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    Subject[PATCH 5.4 038/434] Btrfs: fix removal logic of the tree mod log that leads to use-after-free issues
    Date
    From: Filipe Manana <fdmanana@suse.com>

    commit 6609fee8897ac475378388238456c84298bff802 upstream.

    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
    btrfs_put_tree_mod_seq();

    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: bd989ba359f2ac ("Btrfs: add tree modification log functions")
    CC: stable@vger.kernel.org # 4.4+
    Reviewed-by: Josef Bacik <josef@toxicpanda.com>
    Signed-off-by: Filipe Manana <fdmanana@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/btrfs/ctree.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/fs/btrfs/ctree.c
    +++ b/fs/btrfs/ctree.c
    @@ -383,7 +383,7 @@ void btrfs_put_tree_mod_seq(struct btrfs
    for (node = rb_first(tm_root); node; node = next) {
    next = rb_next(node);
    tm = rb_entry(node, struct tree_mod_elem, node);
    - if (tm->seq > min_seq)
    + if (tm->seq >= min_seq)
    continue;
    rb_erase(node, tm_root);
    kfree(tm);

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