• Tejun Heo's avatar
    writeback: fix use-after-free in finish_writeback_work() · 8e00c4e9
    Tejun Heo authored
    finish_writeback_work() reads @done->waitq after decrementing
    @done->cnt.  However, once @done->cnt reaches zero, @done may be freed
    (from stack) at any moment and @done->waitq can contain something
    unrelated by the time finish_writeback_work() tries to read it.  This
    led to the following crash.
    
      "BUG: kernel NULL pointer dereference, address: 0000000000000002"
      #PF: supervisor write access in kernel mode
      #PF: error_code(0x0002) - not-present page
      PGD 0 P4D 0
      Oops: 0002 [#1] SMP DEBUG_PAGEALLOC
      CPU: 40 PID: 555153 Comm: kworker/u98:50 Kdump: loaded Not tainted
      ...
      Workqueue: writeback wb_workfn (flush-btrfs-1)
      RIP: 0010:_raw_spin_lock_irqsave+0x10/0x30
      Code: 48 89 d8 5b c3 e8 50 db 6b ff eb f4 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 53 9c 5b fa 31 c0 ba 01 00 00 00 <f0> 0f b1 17 75 05 48 89 d8 5b c3 89 c6 e8 fe ca 6b ff eb f2 66 90
      RSP: 0018:ffffc90049b27d98 EFLAGS: 00010046
      RAX: 0000000000000000 RBX: 0000000000000246 RCX: 0000000000000000
      RDX: 0000000000000001 RSI: 0000000000000003 RDI: 0000000000000002
      RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001
      R10: ffff889fff407600 R11: ffff88ba9395d740 R12: 000000000000e300
      R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000000000
      FS:  0000000000000000(0000) GS:ffff88bfdfa00000(0000) knlGS:0000000000000000
      CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
      CR2: 0000000000000002 CR3: 0000000002409005 CR4: 00000000001606e0
      Call Trace:
       __wake_up_common_lock+0x63/0xc0
       wb_workfn+0xd2/0x3e0
       process_one_work+0x1f5/0x3f0
       worker_thread+0x2d/0x3d0
       kthread+0x111/0x130
       ret_from_fork+0x1f/0x30
    
    Fix it by reading and caching @done->waitq before decrementing
    @done->cnt.
    
    Link: http://lkml.kernel.org/r/20190924010631.GH2233839@devbig004.ftw2.facebook.com
    Fixes: 5b9cce4c ("writeback: Generalize and expose wb_completion")
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    Debugged-by: default avatarChris Mason <clm@fb.com>
    Reviewed-by: default avatarJens Axboe <axboe@kernel.dk>
    Cc: Jan Kara <jack@suse.cz>
    Cc: <stable@vger.kernel.org>	[5.2+]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    8e00c4e9
fs-writeback.c 74.4 KB