Re: net/kcm: WARNING in kcm_write_msgs

On Mon, Feb 6, 2017 at 4:43 AM, Dmitry Vyukov [off-list ref] wrote:

[resending as plain text]

Hello,

The following program triggers WARNING in kcm_write_msgs:

WARNING: CPU: 3 PID: 2936 at net/kcm/kcmsock.c:627
kcm_write_msgs+0x12e3/0x1b90 net/kcm/kcmsock.c:627
CPU: 3 PID: 2936 Comm: a.out Not tainted 4.10.0-rc6+ #209
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:15 [inline]
 dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
 panic+0x1fb/0x412 kernel/panic.c:179
 __warn+0x1c4/0x1e0 kernel/panic.c:539
 warn_slowpath_null+0x2c/0x40 kernel/panic.c:582
 kcm_write_msgs+0x12e3/0x1b90 net/kcm/kcmsock.c:627
 kcm_sendmsg+0x163a/0x2200 net/kcm/kcmsock.c:1029
 sock_sendmsg_nosec net/socket.c:635 [inline]
 sock_sendmsg+0xca/0x110 net/socket.c:645
 sock_write_iter+0x326/0x600 net/socket.c:848
 new_sync_write fs/read_write.c:499 [inline]
 __vfs_write+0x483/0x740 fs/read_write.c:512
 vfs_write+0x187/0x530 fs/read_write.c:560
 SYSC_write fs/read_write.c:607 [inline]
 SyS_write+0xfb/0x230 fs/read_write.c:599
 entry_SYSCALL_64_fastpath+0x1f/0xc2

[...]

  syscall(__NR_write, sock2, 0x208aaf27ul, 0x0ul);

Looks like len == 0 case is not handled correctly in kcm_sendmsg().
The attached patch fixes it, but I am not sure if it is correct in all
cases yet, the logic is complicated.