Re: [PATCH 2/2] ovl: fix UAF for ovl_aio_req

[Miklos Szeredi <miklos@szeredi.hu>]

On Mon, 1 Nov 2021 at 05:02, yangerkun <yangerkun@huawei.com> wrote:
>
>
>
> On 2021/10/29 19:40, Miklos Szeredi wrote:
> > On Thu, Oct 28, 2021 at 08:41:28PM +0800, yangerkun wrote:
> >>
> >>
> >> 在 2021/10/18 22:26, yangerkun 写道:
> >>>
> >>>
> >>> 在 2021/10/8 23:13, Miklos Szeredi 写道:
> >>>> On Thu, 30 Sept 2021 at 05:12, yangerkun <yangerkun@huawei.com> wrote:
> >>>>>
> >>>>> Our testcase trigger follow UAF:
> >>>>>
> >>>>> [  153.939147]
> >>>>> ==================================================================
> >>>>> [  153.942199] BUG: KASAN: use-after-free in
> >>>>> ext4_dio_read_iter+0xcc/0x1a0
> >>>>> [  153.943331] Read of size 8 at addr ffff88803b7f1500 by task
> >>>>> fsstress/726
> >>>>> [  153.948182] Call Trace:
> >>>>> [  153.948628]  ? dump_stack_lvl+0x73/0x9f
> >>>>> [  153.950255]  ? ext4_dio_read_iter+0xcc/0x1a0
> >>>>> [  153.950972]  ? ext4_dio_read_iter+0xcc/0x1a0
> >>>>> [  153.951693]  ? kasan_report.cold+0x81/0x165
> >>>>> [  153.952429]  ? ext4_dio_read_iter+0xcc/0x1a0
> >>>>> [  153.953190]  ? __asan_load8+0x74/0x110
> >>>>> [  153.953856]  ? ext4_dio_read_iter+0xcc/0x1a0
> >>>>> [  153.954612]  ? ext4_file_read_iter+0x1df/0x2a0
> >>>>> [  153.955394]  ? ext4_dio_read_iter+0x1a0/0x1a0
> >>>>> [  153.956159]  ? vfs_iocb_iter_read+0xd5/0x330
> >>>>> [  153.956916]  ? ovl_read_iter+0x15f/0x270
> >>>>> [  153.957605]  ? ovl_aio_cleanup_handler+0x2a0/0x2a0
> >>>>> [  153.958436]  ? aio_setup_rw+0xbf/0xe0
> >>>>> [  153.959101]  ? aio_read+0x190/0x2d0
> >>>>> [  153.959750]  ? aio_write+0x3e0/0x3e0
> >>>>> [  153.960404]  ? __kasan_check_read+0x1d/0x30
> >>>>> [  153.961167]  ? ext4_file_getattr+0x116/0x1b0
> >>>>> [  153.961978]  ? __put_user_ns+0x40/0x40
> >>>>> [  153.962714]  ? kasan_poison+0x40/0x90
> >>>>> [  153.963384]  ? set_alloc_info+0x46/0x70
> >>>>> [  153.964102]  ? __kasan_check_write+0x20/0x30
> >>>>> [  153.964856]  ? __fget_files+0x106/0x180
> >>>>> [  153.965571]  ? io_submit_one+0xaa7/0x1480
> >>>>> [  153.966325]  ? aio_poll_complete_work+0x590/0x590
> >>>>> [  153.967713]  ? ioctl_file_clone+0x110/0x110
> >>>>> [  153.969008]  ? vfs_getattr_nosec+0x14a/0x190
> >>>>> [  153.970539]  ? __do_sys_newfstat+0xd6/0xf0
> >>>>> [  153.971713]  ? __ia32_compat_sys_newfstat+0x40/0x40
> >>>>> [  153.973005]  ? __x64_sys_io_submit+0x125/0x3b0
> >>>>> [  153.974282]  ? __ia32_sys_io_submit+0x390/0x390
> >>>>> [  153.975575]  ? __kasan_check_read+0x1d/0x30
> >>>>> [  153.976749]  ? do_syscall_64+0x35/0x80
> >>>>> [  153.978034]  ? entry_SYSCALL_64_after_hwframe+0x44/0xae
> >>>>> [  153.979436]
> >>>>> [  153.979738] Allocated by task 726:
> >>>>> [  153.980352]  kasan_save_stack+0x23/0x60
> >>>>> [  153.981054]  set_alloc_info+0x46/0x70
> >>>>> [  153.981745]  __kasan_slab_alloc+0x4c/0x90
> >>>>> [  153.982492]  kmem_cache_alloc+0x153/0x750
> >>>>> [  153.983953]  ovl_read_iter+0x13b/0x270
> >>>>> [  153.984689]  aio_read+0x190/0x2d0
> >>>>> [  153.985332]  io_submit_one+0xaa7/0x1480
> >>>>> [  153.986060]  __x64_sys_io_submit+0x125/0x3b0
> >>>>> [  153.986878]  do_syscall_64+0x35/0x80
> >>>>> [  153.987590]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> >>>>> [  153.988516]
> >>>>> [  153.988824] Freed by task 750:
> >>>>> [  153.989417]  kasan_save_stack+0x23/0x60
> >>>>> [  153.990140]  kasan_set_track+0x24/0x40
> >>>>> [  153.990871]  kasan_set_free_info+0x30/0x60
> >>>>> [  153.991639]  __kasan_slab_free+0x137/0x210
> >>>>> [  153.992390]  kmem_cache_free+0xf8/0x590
> >>>>> [  153.993303]  ovl_aio_cleanup_handler+0x1ae/0x2a0
> >>>>> [  153.994205]  ovl_aio_rw_complete+0x31/0x60
> >>>>> [  153.995043]  iomap_dio_complete_work+0x4b/0x60
> >>>>> [  153.995898]  iomap_dio_bio_end_io+0x23d/0x270
> >>>>> [  153.996742]  bio_endio+0x40d/0x440
> >>>>> [  153.997376]  blk_update_request+0x38f/0x820
> >>>>> [  153.998160]  scsi_end_request+0x56/0x320
> >>>>> [  153.998872]  scsi_io_completion+0x10a/0xb60
> >>>>> [  153.999655]  scsi_finish_command+0x194/0x2b0
> >>>>> [  154.000464]  scsi_complete+0xd7/0x1f0
> >>>>> [  154.001184]  blk_complete_reqs+0x92/0xb0
> >>>>> [  154.001930]  blk_done_softirq+0x29/0x40
> >>>>> [  154.002696]  __do_softirq+0x133/0x57f
> >>>>>
> >>>>> ext4_dio_read_iter
> >>>>>     ret = iomap_dio_rw(iocb, to, &ext4_iomap_ops, NULL, 0)
> >>>>>     file_accessed(iocb->ki_filp) <== this trigger UAF
> >>>>>
> >>>>> Ret can be -EIOCBQUEUED which means we will not wait for io
> >>>>> completion in
> >>>>> iomap_dio_rw. So the release for ovl_aio_req will be done when io
> >>>>> completion routine call ovl_aio_rw_complete(or ovl_aio_cleanup_handler
> >>>>> in ovl_read_iter will do this). But once io finish soon, we may already
> >>>>> release ovl_aio_req before we call file_access in ext4_dio_read_iter.
> >>>>> This can trigger the upper UAF.
> >>>>>
> >>>>> Fix it by introduce refcount in ovl_aio_req like what aio_kiocb has did.
> >>>>
> >>>> Looks good at a glance.   Will review more fully.
> >>>
> >>> Hi,
> >>>
> >>> Any comments for this patch? :)
> >
> > Thanks for the patch.  I found one issue: the fdput() needs to be moved to the
> > destruction as well, since the iocb->ki_filp reference comes from aio_req->fd.
>
> Hi, it seems we will only use aio_req->iocb in ovl_aio_cleanup_handler
> before we call fdput(aio_req->fd):

Correct.  However I'm taking about this call:
file_accessed(iocb->ki_filp).  If the fdput() is done from
ovl_aio_cleanup_handler() which is called from ->ki_complete() then
this would put the file, agains resulting in a UAF, only inside
file_accessed() this time.   The reason why you weren't seeing this,
is probably that most of the time aio_req->fd is from
file->private_data, which is the real file obtained at open time, and
which isn't actually closed at fdput() (flags == 0).

>
> static void ovl_aio_cleanup_handler(struct ovl_aio_req *aio_req)
> {
>          struct kiocb *iocb = &aio_req->iocb;
>          struct kiocb *orig_iocb = aio_req->orig_iocb;
>
>          if (iocb->ki_flags & IOCB_WRITE) {
>                  struct inode *inode = file_inode(orig_iocb->ki_filp);
>
>                  /* Actually acquired in ovl_write_iter() */
>                  __sb_writers_acquired(file_inode(iocb->ki_filp)->i_sb,
>                                        SB_FREEZE_WRITE);
>                  file_end_write(iocb->ki_filp);
>                  <=== use iocb
>                  ovl_copyattr(ovl_inode_real(inode), inode);
>          }
>
>          orig_iocb->ki_pos = iocb->ki_pos;
>          fdput(aio_req->fd); <=== release aio_req->fd
>          kmem_cache_free(ovl_aio_request_cachep, aio_req);
> }
>
> So call fdput(aio_req->fd) in ovl_aio_cleanup_handler or ovl_aio_put
> seems all ok for me.
>
> >
> > Other than that, it looks good.
> >
> > Minor things:
> >
> > - instead of setting refcount to 1 and incrementing it, just initialize it to 2
> >
> > - code deduplication can come after the fix (if at all), this helps backporting
>
> Agree.
>
> >
> > - cleaned up header comment
> >
> > Here's the updated patch, can you please verify that I didn't break anything?
>
> Thanks! This patch can help pass the test!

Great, thanks.

Miklos