| Message ID | 20190628193442.94745-1-joel@joelfernandes.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] Convert struct pid count to refcount_t | expand |
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
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]
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
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.
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]);
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(-)