On Tue, Jun 23, 2020 at 12:12:40PM +0800, Weilong Chen wrote: > KCSAN report there's a data race risk while using nr_threads. > But according to the comment above it: > ' > /* > * If multiple threads are within copy_process(), then this check > * triggers too late. This doesn't hurt, the check is only there > * to stop root fork bombs. > */ > ' > The concurrency problem is not care. And we needn't to use > READ_ONCE/atomic/etc > to protect it. Meanwhile 'max_threads' is a sysctl variable which can > be modified concurrently while being read, we can use > 'data_race(nr_threads >= max_threads)' to mark both of then. > > BUG: KCSAN: data-race in copy_process / copy_process > > write to 0xffffffff86205cf8 of 4 bytes by task 14779 on cpu 1: > copy_process+0x2eba/0x3c40 kernel/fork.c:2273 > _do_fork+0xfe/0x7a0 kernel/fork.c:2421 > __do_sys_clone kernel/fork.c:2576 [inline] > __se_sys_clone kernel/fork.c:2557 [inline] > __x64_sys_clone+0x130/0x170 kernel/fork.c:2557 > do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > read to 0xffffffff86205cf8 of 4 bytes by task 6944 on cpu 0: > copy_process+0x94d/0x3c40 kernel/fork.c:1954 > _do_fork+0xfe/0x7a0 kernel/fork.c:2421 > __do_sys_clone kernel/fork.c:2576 [inline] > __se_sys_clone kernel/fork.c:2557 [inline] > __x64_sys_clone+0x130/0x170 kernel/fork.c:2557 > do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > Link: https://groups.google.com/forum/#!msg/syzkaller-upstream-mo > deration/thvp7AHs5Ew/aPdYLXfYBQAJ > > Reported-by: syzbot+52fced2d288f8ecd2...@syzkaller.appspotmail.com > Cc: Qian Cai <c...@lca.pw> > Cc: Oleg Nesterov <o...@redhat.com> > Cc: Christian Brauner <christian.brau...@ubuntu.com> > Cc: Marco Elver <el...@google.com> > Signed-off-by: Zefan Li <lize...@huawei.com> > Signed-off-by: Weilong Chen <chenweil...@huawei.com> > ---
I'm going to fix up the commit message a little bit but otherwise I'm going to take this unless I hear objections from someone. Acked-by: Christian Brauner <christian.brau...@ubuntu.com> Thanks! Christian > kernel/fork.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/fork.c b/kernel/fork.c > index 63c8fb2f5ca7..caa9c1f27444 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -1976,7 +1976,7 @@ static __latent_entropy struct task_struct > *copy_process( > * to stop root fork bombs. > */ > retval = -EAGAIN; > - if (nr_threads >= max_threads) > + if (data_race(nr_threads >= max_threads)) > goto bad_fork_cleanup_count; > > delayacct_tsk_init(p); /* Must remain after dup_task_struct() */ > -- > 2.17.1 >
