• Filipe Manana's avatar
    Btrfs: fix race between adding and putting tree mod seq elements and nodes · 7227ff4d
    Filipe Manana authored
    There is a race between adding and removing elements to the tree mod log
    list and rbtree that can lead to use-after-free problems.
    
    Consider the following example that explains how/why the problems happens:
    
    1) Task A has mod log element with sequence number 200. It currently is
       the only element in the mod log list;
    
    2) Task A calls btrfs_put_tree_mod_seq() because it no longer needs to
       access the tree mod log. When it enters the function, it initializes
       'min_seq' to (u64)-1. Then it acquires the lock 'tree_mod_seq_lock'
       before checking if there are other elements in the mod seq list.
       Since the list it empty, 'min_seq' remains set to (u64)-1. Then it
       unlocks the lock 'tree_mod_seq_lock';
    
    3) Before task A acquires the lock 'tree_mod_log_lock', task B adds
       itself to the mod seq list through btrfs_get_tree_mod_seq() and gets a
       sequence number of 201;
    
    4) Some other task, name it task C, modifies a btree and because there
       elements in the mod seq list, it adds a tree mod elem to the tree
       mod log rbtree. That node added to the mod log rbtree is assigned
       a sequence number of 202;
    
    5) Task B, which is doing fiemap and resolving indirect back references,
       calls btrfs get_old_root(), with 'time_seq' == 201, which in turn
       calls tree_mod_log_search() - the search returns the mod log node
       from the rbtree with sequence number 202, created by task C;
    
    6) Task A now acquires the lock 'tree_mod_log_lock', starts iterating
       the mod log rbtree and finds the node with sequence number 202. Since
       202 is less than the previously computed 'min_seq', (u64)-1, it
       removes the node and frees it;
    
    7) Task B still has a pointer to the node with sequence number 202, and
       it dereferences the pointer itself and through the call to
       __tree_mod_log_rewind(), resulting in a use-after-free problem.
    
    This issue can be triggered sporadically with the test case generic/561
    from fstests, and it happens more frequently with a higher number of
    duperemove processes. When it happens to me, it either freezes the VM or
    it produces a trace like the following before crashing:
    
      [ 1245.321140] general protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC PTI
      [ 1245.321200] CPU: 1 PID: 26997 Comm: pool Not tainted 5.5.0-rc6-btrfs-next-52 #1
      [ 1245.321235] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c8995f-prebuilt.qemu.org 04/01/2014
      [ 1245.321287] RIP: 0010:rb_next+0x16/0x50
      [ 1245.321307] Code: ....
      [ 1245.321372] RSP: 0018:ffffa151c4d039b0 EFLAGS: 00010202
      [ 1245.321388] RAX: 6b6b6b6b6b6b6b6b RBX: ffff8ae221363c80 RCX: 6b6b6b6b6b6b6b6b
      [ 1245.321409] RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff8ae221363c80
      [ 1245.321439] RBP: ffff8ae20fcc4688 R08: 0000000000000002 R09: 0000000000000000
      [ 1245.321475] R10: ffff8ae20b120910 R11: 00000000243f8bb1 R12: 0000000000000038
      [ 1245.321506] R13: ffff8ae221363c80 R14: 000000000000075f R15: ffff8ae223f762b8
      [ 1245.321539] FS:  00007fdee1ec7700(0000) GS:ffff8ae236c80000(0000) knlGS:0000000000000000
      [ 1245.321591] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
      [ 1245.321614] CR2: 00007fded4030c48 CR3: 000000021da16003 CR4: 00000000003606e0
      [ 1245.321642] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
      [ 1245.321668] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
      [ 1245.321706] Call Trace:
      [ 1245.321798]  __tree_mod_log_rewind+0xbf/0x280 [btrfs]
      [ 1245.321841]  btrfs_search_old_slot+0x105/0xd00 [btrfs]
      [ 1245.321877]  resolve_indirect_refs+0x1eb/0xc60 [btrfs]
      [ 1245.321912]  find_parent_nodes+0x3dc/0x11b0 [btrfs]
      [ 1245.321947]  btrfs_check_shared+0x115/0x1c0 [btrfs]
      [ 1245.321980]  ? extent_fiemap+0x59d/0x6d0 [btrfs]
      [ 1245.322029]  extent_fiemap+0x59d/0x6d0 [btrfs]
      [ 1245.322066]  do_vfs_ioctl+0x45a/0x750
      [ 1245.322081]  ksys_ioctl+0x70/0x80
      [ 1245.322092]  ? trace_hardirqs_off_thunk+0x1a/0x1c
      [ 1245.322113]  __x64_sys_ioctl+0x16/0x20
      [ 1245.322126]  do_syscall_64+0x5c/0x280
      [ 1245.322139]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
      [ 1245.322155] RIP: 0033:0x7fdee3942dd7
      [ 1245.322177] Code: ....
      [ 1245.322258] RSP: 002b:00007fdee1ec6c88 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
      [ 1245.322294] RAX: ffffffffffffffda RBX: 00007fded40210d8 RCX: 00007fdee3942dd7
      [ 1245.322314] RDX: 00007fded40210d8 RSI: 00000000c020660b RDI: 0000000000000004
      [ 1245.322337] RBP: 0000562aa89e7510 R08: 0000000000000000 R09: 00007fdee1ec6d44
      [ 1245.322369] R10: 0000000000000073 R11: 0000000000000246 R12: 00007fdee1ec6d48
      [ 1245.322390] R13: 00007fdee1ec6d40 R14: 00007fded40210d0 R15: 00007fdee1ec6d50
      [ 1245.322423] Modules linked in: ....
      [ 1245.323443] ---[ end trace 01de1e9ec5dff3cd ]---
    
    Fix this by ensuring that btrfs_put_tree_mod_seq() computes the minimum
    sequence number and iterates the rbtree while holding the lock
    'tree_mod_log_lock' in write mode. Also get rid of the 'tree_mod_seq_lock'
    lock, since it is now redundant.
    
    Fixes: bd989ba3 ("Btrfs: add tree modification log functions")
    Fixes: 097b8a7c ("Btrfs: join tree mod log code with the code holding back delayed refs")
    CC: stable@vger.kernel.org # 4.4+
    Reviewed-by: default avatarJosef Bacik <josef@toxicpanda.com>
    Reviewed-by: default avatarNikolay Borisov <nborisov@suse.com>
    Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
    Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
    7227ff4d
delayed-ref.c 32.4 KB