Re: [PATCH] io_uring: Fix a null-ptr-deref in io_tctx_exit_cb()

[Jens Axboe]

On 12/6/22 2:38?AM, Harshit Mogalapalli wrote:
> 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 ...
> 
> Add a NULL check on tctx to prevent this.

I agree with Vegard that I don't think this is fixing the core of
the issue. I think what is happening here is that we don't run the
task_work in io_uring_cancel_generic() unconditionally, if we don't
need to in the loop above. But we do need to ensure we run it before
we clear current->io_uring.

Do you have a reproducer? If so, can you try the below? I _think_
this is all we need. We can't be hitting the delayed fput path as
the task isn't exiting, and we're dealing with current here.


diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 36cb63e4174f..4791d94c88f5 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -3125,6 +3125,15 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
 
 	io_uring_clean_tctx(tctx);
 	if (cancel_all) {
+		/*
+		 * If we didn't run task_work in the loop above, ensure we
+		 * do so here. If an fput() queued up exit task_work for the
+		 * ring descriptor before we started the exec that led to this
+		 * cancelation, then we need to have that run before we proceed
+		 * with tearing down current->io_uring.
+		 */
+		io_run_task_work();
+
 		/*
 		 * We shouldn't run task_works after cancel, so just leave
 		 * ->in_idle set for normal exit.

-- 
Jens Axboe