On Wed, 9 Oct 2019 at 13:31, Christian Brauner christian.brauner@ubuntu.com wrote:
When assiging and testing taskstats in taskstats_exit() there's a race when writing and reading sig->stats when a thread-group with more than one thread exits:
cpu0: thread catches fatal signal and whole thread-group gets taken down do_exit() do_group_exit() taskstats_exit() taskstats_tgid_alloc() The tasks reads sig->stats without holding sighand lock seeing garbage.
cpu1: task calls exit_group() do_exit() do_group_exit() taskstats_exit() taskstats_tgid_alloc() The task takes sighand lock and assigns new stats to sig->stats.
Fix this by using smp_load_acquire() and smp_store_release().
Reported-by: syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation") Cc: stable@vger.kernel.org Signed-off-by: Christian Brauner christian.brauner@ubuntu.com Reviewed-by: Dmitry Vyukov dvyukov@google.com
/* v1 */ Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.co...
/* v2 */ Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@ubuntu.com
- Dmitry Vyukov dvyukov@google.com, Marco Elver elver@google.com:
- fix the original double-checked locking using memory barriers
/* v3 */ Link: https://lore.kernel.org/r/20191007110117.1096-1-christian.brauner@ubuntu.com...
- Andrea Parri parri.andrea@gmail.com:
- document memory barriers to make checkpatch happy
/* v4 */
- Andrea Parri parri.andrea@gmail.com:
- use smp_load_acquire(), not READ_ONCE()
- update commit message
Acked-by: Marco Elver elver@google.com
Note that this now looks almost like what I suggested, except the return at the end of the function is accessing sig->stats again. In this case, it seems it's fine assuming sig->stats cannot be written elsewhere. Just wanted to point it out to make sure it's considered.
Thanks!
kernel/taskstats.c | 24 +++++++++++++++--------- 1 file changed, 15 insertions(+), 9 deletions(-)
diff --git a/kernel/taskstats.c b/kernel/taskstats.c index 13a0f2e6ebc2..e6b45315607a 100644 --- a/kernel/taskstats.c +++ b/kernel/taskstats.c @@ -554,24 +554,30 @@ static int taskstats_user_cmd(struct sk_buff *skb, struct genl_info *info) static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk) { struct signal_struct *sig = tsk->signal;
struct taskstats *stats;
struct taskstats *stats_new, *stats;
if (sig->stats || thread_group_empty(tsk))goto ret;
/* Pairs with smp_store_release() below. */stats = smp_load_acquire(sig->stats);if (stats || thread_group_empty(tsk))return stats; /* No problem if kmem_cache_zalloc() fails */
stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL);
stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); spin_lock_irq(&tsk->sighand->siglock); if (!sig->stats) {
sig->stats = stats;stats = NULL;
/** Pairs with smp_store_release() above and order the* kmem_cache_zalloc().*/smp_store_release(&sig->stats, stats_new);stats_new = NULL; } spin_unlock_irq(&tsk->sighand->siglock);
if (stats)kmem_cache_free(taskstats_cache, stats);-ret:
if (stats_new)kmem_cache_free(taskstats_cache, stats_new);return sig->stats;}
-- 2.23.0