taskstats: fix data-race

[ Upstream commit 0b8d616fb5a8ffa307b1d3af37f55c15dae14f28 ]

When assiging and testing taskstats in taskstats_exit() there's a race
when setting up and reading sig->stats when a thread-group with more
than one thread exits:

write to 0xffff8881157bbe10 of 8 bytes by task 7951 on cpu 0:
 taskstats_tgid_alloc kernel/taskstats.c:567 [inline]
 taskstats_exit+0x6b7/0x717 kernel/taskstats.c:596
 do_exit+0x2c2/0x18e0 kernel/exit.c:864
 do_group_exit+0xb4/0x1c0 kernel/exit.c:983
 get_signal+0x2a2/0x1320 kernel/signal.c:2734
 do_signal+0x3b/0xc00 arch/x86/kernel/signal.c:815
 exit_to_usermode_loop+0x250/0x2c0 arch/x86/entry/common.c:159
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
 do_syscall_64+0x2d7/0x2f0 arch/x86/entry/common.c:299
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881157bbe10 of 8 bytes by task 7949 on cpu 1:
 taskstats_tgid_alloc kernel/taskstats.c:559 [inline]
 taskstats_exit+0xb2/0x717 kernel/taskstats.c:596
 do_exit+0x2c2/0x18e0 kernel/exit.c:864
 do_group_exit+0xb4/0x1c0 kernel/exit.c:983
 __do_sys_exit_group kernel/exit.c:994 [inline]
 __se_sys_exit_group kernel/exit.c:992 [inline]
 __x64_sys_exit_group+0x2e/0x30 kernel/exit.c:992
 do_syscall_64+0xcf/0x2f0 arch/x86/entry/common.c:296
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

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>
Acked-by: Marco Elver <elver@google.com>
Reviewed-by: Will Deacon <will@kernel.org>
Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://lore.kernel.org/r/20191009114809.8643-1-christian.brauner@ubuntu.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index cbb387a265db01a7bde8d6576316cdeffa6f1103..23df1fbad4b4ed47e74fdea8de3f9fd1991775b0 100644 (file)
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -559,25 +559,33 @@ 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;
+       stats = sig->stats;
+       if (!stats) {
+               /*
+                * Pairs with smp_store_release() above and order the
+                * kmem_cache_zalloc().
+                */
+               smp_store_release(&sig->stats, stats_new);
+               stats = stats_new;
+               stats_new = NULL;
        }
        spin_unlock_irq(&tsk->sighand->siglock);
 
-       if (stats)
-               kmem_cache_free(taskstats_cache, stats);
-ret:
-       return sig->stats;
+       if (stats_new)
+               kmem_cache_free(taskstats_cache, stats_new);
+
+       return stats;
 }
 
 /* Send pid data out on exit */