Fix DR_OVERRIDDEN use-after-free race in dbuf_sync_leaf

[tuxoko]

In dbuf_sync_leaf, we clone the arc_buf in dr if we share it with db except for overridden case. However, this exception causes a race where dbuf_new_size could free the arc_buf after the last dereference of *datap and causes use-after-free. We fix this by cloning the buf regardless if it's overridden.

The race:

P0                                     P1

                                       dbuf_hold_impl()
                                         // dbuf_hold_copy passed
                                         // because db_data_pending NULL

dbuf_sync_leaf()
  // doesn't clone *datap
  // *datap derefed to db_buf
  dbuf_write(*datap)

                                       dbuf_new_size()
                                         dmu_buf_will_dirty()
                                           dbuf_fix_old_data()
                                             // alloc new buf for P0 dr
                                             // but can't change *datap

                                         arc_alloc_buf()
                                         arc_buf_destroy()
                                           // alloc new buf for db_buf
                                           // and destroy old buf

  dbuf_write() // continue
    abd_get_from_buf(data->b_data,
    arc_buf_size(data))
      // use-after-free

Here's an example when it happens:

BUG: kernel NULL pointer dereference, address: 000000000000002e
RIP: 0010:arc_buf_size+0x1c/0x30 [zfs]
Call Trace:
 dbuf_write+0x3ff/0x580 [zfs]
 dbuf_sync_leaf+0x13c/0x530 [zfs]
 dbuf_sync_list+0xbf/0x120 [zfs]
 dnode_sync+0x3ea/0x7a0 [zfs]
 sync_dnodes_task+0x71/0xa0 [zfs]
 taskq_thread+0x2b8/0x4e0 [spl]
 kthread+0x112/0x130
 ret_from_fork+0x1f/0x30

Motivation and Context

Description

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[amotin]

This check is ~14 years old, and I can only guess its idea was to avoid extra copying, since in most cases overridden writes do not require the data access. But looking on zio_write_bp_init() code it seems overridden write can be turned into regular one in some cases of dedup. I wonder if those cases are still valid and necessary, but if so, this patch looks right, even though it is likely resource-consuming for no reason.