On Fri, Apr 29, 2016 at 01:57:43PM +0100, David Howells wrote: > (4) File creation time (st_btime*), data version (st_version), inode > generation number (st_gen). > > These will be returned if available whether the caller asked for them or > not. The corresponding bits in st_mask will be set or cleared as > appropriate to indicate a valid value. IMO, exposing the inode generation number to anyone is a potential security problem because they are used in file handles. Most file handles provided by filesystems are simply an encoding of the inode number + generation number, plus maybe the ino+gen of the parent dir if the NFS server is configured to do this. This makes it trivial for an attacker to guess what the likely generation numbers are going to be for inode numbers surrounding any given inode, hence greatly reducing the search space for guessing valid file handles. We've known this to be a problem for a long time - file handles are not cryptographically secure, so exposing information like this by default make guessing handles successfully almost trivial for many filesystems. In the latest XFS filesystem format, we randomise the generation value during every inode allocation to make it hard to guess the handle of adjacent inodes from an existing ino+gen pair, or even from life time to life time of the same inode. We don't use a secure random number generator (prandom_u32()) so it's still possible to guess with enough trial and observation. However, it makes it several orders of magnitude harder to guess and requires knowledge of inode allocation order to guess correctly once the random number sequence has been deduced and so makes brute force the only real option for guessing a valid handle for an inode. However, this is definitely a problem for the older format where each cluster of inodes was initialised with the same seed at cluster allocation time and the generation number was simply incremented for each life time. Most filesystems use a similar method for seeding and incrementing generation numbers, so once the generation numbers are exposed it makes handles trivial to calculate successfully. > If the caller didn't ask for them, then they may be approximated. For > example, NFS won't waste any time updating them from the server, unless > as a byproduct of updating something requested. I would suggest that exposing them from the NFS server is something we most definitely don't want to do because they are the only thing that keeps remote users from guessing filehandles with ease.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
