_set(&pid->count, 1);
+	refcount_set(&pid->count, 1);
 	for (type = 0; type < PIDTYPE_MAX; ++type)
 		INIT_HLIST_HEAD(&pid->tasks[type]);