Re: Race conditions

[jmb () kryten Atinc COM (Jonathan M. Bresler)]

On Wed, 7 Dec 1994, der Mouse wrote:

> To open a file, which should already exist:
>
>       - lstat() the path, check that lstat succeeded
>       - check that it's acceptable (eg, not a symlink :-)
>       - open() (without O_CREAT), check that the open succeeded
>       - fstat() the fd returned by open
>       - if the lstat and fstat st_ino and st_dev fields match,
>         accept.

        if you want to allow symlinks but not allow the race, try:

        -open() the file without privelege, save the inode #
        -open() the file withe privelege, compare the inode #'s

        if the symlink has been switched on you (the cracker won the 
race), the inode #'s will differ.  if he did the switch before the first 
open(), that open() will fail.

Jonathan M. Bresler  jmb () kryten atinc com    | Analysis & Technology, Inc.  
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