Re: io-uring: KASAN failure, presumably

[Jens Axboe]

On 9/11/21 8:34 PM, Nadav Amit wrote:
> Hello Jens (& Pavel),
> 
> I hope you are having a nice weekend. I ran into a KASAN failure in io-uring
> which I think is not "my fault".
> 
> The failure does not happen very infrequently, so my analysis is based on
> reading the code. IIUC the failure, then I do not understand the code well
> enough, as to say I do not understand how it was supposed to work. I would
> appreciate your feedback.
> 
> The failure happens on my own custom kernel (do not try to correlate the line
> numbers). The gist of the splat is:
> 
> [84142.034456] ==================================================================
> [84142.035552] BUG: KASAN: use-after-free in io_req_complete_post (fs/io_uring.c:1629)
> [84142.036473] Read of size 4 at addr ffff8881a1577e60 by task memcached/246246
> [84142.037415]
> [84142.037621] CPU: 0 PID: 246246 Comm: memcached Not tainted 5.13.1+ #236
> [84142.038509] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/22/2020
> [84142.040151] Call Trace:      
> [84142.040495] dump_stack (lib/dump_stack.c:122)
> [84142.040962] print_address_description.constprop.0 (mm/kasan/report.c:234)
> [84142.041751] ? io_req_complete_post (fs/io_uring.c:1629)
> [84142.042365] kasan_report.cold (mm/kasan/report.c:420 mm/kasan/report.c:436)
> [84142.042921] ? io_req_complete_post (fs/io_uring.c:1629)
> [84142.043534] __asan_load4 (mm/kasan/generic.c:252) 
> [84142.044008] io_req_complete_post (fs/io_uring.c:1629) 
> [84142.044609] __io_complete_rw.isra.0 (fs/io_uring.c:2525) 
> [84142.045264] ? lockdep_hardirqs_on_prepare (kernel/locking/lockdep.c:4123) 
> [84142.045949] io_complete_rw (fs/io_uring.c:2532) 
> [84142.046447] handle_userfault (fs/userfaultfd.c:778) 
> 
> [snip]
> 
> [84142.072667] Freed by task 246231:
> [84142.073197] kasan_save_stack (mm/kasan/common.c:39)
> [84142.073896] kasan_set_track (mm/kasan/common.c:46)
> [84142.074421] kasan_set_free_info (mm/kasan/generic.c:359)
> [84142.075015] __kasan_slab_free (mm/kasan/common.c:362 mm/kasan/common.c:325 mm/kasan/common.c:368)
> [84142.075578] kmem_cache_free (mm/slub.c:1608 mm/slub.c:3168 mm/slub.c:3184)
> [84142.076116] __io_free_req (./arch/x86/include/asm/preempt.h:80 ./include/linux/rcupdate.h:68 ./include/linux/rcupdate.h:655 ./include/linux/percpu-refcount.h:317 ./include/linux/percpu-refcount.h:338 fs/io_uring.c:1802)
> [84142.076641] io_free_req (fs/io_uring.c:2113)
> [84142.077110] __io_queue_sqe (fs/io_uring.c:2208 fs/io_uring.c:6533)
> [84142.077628] io_queue_sqe (fs/io_uring.c:6568)
> [84142.078121] io_submit_sqes (fs/io_uring.c:6730 fs/io_uring.c:6838)
> [84142.078665] __x64_sys_io_uring_enter (fs/io_uring.c:9428 fs/io_uring.c:9369 fs/io_uring.c:9369)
> [84142.079463] do_syscall_64 (arch/x86/entry/common.c:47)
> [84142.079967] entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:112)
> 
> 
> I believe the issue is related to the handling of REQ_F_REISSUE and
> specifically to commit 230d50d448acb ("io_uring: move reissue into regular IO
> path"). There seems to be a race between io_write()/io_read()
> and __io_complete_rw()/kiocb_done().
> 
> __io_complete_rw() sets REQ_F_REIUSSE:
> 
>                if ((res == -EAGAIN || res == -EOPNOTSUPP) &&
>                     io_rw_should_reissue(req)) {
>                         req->flags |= REQ_F_REISSUE;
>                         return;
>                }
> 
> And then kiocb_done() then checks REQ_F_REISSUE and clear it:
> 
>         if (check_reissue && req->flags & REQ_F_REISSUE) {
>                 req->flags &= ~REQ_F_REISSUE;
>                 ...
> 
> 
> These two might race with io_write() for instance, which issues the I/O
> (__io_complete_rw() and kiocb_done() might run immediately after
> call_write_iter() is called) and then check and clear REQ_F_REISSUE.
> 
>         if (req->file->f_op->write_iter)
>                 ret2 = call_write_iter(req->file, kiocb, iter);
>         else if (req->file->f_op->write)
>                 ret2 = loop_rw_iter(WRITE, req, iter);
>         else
>                 ret2 = -EINVAL;
> 
>         if (req->flags & REQ_F_REISSUE) {
>                 req->flags &= ~REQ_F_REISSUE;
>                 ret2 = -EAGAIN;
>         }
> 
> 
> So if call_write_iter() returns -EIOCBQUEUED, this return value can be
> lost/ignored if kiocb_done() was called with result of -EAGAIN. Presumably,
> other bad things might happen due to the fact both io_write() and
> kiocb_done() see REQ_F_REISSUE set.
> 
> You might ask why, after enqueuing the IO for async execution, kiocb_done()
> would be called with -EAGAIN as a result. Indeed, this might be more
> unique to my use-case that is under development (userfaultfd might
> return -EAGAIN if the mappings undergoing changes; presumably -EBUSY or some
> wait-queue would be better.) Having said that, the current behavior still
> seems valid.
> 
> So I do not understand the check for REQ_F_REISSUE in io_write()/io_read().
> Shouldn't it just be removed? I do not suppose you want to do
> bit-test-and-clear to avoid such a race.

Just a note to say that I've seen this, I'll take a deeper look at this
tomorrow.

-- 
Jens Axboe