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    Subject[PATCH 5.5 219/257] ext4: fix a data race at inode->i_blocks
    Date
    From: Qian Cai <cai@lca.pw>

    commit 28936b62e71e41600bab319f262ea9f9b1027629 upstream.

    inode->i_blocks could be accessed concurrently as noticed by KCSAN,

    BUG: KCSAN: data-race in ext4_do_update_inode [ext4] / inode_add_bytes

    write to 0xffff9a00d4b982d0 of 8 bytes by task 22100 on cpu 118:
    inode_add_bytes+0x65/0xf0
    __inode_add_bytes at fs/stat.c:689
    (inlined by) inode_add_bytes at fs/stat.c:702
    ext4_mb_new_blocks+0x418/0xca0 [ext4]
    ext4_ext_map_blocks+0x1a6b/0x27b0 [ext4]
    ext4_map_blocks+0x1a9/0x950 [ext4]
    _ext4_get_block+0xfc/0x270 [ext4]
    ext4_get_block_unwritten+0x33/0x50 [ext4]
    __block_write_begin_int+0x22e/0xae0
    __block_write_begin+0x39/0x50
    ext4_write_begin+0x388/0xb50 [ext4]
    ext4_da_write_begin+0x35f/0x8f0 [ext4]
    generic_perform_write+0x15d/0x290
    ext4_buffered_write_iter+0x11f/0x210 [ext4]
    ext4_file_write_iter+0xce/0x9e0 [ext4]
    new_sync_write+0x29c/0x3b0
    __vfs_write+0x92/0xa0
    vfs_write+0x103/0x260
    ksys_write+0x9d/0x130
    __x64_sys_write+0x4c/0x60
    do_syscall_64+0x91/0xb05
    entry_SYSCALL_64_after_hwframe+0x49/0xbe

    read to 0xffff9a00d4b982d0 of 8 bytes by task 8 on cpu 65:
    ext4_do_update_inode+0x4a0/0xf60 [ext4]
    ext4_inode_blocks_set at fs/ext4/inode.c:4815
    ext4_mark_iloc_dirty+0xaf/0x160 [ext4]
    ext4_mark_inode_dirty+0x129/0x3e0 [ext4]
    ext4_convert_unwritten_extents+0x253/0x2d0 [ext4]
    ext4_convert_unwritten_io_end_vec+0xc5/0x150 [ext4]
    ext4_end_io_rsv_work+0x22c/0x350 [ext4]
    process_one_work+0x54f/0xb90
    worker_thread+0x80/0x5f0
    kthread+0x1cd/0x1f0
    ret_from_fork+0x27/0x50

    4 locks held by kworker/u256:0/8:
    #0: ffff9a025abc4328 ((wq_completion)ext4-rsv-conversion){+.+.}, at: process_one_work+0x443/0xb90
    #1: ffffab5a862dbe20 ((work_completion)(&ei->i_rsv_conversion_work)){+.+.}, at: process_one_work+0x443/0xb90
    #2: ffff9a025a9d0f58 (jbd2_handle){++++}, at: start_this_handle+0x1c1/0x9d0 [jbd2]
    #3: ffff9a00d4b985d8 (&(&ei->i_raw_lock)->rlock){+.+.}, at: ext4_do_update_inode+0xaa/0xf60 [ext4]
    irq event stamp: 3009267
    hardirqs last enabled at (3009267): [<ffffffff980da9b7>] __find_get_block+0x107/0x790
    hardirqs last disabled at (3009266): [<ffffffff980da8f9>] __find_get_block+0x49/0x790
    softirqs last enabled at (3009230): [<ffffffff98a0034c>] __do_softirq+0x34c/0x57c
    softirqs last disabled at (3009223): [<ffffffff97cc67a2>] irq_exit+0xa2/0xc0

    Reported by Kernel Concurrency Sanitizer on:
    CPU: 65 PID: 8 Comm: kworker/u256:0 Tainted: G L 5.6.0-rc2-next-20200221+ #7
    Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019
    Workqueue: ext4-rsv-conversion ext4_end_io_rsv_work [ext4]

    The plain read is outside of inode->i_lock critical section which
    results in a data race. Fix it by adding READ_ONCE() there.

    Link: https://lore.kernel.org/r/20200222043258.2279-1-cai@lca.pw
    Signed-off-by: Qian Cai <cai@lca.pw>
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>
    Cc: stable@kernel.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/ext4/inode.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/fs/ext4/inode.c
    +++ b/fs/ext4/inode.c
    @@ -4783,7 +4783,7 @@ static int ext4_inode_blocks_set(handle_
    struct ext4_inode_info *ei)
    {
    struct inode *inode = &(ei->vfs_inode);
    - u64 i_blocks = inode->i_blocks;
    + u64 i_blocks = READ_ONCE(inode->i_blocks);
    struct super_block *sb = inode->i_sb;

    if (i_blocks <= ~0U) {

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