Re: [PATCH] net: scm: Fix a possible sleep-in-atomic-context bug in scm_fp_copy()

[David Miller <davem@davemloft.net>]

From: Jia-Ju Bai <baijiaju1990@gmail.com>
Date: Sat,  1 Sep 2018 18:00:26 +0800

> The kernel module may sleep with holding a spinlock.
> 
> The function call paths (from bottom to top) in Linux-4.16 are:
> 
> [FUNC] kmalloc(GFP_KERNEL)
> net/core/scm.c, 85: kmalloc in scm_fp_copy
> net/core/scm.c, 161: scm_fp_copy in __scm_send
> ./include/net/scm.h, 88: __scm_send in scm_send
> net/unix/af_unix.c, 1600: scm_send in maybe_init_creds
> net/unix/af_unix.c, 1983: maybe_init_creds in unix_stream_sendpage
> net/unix/af_unix.c, 1973: spin_lock in unix_stream_sendpage

Please, do a full analysis of the code for these changes you are
submitting.

Read maybe_init_creds(), it sets msg.msg_controllen to zero.

	struct msghdr msg = { .msg_controllen = 0 };

When that is zero, __scm__send() is never called.

static __inline__ int scm_send(struct socket *sock, struct msghdr *msg,
			       struct scm_cookie *scm, bool forcecreds)
{
 ...
	if (msg->msg_controllen <= 0)
		return 0;
	return __scm_send(sock, msg, scm);

If this bug existed, sleeping in atomic warnings would be triggering
all the time and people would report that.