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To:     Hawkins Jiawei <yin31149@gmail.com>
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Subject: Re: [PATCH v2] net/smc: fix refcount bug in sk_psock_get (2)
Date:   Mon, 01 Aug 2022 11:09:23 +0200
In-reply-to: <20220730085654.2598-1-yin31149@gmail.com>
Message-ID: <87wnbsjpdb.fsf@cloudflare.com>
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Hi,

On Sat, Jul 30, 2022 at 04:56 PM +08, Hawkins Jiawei wrote:
> Syzkaller reports refcount bug as follows:
> ------------[ cut here ]------------
> refcount_t: saturated; leaking memory.
> WARNING: CPU: 1 PID: 3605 at lib/refcount.c:19 refcount_warn_saturate+0xf4/0x1e0 lib/refcount.c:19
> Modules linked in:
> CPU: 1 PID: 3605 Comm: syz-executor208 Not tainted 5.18.0-syzkaller-03023-g7e062cda7d90 #0
> ...
> Call Trace:
>  <TASK>
>  __refcount_add_not_zero include/linux/refcount.h:163 [inline]
>  __refcount_inc_not_zero include/linux/refcount.h:227 [inline]
>  refcount_inc_not_zero include/linux/refcount.h:245 [inline]
>  sk_psock_get+0x3bc/0x410 include/linux/skmsg.h:439
>  tls_data_ready+0x6d/0x1b0 net/tls/tls_sw.c:2091
>  tcp_data_ready+0x106/0x520 net/ipv4/tcp_input.c:4983
>  tcp_data_queue+0x25f2/0x4c90 net/ipv4/tcp_input.c:5057
>  tcp_rcv_state_process+0x1774/0x4e80 net/ipv4/tcp_input.c:6659
>  tcp_v4_do_rcv+0x339/0x980 net/ipv4/tcp_ipv4.c:1682
>  sk_backlog_rcv include/net/sock.h:1061 [inline]
>  __release_sock+0x134/0x3b0 net/core/sock.c:2849
>  release_sock+0x54/0x1b0 net/core/sock.c:3404
>  inet_shutdown+0x1e0/0x430 net/ipv4/af_inet.c:909
>  __sys_shutdown_sock net/socket.c:2331 [inline]
>  __sys_shutdown_sock net/socket.c:2325 [inline]
>  __sys_shutdown+0xf1/0x1b0 net/socket.c:2343
>  __do_sys_shutdown net/socket.c:2351 [inline]
>  __se_sys_shutdown net/socket.c:2349 [inline]
>  __x64_sys_shutdown+0x50/0x70 net/socket.c:2349
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x46/0xb0
>  </TASK>
>
> During SMC fallback process in connect syscall, kernel will
> replaces TCP with SMC. In order to forward wakeup
> smc socket waitqueue after fallback, kernel will sets
> clcsk->sk_user_data to origin smc socket in
> smc_fback_replace_callbacks().
>
> Later, in shutdown syscall, kernel will calls
> sk_psock_get(), which treats the clcsk->sk_user_data
> as sk_psock type, triggering the refcnt warning.
>
> So, the root cause is that smc and psock, both will use
> sk_user_data field. So they will mismatch this field
> easily.
>
> This patch solves it by using another bit(defined as
> SK_USER_DATA_NOTPSOCK) in PTRMASK, to mark whether
> sk_user_data points to a sk_psock object or not.
> This patch depends on a PTRMASK introduced in commit f1ff5ce2cd5e
> ("net, sk_msg: Clear sk_user_data pointer on clone if tagged").
>
> Fixes: 341adeec9ada ("net/smc: Forward wakeup to smc socket waitqueue after fallback")
> Fixes: a60a2b1e0af1 ("net/smc: reduce active tcp_listen workers")
> Reported-and-tested-by: syzbot+5f26f85569bd179c18ce@syzkaller.appspotmail.com
> Suggested-by: Jakub Kicinski <kuba@kernel.org>
> Acked-by: Wen Gu <guwen@linux.alibaba.com>
> Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
> ---

Since using psock is not the common case, I'm wondering if it makes more
sense to have an inverse flag - SK_USER_DATA_PSOCK. Flag would be set by
the psock code on assignment to sk_user_data.

This way we would also avoid some confusion. With the change below, the
SK_USER_DATA_NOTPSOCK is not *always* set when sk_user_data holds a
non-psock pointer. Only when SMC sets it.

If we go with the current approach, the rest of sites, execpt for psock,
that assign to sk_user_data should be updated to set
SK_USER_DATA_NOTPSOCK as well, IMO.

That is why I'd do it the other way.

[...]