Quoting Jan Kara (jack@xxxxxxx): > Hi, > > On Sun 25-10-09 07:29:53, Pavel Machek wrote: > > ...yes, they do exist, in /proc/self/fd/* . Unfortunately, their > > permissions are not actually checked during open, resulting in > > (obscure) security hole: if you have fd open for reading, you can > > reopen it for write, even through unix permissions would not allow > > that. > > > > Now... I'd like to close the hole. One way would be to actually check > > symlink permissions on open -- because those symlinks already have > > correct permissions. > Hmm, I'm not sure I understand the problem. Symlink is just a file > containing a path. So if you try to open a symlink, you will actually open > a file to which the path points. So what security problem is here? Either > you can open the file symlink points to for writing or you cannot... > Anyway, if you want to play with this, > fs/proc/base.c:proc_pid_follow_link > is probably the function you are interested in. The problem he's trying to address is that users may try to protect a file by doing chmod 700 on the parent dir, but leave the file itself accessible. They don't realize that merely having a task with an open fd to that file gives other users another path to the file. Whether or not that's actually a problem is open to debate, but I think he's right that many users aren't aware of it. -serge -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
