Messages in this thread | | | From | Kees Cook <> | Date | Wed, 27 Mar 2019 17:06:04 -0700 | Subject | Re: [PATCH] Convert struct pid count to refcount_t |
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On Wed, Mar 27, 2019 at 7:53 AM 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. No change in behavior if > CONFIG_REFCOUNT_FULL=n. > > Cc: keescook@chromium.org > Cc: kernel-team@android.com > Cc: kernel-hardening@lists.openwall.com > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > [...] > diff --git a/kernel/pid.c b/kernel/pid.c > index 20881598bdfa..2095c7da644d 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 +106,8 @@ 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_read(&pid->count) == 1) || > + refcount_dec_and_test(&pid->count)) {
Why is this (and the original code) safe in the face of a race against get_pid()? i.e. shouldn't this only use refcount_dec_and_test()? I don't see this code pattern anywhere else in the kernel.
Beyond that, yes please. :)
-Kees
> kmem_cache_free(ns->pid_cachep, pid); > put_pid_ns(ns); > } > @@ -210,7 +210,7 @@ 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]); > > -- > 2.21.0.392.gf8f6787159e-goog >
-- Kees Cook
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