On Tue, Jul 10, 2018 at 4:59 PM Andy Lutomirski <[email protected]> wrote: > > [cc Jann - you love this stuff] > > > On Jul 10, 2018, at 3:44 PM, David Howells <[email protected]> wrote: > > > > Provide an fsopen() system call that starts the process of preparing to > > create a superblock that will then be mountable, using an fd as a context > > handle. fsopen() is given the name of the filesystem that will be used: > > > > int mfd = fsopen(const char *fsname, unsigned int flags); > > This is great in principle, but I think you’re seriously playing with fire > with the API. > > > > > where flags can be 0 or FSOPEN_CLOEXEC. > > > > For example: > > > > sfd = fsopen("ext4", FSOPEN_CLOEXEC); > > write(sfd, "s /dev/sdb1"); // note I'm ignoring write's length arg > > Imagine some malicious program passes sfd as stdout to a setuid program. That > program gets persuaded to write “s /etc/shadow”. What happens? You’re okay > as long as *every single fs* gets it right, but that’s asking a lot. > > > write(sfd, "o noatime"); > > write(sfd, "o acl"); > > write(sfd, "o user_attr"); > > write(sfd, "o iversion"); > > write(sfd, "o "); > > write(sfd, "r /my/container"); // root inside the fs > > write(sfd, "x create"); // create the superblock > > From cursory inspection of a bunch of the code, I think the expectation is > that the actual device access happens in the “x” action. This is not okay. > You can’t do this kind of thing in a write() handler, unless you somehow make > every single access using f_cred, which is a real pain. > > I think the right solution is one of: > > (a) Pass a netlink-formatted blob to fsopen() and do the whole thing in one > syscall. I don’t mean using netlink sockets — just the nlattr format. Or you > could use a different format. The part that matters is using just one syscall > to do the whole thing. > > (b) Keep the current structure but use a new syscall instead of write(). > > (c) Keep using write() but literally just buffer the data. Then have a new > syscall to commit it. In other words, replace “x” with a syscall and call > all the fs_context_operations helpers in that context instead of from write().
I also love ioctls, so I think you could also use an ioctl to do the commit? You can do anything (well, almost anything) that you can do in syscall context in ioctl context, too; and when you already have a file descriptor of a specific type that you want to perform an operation on, an ioctl works just fine.
