On 12/6/22 2:30 PM, Jens Axboe wrote: > On 12/6/22 2:15?PM, Jens Axboe wrote: >> 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. > > While I think the above is the right description of what happens, I > think there's still a race with the proposed solution. If the task_work > gets added right after the newly inserted io_run_task_work(), then we'll > still crash when the targeted task exits to userspace and runs the > task_work. > > It should actually be fine to add that NULL check in io_tctx_exit_cb. We > can't be racing here, as both the clear and io_tctx_exit_cb() are run by > current itself. It's really just an ordering issue. I've queued it up with an improved commit message, and also a code comment: https://git.kernel.dk/cgit/linux-block/commit/?h=for-6.2/io_uring-next&id=6d1b48314b989d059642958fc94ef0a58b25fc8c -- Jens Axboe
