io_uring/msg_ring: Fix NULL pointer dereference in io_msg_send_fd()

In the Linux kernel, the following vulnerability has been resolved:

io_uring/msg_ring: Fix NULL pointer dereference in io_msg_send_fd()

Syzkaller produced the below call trace:

BUG: KASAN: null-ptr-deref in io_msg_ring+0x3cb/0x9f0

Write of size 8 at addr 0000000000000070 by task repro/16399

CPU: 0 PID: 16399 Comm: repro Not tainted 6.1.0-rc1 #28

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7

Call Trace:

<TASK>

dump_stack_lvl+0xcd/0x134

? io_msg_ring+0x3cb/0x9f0

kasan_report+0xbc/0xf0

? io_msg_ring+0x3cb/0x9f0

kasan_check_range+0x140/0x190

io_msg_ring+0x3cb/0x9f0

? io_msg_ring_prep+0x300/0x300

io_issue_sqe+0x698/0xca0

io_submit_sqes+0x92f/0x1c30

__do_sys_io_uring_enter+0xae4/0x24b0

....

RIP: 0033:0x7f2eaf8f8289

RSP: 002b:00007fff40939718 EFLAGS: 00000246 ORIG_RAX: 00000000000001aa

RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f2eaf8f8289

RDX: 0000000000000000 RSI: 0000000000006f71 RDI: 0000000000000004

RBP: 00007fff409397a0 R08: 0000000000000000 R09: 0000000000000039

R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004006d0

R13: 00007fff40939880 R14: 0000000000000000 R15: 0000000000000000

</TASK>

Kernel panic - not syncing: panic_on_warn set ...

We don't have a NULL check on file_ptr in io_msg_send_fd() function,

so when file_ptr is NUL src_file is also NULL and get_file()

dereferences a NULL pointer and leads to above crash.

Add a NULL check to fix this issue.