Add support for wrapping binaries (rpm, dracut, grubby)

We need to be friendlier to people who are transitioning from
"traditional" yum managed systems.  This patchset starts to lay
out the groundwork for supporting "intercepting" binaries that
are in the tree.

For backwards compatibility, this feature is disabled by default,
to enable it, one can add `cliwrap: true` to the manifest.

To start with for example, we wrap `/usr/bin/rpm` and cause it
to drop privileges.  This way it can't corrupt anything; we're
not just relying on the read-only bind mount.  For example nothing
will accidentally get written to `/var/lib/rpm`.

Now a tricky thing with this one is we *do* want it to write if
we're in an unlocked state.

There are various other examples of binaries we want to intercept,
among them:

 - `grubby` -> `rpm-ostree kargs`
 - `dracut` -> `rpm-ostree initramfs`
 - `yum` -> well...we'll talk about that later

Makefile-rpm-ostree.am

@@ -29,6 +29,7 @@ rpm_ostree_SOURCES = src/app/main.c \
 	src/app/rpmostree-builtin-reload.c \
 	src/app/rpmostree-builtin-rebase.c \
 	src/app/rpmostree-builtin-cancel.c \
+	src/app/rpmostree-builtin-cliwrap.c \
 	src/app/rpmostree-builtin-cleanup.c \
 	src/app/rpmostree-builtin-initramfs.c \
 	src/app/rpmostree-builtin-livefs.c \
@@ -102,7 +103,7 @@ librpmostree_rust_path = @abs_top_builddir@/target/@RUST_TARGET_SUBDIR@/librpmos
 # If the target directory exists, and isn't owned by our uid, then
 # we exit with a fatal error, since someone probably did `make && sudo make install`,
 # and in this case cargo will download into ~/.root which we don't want.
-LIBRPMOSTREE_RUST_SRCS = $(wildcard rust/src/*.rs) rust/cbindgen.toml
+LIBRPMOSTREE_RUST_SRCS = $(shell find rust/src/ -name '*.rs') rust/cbindgen.toml
 $(librpmostree_rust_path): Makefile $(LIBRPMOSTREE_RUST_SRCS)
 	cd $(top_srcdir)/rust && \
 	  export CARGO_TARGET_DIR=@abs_top_builddir@/target && \

docs/manual/treefile.md

@@ -94,6 +94,12 @@ It supports the following parameters:
     specific filesystem drivers are included.  If not specified,
     `--no-hostonly` will be used.
 
+ * `cliwrap`: boolean, optional.  Defaults to `false`.  If enabled,
+    rpm-ostree will replace binaries such as `/usr/bin/rpm` with
+    wrappers that intercept unsafe operations, or adjust functionality.
+
+    The default is `false` out of conservatism; you likely want to enable this.
+
  * `remove-files`: Array of files to delete from the generated tree.
 
  * `remove-from-packages`: Array, optional: Delete from specified packages

rust/src/cliwrap.rs

@@ -0,0 +1,125 @@
+/*
+ * Copyright (C) 2019 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+use std::io::prelude::*;
+use std::{io, path};
+use anyhow::{Result, bail};
+
+use rayon::prelude::*;
+use openat_ext::OpenatDirExt;
+mod cliutil;
+mod rpm;
+mod grubby;
+mod dracut;
+
+/// Location for the underlying (not wrapped) binaries.
+pub const CLIWRAP_DESTDIR : &'static str = "usr/libexec/rpm-ostree/wrapped";
+
+/// Our list of binaries that will be wrapped.  Must be a relative path.
+static WRAPPED_BINARIES : &[&str] = &["usr/bin/rpm",
+                                      "usr/bin/dracut",
+                                      "usr/sbin/grubby"];
+
+/// Main entrypoint for cliwrap
+fn cliwrap_main(args: &Vec<String>) -> Result<()> {
+    // We'll panic here if the vector is empty, but that is intentional;
+    // the outer code should always pass us at least one arg.
+    let name = args[0].as_str();
+    let name = match std::path::Path::new(name).file_name() {
+        Some(name) => name,
+        None => bail!("Invalid wrapped binary: {}", name),
+    };
+    // We know we had a string from above
+    let name = name.to_str().unwrap();
+
+    let args : Vec<&str> = args.iter().skip(1).map(|v| v.as_str()).collect();
+
+    // If we're not booted into ostree, just run the child directly.
+    if !cliutil::is_ostree_booted() {
+        cliutil::exec_real_wrapped(name, &args)
+    } else {
+        match name {
+            "rpm" => self::rpm::main(&args),
+            "dracut" => self::dracut::main(&args),
+            "grubby" => self::grubby::main(&args),
+            _ => bail!("Unknown wrapped binary: {}", name),
+        }
+    }
+}
+
+/// Move the real binaries to a subdir, and replace them with
+/// a shell script that calls our wrapping code.
+fn write_wrappers(rootfs_dfd: &openat::Dir) -> Result<()> {
+    let destdir = std::path::Path::new(CLIWRAP_DESTDIR);
+    rootfs_dfd.ensure_dir(destdir.parent().unwrap(), 0o755)?;
+    rootfs_dfd.ensure_dir(destdir, 0o755)?;
+    WRAPPED_BINARIES.par_iter().try_for_each(|&bin| {
+        let binpath = path::Path::new(bin);
+
+        if !rootfs_dfd.exists(binpath)? {
+            return Ok(());
+        }
+
+        let name = binpath.file_name().unwrap().to_str().unwrap();
+        let destpath = format!("{}/{}", CLIWRAP_DESTDIR, name);
+        rootfs_dfd.local_rename(bin, destpath.as_str())?;
+
+        let f = rootfs_dfd.write_file(binpath, 0o755)?;
+        let mut f = io::BufWriter::new(f);
+        f.write(b"#!/bin/sh
+# Wrapper created by rpm-ostree to override
+# behavior of the underlying binary.  For more
+# information see `man rpm-ostree`.
+exec /usr/bin/rpm-ostree cliwrap $0 \"$@\"
+")?;
+        f.flush()?;
+        Ok(())
+    })
+}
+
+mod ffi {
+    use super::*;
+    use std::ffi::CString;
+    use crate::ffiutil::*;
+    use glib;
+    use libc;
+    use anyhow::Context;
+    use lazy_static::lazy_static;
+
+    #[no_mangle]
+    pub extern "C" fn ror_cliwrap_write_wrappers(rootfs_dfd: libc::c_int, gerror: *mut *mut glib_sys::GError) -> libc::c_int {
+        let rootfs_dfd = ffi_view_openat_dir(rootfs_dfd);
+        int_glib_error(write_wrappers(&rootfs_dfd).with_context(|| format!("cli wrapper replacement failed")), gerror)
+    }
+
+    #[no_mangle]
+    pub extern "C" fn ror_cliwrap_entrypoint(argv: *mut *mut libc::c_char,
+                                             gerror: *mut *mut glib_sys::GError) -> libc::c_int {
+        let v: Vec<String> = unsafe { glib::translate::FromGlibPtrContainer::from_glib_none(argv) };
+        int_glib_error(cliwrap_main(&v), gerror)
+    }
+
+    #[no_mangle]
+    pub extern "C" fn ror_cliwrap_destdir() -> *const libc::c_char {
+        lazy_static! {
+            static ref CLIWRAP_DESTDIR_C: CString = CString::new(CLIWRAP_DESTDIR).unwrap();
+        }
+        CLIWRAP_DESTDIR_C.as_ptr()
+    }
+}
+pub use self::ffi::*;

rust/src/cliwrap/cliutil.rs

@@ -0,0 +1,96 @@
+use anyhow::Result;
+use std::{thread, time, path};
+use std::os::unix::process::CommandExt;
+use nix::sys::statvfs;
+
+use crate::cliwrap;
+
+/// Returns true if the current process is booted via ostree.
+pub fn is_ostree_booted() -> bool {
+    path::Path::new("/run/ostree-booted").exists()
+}
+
+/// Returns true if /usr is not a read-only bind mount
+pub fn is_unlocked() -> Result<bool> {
+    Ok(!statvfs::statvfs("/usr")?.flags().contains(statvfs::FsFlags::ST_RDONLY))
+}
+
+/// Returns true if the current process is running as root.
+pub fn am_privileged() -> bool {
+    nix::unistd::getuid() == nix::unistd::Uid::from_raw(0)
+}
+
+/// Return the absolute path to the underlying wrapped binary
+fn get_real_bin(bin_name: &str) -> String {
+    format!("/{}/{}", cliwrap::CLIWRAP_DESTDIR, bin_name)
+}
+
+/// Wrapper for execv which accepts strings
+pub fn exec_real_wrapped<T: AsRef<str> + std::fmt::Display>(bin_name: T, argv: &[T]) -> Result<()> {
+    let bin_name = bin_name.as_ref();
+    let real_bin = get_real_bin(bin_name);
+    let mut proc = std::process::Command::new(real_bin);
+    proc.args(argv.iter().map(|s|s.as_ref()));
+    Err(proc.exec().into())
+}
+
+/// Run a subprocess synchronously as user `bin` (dropping all capabilities).
+pub fn run_unprivileged<T: AsRef<str>>(
+    with_warning: bool,
+    target_bin: &str,
+    argv: &[T],
+) -> Result<()> {
+    // `setpriv` is in util-linux; we could do this internally, but this is easier.
+    let setpriv_argv = &[
+        "setpriv",
+        "--no-new-privs",
+        "--reuid=bin",
+        "--regid=bin",
+        "--init-groups",
+        "--bounding-set",
+        "-all",
+        "--",
+    ];
+
+    let argv: Vec<&str> = argv.into_iter().map(AsRef::as_ref).collect();
+    let drop_privileges = am_privileged ();
+    let app_name = "rpm-ostree";
+    if with_warning {
+        let delay_s = 5;
+        eprintln!(
+            "{name}: NOTE: This system is ostree based.",
+            name = app_name
+        );
+        if drop_privileges {
+            eprintln!(r#"{name}: Dropping privileges as `{bin}` was executed with not "known safe" arguments."#,
+                     name=app_name, bin = target_bin);
+        } else {
+            eprintln!(
+                r#"{name}: Wrapped binary "{bin}" was executed with not "known safe" arguments."#,
+                name = app_name,
+                bin = target_bin
+            );
+        }
+        eprintln!(
+            r##"{name}: You may invoke the real `{bin}` binary in `/{wrap_destdir}/{bin}`.
+{name}: Continuing execution in {delay} seconds.
+"##,
+            name = app_name,
+            wrap_destdir = cliwrap::CLIWRAP_DESTDIR,
+            bin = target_bin,
+            delay = delay_s,
+        );
+        thread::sleep(time::Duration::from_secs(delay_s));
+    }
+
+    if drop_privileges {
+        let real_bin = get_real_bin(target_bin);
+        let mut proc = std::process::Command::new("setpriv");
+        proc.args(setpriv_argv);
+        proc.arg(real_bin);
+        proc.args(argv);
+        Err(proc.exec().into())
+    } else {
+        exec_real_wrapped(target_bin, &argv)
+    }
+}

rust/src/cliwrap/dracut.rs

@@ -0,0 +1,9 @@
+use anyhow::Result;
+
+/// Primary entrypoint to running our wrapped `dracut` handling.
+pub(crate) fn main(_argv: &[&str]) -> Result<()> {
+    eprintln!("This system is rpm-ostree based; initramfs handling is
+integrated with the underlying ostree transaction mechanism.
+Use `rpm-ostree initramfs` to control client-side initramfs generation.");
+    std::process::exit(1);
+}

rust/src/cliwrap/grubby.rs

@@ -0,0 +1,8 @@
+use anyhow::Result;
+
+/// Primary entrypoint to running our wrapped `grubby` handling.
+pub(crate) fn main(_argv: &[&str]) -> Result<()> {
+    eprintln!("This system is rpm-ostree based; grubby is not used.
+Use `rpm-ostree kargs` instead.");
+    std::process::exit(1);
+}