• Jan Kara's avatar
    udf: Fix data corruption on file type conversion · 09ebb17a
    Jan Kara authored
    UDF has two types of files - files with data stored in inode (ICB in
    UDF terminology) and files with data stored in external data blocks. We
    convert file from in-inode format to external format in
    udf_file_aio_write() when we find out data won't fit into inode any
    longer. However the following race between two O_APPEND writes can happen:
    
    CPU1					CPU2
    udf_file_aio_write()			udf_file_aio_write()
      down_write(&iinfo->i_data_sem);
      checks that i_size + count1 fits within inode
        => no need to convert
      up_write(&iinfo->i_data_sem);
    					  down_write(&iinfo->i_data_sem);
    					  checks that i_size + count2 fits
    					    within inode => no need to convert
    					  up_write(&iinfo->i_data_sem);
      generic_file_aio_write()
        - extends file by count1 bytes
    					  generic_file_aio_write()
    					    - extends file by count2 bytes
    
    Clearly if count1 + count2 doesn't fit into the inode, we overwrite
    kernel buffers beyond inode, possibly corrupting the filesystem as well.
    
    Fix the problem by acquiring i_mutex before checking whether write fits
    into the inode and using __generic_file_aio_write() afterwards which
    puts check and write into one critical section.
    Reported-by: default avatarAl Viro <viro@ZenIV.linux.org.uk>
    Signed-off-by: default avatarJan Kara <jack@suse.cz>
    09ebb17a
file.c 7.09 KB