Re: [PATCH v1 net 11/16] net: Fix a data-race around sysctl_mem.

[Steven Rostedt]

On Wed, 6 Jul 2022 09:17:07 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> On Tue, 5 Jul 2022 22:21:25 -0700
> Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:
> 
> > --- a/include/trace/events/sock.h
> > +++ b/include/trace/events/sock.h
> > @@ -122,9 +122,9 @@ TRACE_EVENT(sock_exceed_buf_limit,
> >  
> >  	TP_printk("proto:%s sysctl_mem=%ld,%ld,%ld allocated=%ld sysctl_rmem=%d rmem_alloc=%d sysctl_wmem=%d wmem_alloc=%d wmem_queued=%d kind=%s",
> >  		__entry->name,
> > -		__entry->sysctl_mem[0],
> > -		__entry->sysctl_mem[1],
> > -		__entry->sysctl_mem[2],
> > +		READ_ONCE(__entry->sysctl_mem[0]),
> > +		READ_ONCE(__entry->sysctl_mem[1]),
> > +		READ_ONCE(__entry->sysctl_mem[2]),  
> 
> This is not reading anything to do with sysctl. It's reading the content of
> what was recorded in the ring buffer.
> 
> That is, the READ_ONCE() here is not necessary, and if anything will break
> user space parsing, as this is exported to user space to tell it how to
> read the binary format in the ring buffer.

I take that back. Looking at the actual trace event, it is pointing to
sysctl memory, which is a major bug.

TRACE_EVENT(sock_exceed_buf_limit,

        TP_PROTO(struct sock *sk, struct proto *prot, long allocated, int kind),

        TP_ARGS(sk, prot, allocated, kind),

        TP_STRUCT__entry(
                __array(char, name, 32)
                __field(long *, sysctl_mem)

sysctl_mem is a pointer.

                __field(long, allocated)
                __field(int, sysctl_rmem)
                __field(int, rmem_alloc)
                __field(int, sysctl_wmem)
                __field(int, wmem_alloc)
                __field(int, wmem_queued)
                __field(int, kind)
        ),

        TP_fast_assign(
                strncpy(__entry->name, prot->name, 32);

                __entry->sysctl_mem = prot->sysctl_mem;


They save the pointer **IN THE RING BUFFER**!!!

                __entry->allocated = allocated;
                __entry->sysctl_rmem = sk_get_rmem0(sk, prot);
                __entry->rmem_alloc = atomic_read(&sk->sk_rmem_alloc);
                __entry->sysctl_wmem = sk_get_wmem0(sk, prot);
                __entry->wmem_alloc = refcount_read(&sk->sk_wmem_alloc);
                __entry->wmem_queued = READ_ONCE(sk->sk_wmem_queued);
                __entry->kind = kind;
        ),

        TP_printk("proto:%s sysctl_mem=%ld,%ld,%ld allocated=%ld sysctl_rmem=%d rmem_alloc=%d sysctl_wmem=%d wmem_alloc=%d wmem_queued=%d kind=%s",
                __entry->name,
                __entry->sysctl_mem[0],
                __entry->sysctl_mem[1],
                __entry->sysctl_mem[2],

They are now reading a stale pointer, which can be read at any time. That
is, you get the information of what is in sysctl_mem at the time the ring
buffer is read (which is useless from user space), and not at the time of
the event.

Thanks for pointing this out. This needs to be fixed.

-- Steve


                __entry->allocated,
                __entry->sysctl_rmem,
                __entry->rmem_alloc,
                __entry->sysctl_wmem,
                __entry->wmem_alloc,
                __entry->wmem_queued,
                show_skmem_kind_names(__entry->kind)
        )