Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

On Tue, Jul 10, 2018 at 4:59 PM Andy Lutomirski [off-list ref] wrote:

[cc Jann - you love this stuff]

quoted

On Jul 10, 2018, at 3:44 PM, David Howells [off-list ref] 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.

quoted

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.

quoted

   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.