io_uring: Fix a null-ptr-deref in io_tctx_exit_cb()

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

io_uring: Fix a null-ptr-deref in io_tctx_exit_cb()

Syzkaller reports a NULL deref bug as follows:

BUG: KASAN: null-ptr-deref in io_tctx_exit_cb+0x53/0xd3

Read of size 4 at addr 0000000000000138 by task file1/1955

CPU: 1 PID: 1955 Comm: file1 Not tainted 6.1.0-rc7-00103-gef4d3ea40565 #75

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7 04/01/2014

Call Trace:

<TASK>

dump_stack_lvl+0xcd/0x134

? io_tctx_exit_cb+0x53/0xd3

kasan_report+0xbb/0x1f0

? io_tctx_exit_cb+0x53/0xd3

kasan_check_range+0x140/0x190

io_tctx_exit_cb+0x53/0xd3

task_work_run+0x164/0x250

? task_work_cancel+0x30/0x30

get_signal+0x1c3/0x2440

? lock_downgrade+0x6e0/0x6e0

? lock_downgrade+0x6e0/0x6e0

? exit_signals+0x8b0/0x8b0

? do_raw_read_unlock+0x3b/0x70

? do_raw_spin_unlock+0x50/0x230

arch_do_signal_or_restart+0x82/0x2470

? kmem_cache_free+0x260/0x4b0

? putname+0xfe/0x140

? get_sigframe_size+0x10/0x10

? do_execveat_common.isra.0+0x226/0x710

? lockdep_hardirqs_on+0x79/0x100

? putname+0xfe/0x140

? do_execveat_common.isra.0+0x238/0x710

exit_to_user_mode_prepare+0x15f/0x250

syscall_exit_to_user_mode+0x19/0x50

do_syscall_64+0x42/0xb0

entry_SYSCALL_64_after_hwframe+0x63/0xcd

RIP: 0023:0x0

Code: Unable to access opcode bytes at 0xffffffffffffffd6.

RSP: 002b:00000000fffb7790 EFLAGS: 00000200 ORIG_RAX: 000000000000000b

RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000

RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000

RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000

R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000

R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000

</TASK>

Kernel panic - not syncing: panic_on_warn set ...

This happens because the adding of task_work from io_ring_exit_work()

isn't synchronized with canceling all work items from eg exec. The

execution of the two are ordered in that they are both run by the task

itself, but if io_tctx_exit_cb() is queued while we're canceling all

work items off exec AND gets executed when the task exits to userspace

rather than in the main loop in io_uring_cancel_generic(), then we can

find current->io_uring == NULL and hit the above crash.

It's safe to add this NULL check here, because the execution of the two

paths are done by the task itself.

[axboe: add code comment and also put an explanation in the commit msg]