• Filipe Manana's avatar
    Btrfs: fix removal logic of the tree mod log that leads to use-after-free issues · 6609fee8
    Filipe Manana authored
    When a tree mod log user no longer needs to use the tree it calls
    btrfs_put_tree_mod_seq() to remove itself from the list of users and
    delete all no longer used elements of the tree's red black tree, which
    should be all elements with a sequence number less then our equals to
    the caller's sequence number. However the logic is broken because it
    can delete and free elements from the red black tree that have a
    sequence number greater then the caller's sequence number:
    1) At a point in time we have sequence numbers 1, 2, 3 and 4 in the
       tree mod log;
    2) The task which got assigned the sequence number 1 calls
    3) Sequence number 1 is deleted from the list of sequence numbers;
    4) The current minimum sequence number is computed to be the sequence
       number 2;
    5) A task using sequence number 2 is at tree_mod_log_rewind() and gets
       a pointer to one of its elements from the red black tree through
       a call to tree_mod_log_search();
    6) The task with sequence number 1 iterates the red black tree of tree
       modification elements and deletes (and frees) all elements with a
       sequence number less then or equals to 2 (the computed minimum sequence
       number) - it ends up only leaving elements with sequence numbers of 3
       and 4;
    7) The task with sequence number 2 now uses the pointer to its element,
       already freed by the other task, at __tree_mod_log_rewind(), resulting
       in a use-after-free issue. When CONFIG_DEBUG_PAGEALLOC=y it produces
       a trace like the following:
      [16804.546854] general protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC PTI
      [16804.547451] CPU: 0 PID: 28257 Comm: pool Tainted: G        W         5.4.0-rc8-btrfs-next-51 #1
      [16804.548059] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c8995f-prebuilt.qemu.org 04/01/2014
      [16804.548666] RIP: 0010:rb_next+0x16/0x50
      [16804.550581] RSP: 0018:ffffb948418ef9b0 EFLAGS: 00010202
      [16804.551227] RAX: 6b6b6b6b6b6b6b6b RBX: ffff90e0247f6600 RCX: 6b6b6b6b6b6b6b6b
      [16804.551873] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff90e0247f6600
      [16804.552504] RBP: ffff90dffe0d4688 R08: 0000000000000001 R09: 0000000000000000
      [16804.553136] R10: ffff90dffa4a0040 R11: 0000000000000000 R12: 000000000000002e
      [16804.553768] R13: ffff90e0247f6600 R14: 0000000000001663 R15: ffff90dff77862b8
      [16804.554399] FS:  00007f4b197ae700(0000) GS:ffff90e036a00000(0000) knlGS:0000000000000000
      [16804.555039] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
      [16804.555683] CR2: 00007f4b10022000 CR3: 00000002060e2004 CR4: 00000000003606f0
      [16804.556336] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
      [16804.556968] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
      [16804.557583] Call Trace:
      [16804.558207]  __tree_mod_log_rewind+0xbf/0x280 [btrfs]
      [16804.558835]  btrfs_search_old_slot+0x105/0xd00 [btrfs]
      [16804.559468]  resolve_indirect_refs+0x1eb/0xc70 [btrfs]
      [16804.560087]  ? free_extent_buffer.part.19+0x5a/0xc0 [btrfs]
      [16804.560700]  find_parent_nodes+0x388/0x1120 [btrfs]
      [16804.561310]  btrfs_check_shared+0x115/0x1c0 [btrfs]
      [16804.561916]  ? extent_fiemap+0x59d/0x6d0 [btrfs]
      [16804.562518]  extent_fiemap+0x59d/0x6d0 [btrfs]
      [16804.563112]  ? __might_fault+0x11/0x90
      [16804.563706]  do_vfs_ioctl+0x45a/0x700
      [16804.564299]  ksys_ioctl+0x70/0x80
      [16804.564885]  ? trace_hardirqs_off_thunk+0x1a/0x20
      [16804.565461]  __x64_sys_ioctl+0x16/0x20
      [16804.566020]  do_syscall_64+0x5c/0x250
      [16804.566580]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
      [16804.567153] RIP: 0033:0x7f4b1ba2add7
      [16804.568907] RSP: 002b:00007f4b197adc88 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
      [16804.569513] RAX: ffffffffffffffda RBX: 00007f4b100210d8 RCX: 00007f4b1ba2add7
      [16804.570133] RDX: 00007f4b100210d8 RSI: 00000000c020660b RDI: 0000000000000003
      [16804.570726] RBP: 000055de05a6cfe0 R08: 0000000000000000 R09: 00007f4b197add44
      [16804.571314] R10: 0000000000000000 R11: 0000000000000246 R12: 00007f4b197add48
      [16804.571905] R13: 00007f4b197add40 R14: 00007f4b100210d0 R15: 00007f4b197add50
      [16804.575623] ---[ end trace 87317359aad4ba50 ]---
    Fix this by making btrfs_put_tree_mod_seq() skip deletion of elements that
    have a sequence number equals to the computed minimum sequence number, and
    not just elements with a sequence number greater then that minimum.
    Fixes: bd989ba3
     ("Btrfs: add tree modification log functions")
    CC: stable@vger.kernel.org # 4.4+
    Reviewed-by: default avatarJosef Bacik <josef@toxicpanda.com>
    Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
    Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>