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    Subject[PATCH 5.7 135/166] btrfs: reset tree root pointer after error in init_tree_roots
    Date
    From: Josef Bacik <josef@toxicpanda.com>

    commit 0465337c5599bbe360cdcff452992a1a6b7ed2d4 upstream.

    Eric reported an issue where mounting -o recovery with a fuzzed fs
    resulted in a kernel panic. This is because we tried to free the tree
    node, except it was an error from the read. Fix this by properly
    resetting the tree_root->node == NULL in this case. The panic was the
    following

    BTRFS warning (device loop0): failed to read tree root
    BUG: kernel NULL pointer dereference, address: 000000000000001f
    RIP: 0010:free_extent_buffer+0xe/0x90 [btrfs]
    Call Trace:
    free_root_extent_buffers.part.0+0x11/0x30 [btrfs]
    free_root_pointers+0x1a/0xa2 [btrfs]
    open_ctree+0x1776/0x18a5 [btrfs]
    btrfs_mount_root.cold+0x13/0xfa [btrfs]
    ? selinux_fs_context_parse_param+0x37/0x80
    legacy_get_tree+0x27/0x40
    vfs_get_tree+0x25/0xb0
    fc_mount+0xe/0x30
    vfs_kern_mount.part.0+0x71/0x90
    btrfs_mount+0x147/0x3e0 [btrfs]
    ? cred_has_capability+0x7c/0x120
    ? legacy_get_tree+0x27/0x40
    legacy_get_tree+0x27/0x40
    vfs_get_tree+0x25/0xb0
    do_mount+0x735/0xa40
    __x64_sys_mount+0x8e/0xd0
    do_syscall_64+0x4d/0x90
    entry_SYSCALL_64_after_hwframe+0x44/0xa9

    Nik says: this is problematic only if we fail on the last iteration of
    the loop as this results in init_tree_roots returning err value with
    tree_root->node = -ERR. Subsequently the caller does: fail_tree_roots
    which calls free_root_pointers on the bogus value.

    Reported-by: Eric Sandeen <sandeen@redhat.com>
    Fixes: b8522a1e5f42 ("btrfs: Factor out tree roots initialization during mount")
    CC: stable@vger.kernel.org # 5.5+
    Reviewed-by: Nikolay Borisov <nborisov@suse.com>
    Signed-off-by: Josef Bacik <josef@toxicpanda.com>
    Reviewed-by: David Sterba <dsterba@suse.com>
    [ add details how the pointer gets dereferenced ]
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/btrfs/disk-io.c | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    --- a/fs/btrfs/disk-io.c
    +++ b/fs/btrfs/disk-io.c
    @@ -2583,10 +2583,12 @@ static int __cold init_tree_roots(struct
    !extent_buffer_uptodate(tree_root->node)) {
    handle_error = true;

    - if (IS_ERR(tree_root->node))
    + if (IS_ERR(tree_root->node)) {
    ret = PTR_ERR(tree_root->node);
    - else if (!extent_buffer_uptodate(tree_root->node))
    + tree_root->node = NULL;
    + } else if (!extent_buffer_uptodate(tree_root->node)) {
    ret = -EUCLEAN;
    + }

    btrfs_warn(fs_info, "failed to read tree root");
    continue;

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