On Fri, Jun 05, 2020 at 07:54:36AM +0000, Sargun Dhillon wrote: > On Thu, Jun 04, 2020 at 02:52:26PM +0200, Christian Brauner wrote: > > On Wed, Jun 03, 2020 at 07:22:57PM -0700, Kees Cook wrote: > > > On Thu, Jun 04, 2020 at 03:24:52AM +0200, Christian Brauner wrote: > > > > On Tue, Jun 02, 2020 at 06:10:41PM -0700, Sargun Dhillon wrote: > > > > > Previously there were two chunks of code where the logic to receive > > > > > file > > > > > descriptors was duplicated in net. The compat version of copying > > > > > file descriptors via SCM_RIGHTS did not have logic to update cgroups. > > > > > Logic to change the cgroup data was added in: > > > > > commit 48a87cc26c13 ("net: netprio: fd passed in SCM_RIGHTS datagram > > > > > not set correctly") > > > > > commit d84295067fc7 ("net: net_cls: fd passed in SCM_RIGHTS datagram > > > > > not set correctly") > > > > > > > > > > This was not copied to the compat path. This commit fixes that, and > > > > > thus > > > > > should be cherry-picked into stable. > > > > > > > > > > This introduces a helper (file_receive) which encapsulates the logic > > > > > for > > > > > handling calling security hooks as well as manipulating cgroup > > > > > information. > > > > > This helper can then be used other places in the kernel where file > > > > > descriptors are copied between processes > > > > > > > > > > I tested cgroup classid setting on both the compat (x32) path, and the > > > > > native path to ensure that when moving the file descriptor the classid > > > > > is set. > > > > > > > > > > Signed-off-by: Sargun Dhillon <sar...@sargun.me> > > > > > Suggested-by: Kees Cook <keesc...@chromium.org> > > > > > Cc: Al Viro <v...@zeniv.linux.org.uk> > > > > > Cc: Christian Brauner <christian.brau...@ubuntu.com> > > > > > Cc: Daniel Wagner <daniel.wag...@bmw-carit.de> > > > > > Cc: David S. Miller <da...@davemloft.net> > > > > > Cc: Jann Horn <ja...@google.com>, > > > > > Cc: John Fastabend <john.r.fastab...@intel.com> > > > > > Cc: Tejun Heo <t...@kernel.org> > > > > > Cc: Tycho Andersen <ty...@tycho.ws> > > > > > Cc: sta...@vger.kernel.org > > > > > Cc: cgro...@vger.kernel.org > > > > > Cc: linux-fsde...@vger.kernel.org > > > > > Cc: linux-kernel@vger.kernel.org > > > > > --- > > > > > fs/file.c | 35 +++++++++++++++++++++++++++++++++++ > > > > > include/linux/file.h | 1 + > > > > > net/compat.c | 10 +++++----- > > > > > net/core/scm.c | 14 ++++---------- > > > > > 4 files changed, 45 insertions(+), 15 deletions(-) > > > > > > > > > > > > > This is all just a remote version of fd_install(), yet it deviates from > > > > fd_install()'s semantics and naming. That's not great imho. What about > > > > naming this something like: > > > > > > > > fd_install_received() > > > > > > > > and move the get_file() out of there so it has the same semantics as > > > > fd_install(). It seems rather dangerous to have a function like > > > > fd_install() that consumes a reference once it returned and another > > > > version of this that is basically the same thing but doesn't consume a > > > > reference because it takes its own. Seems an invitation for confusion. > > > > Does that make sense? > > > > > > We have some competing opinions on this, I guess. What I really don't > > > like is the copy/pasting of the get_unused_fd_flags() and > > > put_unused_fd() needed by (nearly) all the callers. If it's a helper, it > > > should help. Specifically, I'd like to see this: > > > > > > int file_receive(int fd, unsigned long flags, struct file *file, > > > int __user *fdptr) > > > > I still fail to see what this whole put_user() handling buys us at all > > and why this function needs to be anymore complicated then simply: > > > > fd_install_received(int fd, struct file *file) > > { > > security_file_receive(file); > > > > sock = sock_from_file(fd, &err); > > if (sock) { > > sock_update_netprioidx(&sock->sk->sk_cgrp_data); > > sock_update_classid(&sock->sk->sk_cgrp_data); > > } > > > > fd_install(); > > return; > > } > > > > exactly like fd_install() but for received files. > > > > For scm you can fail somewhere in the middle of putting any number of > > file descriptors so you're left in a state with only a subset of > > requested file descriptors installed so it's not really useful there. > > And if you manage to install an fd but then fail to put_user() it > > userspace can simply check it's fds via proc and has to anyway on any > > scm message error. If you fail an scm message userspace better check > > their fds. > > For seccomp maybe but even there I doubt it and I still maintain that > > userspace screwing this up is on them which is how we do this most of > > the time. And for pidfd_getfd() this whole put_user() thing doesn't > > matter at all. > > > > It's much easier and clearer if we simply have a fd_install() - > > fd_install_received() parallelism where we follow an established > > convention. _But_ if that blocks you from making this generic enough > > then at least the replace_fd() vs fd_install() logic seems it shouldn't > > be in there. > > > > And the function name really needs to drive home the point that it > > installs an fd into the tasks fdtable no matter what version you go > > with. file_receive() is really not accurate enough for this at all. > > > > > { > > > struct socket *sock; > > > int err; > > > > > > err = security_file_receive(file); > > > if (err) > > > return err; > > > > > > if (fd < 0) { > > > /* Install new fd. */ > > > int new_fd; > > > > > > err = get_unused_fd_flags(flags); > > > if (err < 0) > > > return err; > > > new_fd = err; > > > > > > /* Copy fd to any waiting user memory. */ > > > if (fdptr) { > > > err = put_user(new_fd, fdptr); > > > if (err < 0) { > > > put_unused_fd(new_fd); > > > return err; > > > } > > > } > > > fd_install(new_fd, get_file(file)); > > > fd = new_fd; > > > } else { > > > /* Replace existing fd. */ > > > err = replace_fd(fd, file, flags); > > > if (err) > > > return err; > > > } > > > > > > /* Bump the cgroup usage counts. */ > > > sock = sock_from_file(fd, &err); > > > if (sock) { > > > sock_update_netprioidx(&sock->sk->sk_cgrp_data); > > > sock_update_classid(&sock->sk->sk_cgrp_data); > > > } > > > > > > return fd; > > > } > > > > > > If everyone else *really* prefers keeping the get_unused_fd_flags() / > > > put_unused_fd() stuff outside the helper, then I guess I'll give up, > > > but I think it is MUCH cleaner this way -- all 4 users trim down lots > > > of code duplication. > > > > > > -- > > > Kees Cook > How about this: > > > static int do_dup2(struct files_struct *files, > struct file *file, unsigned fd, unsigned flags) > __releases(&files->file_lock) > { > struct file *tofree; > struct fdtable *fdt; > > ... > > /* > * New bit, allowing the file to be null. Doesn't have the same > * "sanity check" bits from __alloc_fd > */ > if (likely(file)) > get_file(file); > rcu_assign_pointer(fdt->fd[fd], file); > > __set_open_fd(fd, fdt);
IIUC, this means we can get the fdt into a state of an open fd with a NULL file... is that okay? That feels like something Al might rebel at. :) > > ... > } > > /* > * File Receive - Receive a file from another process > * > * Encapsulates the logic to handle receiving a file from another task. It > * does not install the file descriptor. That is delegated to the user. If > * an error occurs that results in the file descriptor not being installed, > * they must put_unused_fd. > * > * fd should be >= 0 if you intend on replacing a file descriptor, or > * alternatively -1 if you want file_receive to allocate an FD for you > * > * Returns the fd number on success. > * Returns negative error code on failure. > * > */ > int file_receive(int fd, unsigned int flags, struct file *file) > { > int err; > struct socket *sock; > struct files_struct *files = current->files; > > err = security_file_receive(file); > if (err) > return err; > > if (fd >= 0) { > if (fd >= rlimit(RLIMIT_NOFILE)) > return -EBADF; > > spin_lock(&files->file_lock); > err = expand_files(files, fd); > if (err < 0) { > goto out_unlock; > } > > err = do_dup2(files, NULL, fd, flags); > if (err) > return err; This seems like we're duplicating some checks and missing others -- I really think we need to do this using the existing primitives. But I'd really like some review or commentary from Al. We can do this a bunch of ways, and I'd like to know which way looks best to him. :( > This way there is: > 1. No "put_user" logic in file_receive > 2. Minimal (single) branching logic, unless there's something in between > the file_receive and installing the FD, such as put_user. > 3. Doesn't implement fd_install, so there's no ambiguity about it being > file_install_received vs. just the receive logic. I still wonder if we should refactor SCM_RIGHTS to just delay put_user failures, which would simplify a bunch. It's a behavior change, but it seems from Al and Jann that it just shouldn't matter. (And if it does, we'll hear about it.) -- Kees Cook