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jailer: use Rust-like strings for paths
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jailer keeps track of various paths, e.g. device files under /dev. It
represents these paths as C-like NULL terminated strings, because we
use these paths while calling directly system calls. This requires us
to do conversions between C-like and Rust-like strings quite often.

This commit reverses the logic to store the paths as Rust strings and
only convert them when we need to perform a system call, using the
CString type. This is much safer (in terms of Rust-safety), it allows
for more Rust-idiomatic code and requires less conversions along the
way.

Signed-off-by: Babis Chalios <[email protected]>
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@bchalios
bchalios committed Sep 5, 2023
1 parent a801153 commit 93207ff
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Showing 2 changed files with 38 additions and 60 deletions.
96 changes: 37 additions & 59 deletions src/jailer/src/env.rs
View file
Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
// Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0

use std::ffi::{CStr, OsString};
use std::ffi::{CString, OsString};
use std::fs::{self, canonicalize, File, OpenOptions, Permissions};
use std::io::Write;
use std::os::unix::fs::PermissionsExt;
Expand All @@ -27,19 +27,19 @@ const STDERR_FILENO: libc::c_int = 2;
// Kernel-based virtual machine (hardware virtualization extensions)
// minor/major numbers are taken from
// https://www.kernel.org/doc/html/latest/admin-guide/devices.html
const DEV_KVM_WITH_NUL: &[u8] = b"/dev/kvm\0";
const DEV_KVM_WITH_NUL: &str = "/dev/kvm";
const DEV_KVM_MAJOR: u32 = 10;
const DEV_KVM_MINOR: u32 = 232;

// TUN/TAP device minor/major numbers are taken from
// www.kernel.org/doc/Documentation/networking/tuntap.txt
const DEV_NET_TUN_WITH_NUL: &[u8] = b"/dev/net/tun\0";
const DEV_NET_TUN_WITH_NUL: &str = "/dev/net/tun";
const DEV_NET_TUN_MAJOR: u32 = 10;
const DEV_NET_TUN_MINOR: u32 = 200;

// Random number generator device minor/major numbers are taken from
// https://www.kernel.org/doc/Documentation/admin-guide/devices.txt
const DEV_URANDOM_WITH_NUL: &[u8] = b"/dev/urandom\0";
const DEV_URANDOM_WITH_NUL: &str = "/dev/urandom";
const DEV_URANDOM_MAJOR: u32 = 1;
const DEV_URANDOM_MINOR: u32 = 9;

Expand All @@ -48,7 +48,7 @@ const DEV_URANDOM_MINOR: u32 = 9;
// We need /run for the default location of the api socket.
// Since libc::chown is not recursive, we cannot specify only /dev/net as we want
// to walk through the entire folder hierarchy.
const FOLDER_HIERARCHY: [&[u8]; 4] = [b"/\0", b"/dev\0", b"/dev/net\0", b"/run\0"];
const FOLDER_HIERARCHY: [&str; 4] = ["/", "/dev", "/dev/net", "/run"];
const FOLDER_PERMISSIONS: u32 = 0o700;

// When running with `--new-pid-ns` flag, the PID of the process running the exec_file differs
Expand Down Expand Up @@ -357,19 +357,18 @@ impl Env {

fn mknod_and_own_dev(
&self,
dev_path_str: &'static [u8],
dev_path_str: &'static str,
dev_major: u32,
dev_minor: u32,
) -> Result<(), JailerError> {
let dev_path =
CStr::from_bytes_with_nul(dev_path_str).map_err(JailerError::FromBytesWithNul)?;
let dev_path = CString::new(dev_path_str).unwrap();
// As per sysstat.h:
// S_IFCHR -> character special device
// S_IRUSR -> read permission, owner
// S_IWUSR -> write permission, owner
// See www.kernel.org/doc/Documentation/networking/tuntap.txt, 'Configuration' chapter for
// more clarity.
// SAFETY: This is safe because dev_path is CStr, and hence null-terminated.
// SAFETY: This is safe because dev_path is CString, and hence null-terminated.
SyscallReturnCode(unsafe {
libc::mknod(
dev_path.as_ptr(),
Expand All @@ -378,12 +377,7 @@ impl Env {
)
})
.into_empty_result()
.map_err(|err| {
JailerError::MknodDev(
err,
std::str::from_utf8(dev_path_str).expect("Cannot convert from UTF-8"),
)
})?;
.map_err(|err| JailerError::MknodDev(err, dev_path_str.to_owned()))?;

// SAFETY: This is safe because dev_path is CStr, and hence null-terminated.
SyscallReturnCode(unsafe { libc::chown(dev_path.as_ptr(), self.uid(), self.gid()) })
Expand All @@ -394,25 +388,22 @@ impl Env {
})
}

fn setup_jailed_folder(&self, folder: &[u8]) -> Result<(), JailerError> {
let folder_cstr =
CStr::from_bytes_with_nul(folder).map_err(JailerError::FromBytesWithNul)?;

// Safe to unwrap as the byte sequence is UTF-8 validated above.
let path = folder_cstr.to_str().unwrap();
let path_buf = PathBuf::from(path);
fs::create_dir_all(path).map_err(|err| JailerError::CreateDir(path_buf.clone(), err))?;
fs::set_permissions(path, Permissions::from_mode(FOLDER_PERMISSIONS))
.map_err(|err| JailerError::Chmod(path_buf.clone(), err))?;
fn setup_jailed_folder(&self, folder: impl AsRef<Path>) -> Result<(), JailerError> {
let folder_path = folder.as_ref();
fs::create_dir_all(folder_path)
.map_err(|err| JailerError::CreateDir(folder_path.to_owned(), err))?;
fs::set_permissions(folder_path, Permissions::from_mode(FOLDER_PERMISSIONS))
.map_err(|err| JailerError::Chmod(folder_path.to_owned(), err))?;

let c_path = CString::new(folder_path.to_str().unwrap()).unwrap();
#[cfg(target_arch = "x86_64")]
let folder_bytes_ptr = folder.as_ptr().cast::<i8>();
let folder_bytes_ptr = c_path.as_ptr().cast::<i8>();
#[cfg(target_arch = "aarch64")]
let folder_bytes_ptr = folder.as_ptr();
let folder_bytes_ptr = c_path.as_ptr();
// SAFETY: This is safe because folder was checked for a null-terminator.
SyscallReturnCode(unsafe { libc::chown(folder_bytes_ptr, self.uid(), self.gid()) })
.into_empty_result()
.map_err(|err| JailerError::ChangeFileOwner(path_buf, err))
.map_err(|err| JailerError::ChangeFileOwner(folder_path.to_owned(), err))
}

fn copy_exec_to_chroot(&mut self) -> Result<OsString, JailerError> {
Expand Down Expand Up @@ -654,7 +645,6 @@ mod tests {
#![allow(clippy::undocumented_unsafe_blocks)]

use std::os::linux::fs::MetadataExt;
use std::os::unix::ffi::OsStrExt;

use utils::tempdir::TempDir;
use utils::tempfile::TempFile;
Expand Down Expand Up @@ -965,11 +955,13 @@ mod tests {
let env = create_env();

// Error case: non UTF-8 paths.
let bad_string: &[u8] = &[0, 102, 111, 111, 0]; // A leading nul followed by 'f', 'o', 'o'
let bad_string_bytes: Vec<u8> = vec![0, 102, 111, 111, 0]; // A leading nul followed by 'f', 'o', 'o'
let bad_string = String::from_utf8(bad_string_bytes).unwrap();
assert_eq!(
format!("{}", env.setup_jailed_folder(bad_string).err().unwrap()),
"Failed to decode string from byte array: data provided contains an interior nul byte \
at byte pos 0"
format!(
"Failed to create directory \\0foo\\0: file name contained an unexpected NUL byte"
)
);

// Error case: inaccessible path - can't be triggered with unit tests running as root.
Expand All @@ -979,19 +971,10 @@ mod tests {
// );

// Success case.
let foo_dir = TempDir::new().unwrap();
let mut foo_path = foo_dir.as_path().as_os_str().as_bytes().to_vec();
foo_path.push(0);
foo_dir.remove().unwrap();
assert!(env.setup_jailed_folder(foo_path.as_slice()).is_ok());

let metadata = fs::metadata(
CStr::from_bytes_with_nul(foo_path.as_slice())
.unwrap()
.to_str()
.unwrap(),
)
.unwrap();
let foo_dir = TempDir::new().unwrap().as_path().to_owned();
assert!(env.setup_jailed_folder(foo_dir.as_path()).is_ok());

let metadata = fs::metadata(&foo_dir).unwrap();
// The mode bits will also have S_IFDIR set because the path belongs to a directory.
assert_eq!(
metadata.permissions().mode(),
Expand All @@ -1015,29 +998,24 @@ mod tests {
assert!(mock_cgroups.add_v1_mounts().is_ok());
let env = create_env();

// Ensure path buffers without NULL-termination are handled well.
assert!(env.mknod_and_own_dev(b"/some/path", 0, 0).is_err());

// Ensure device nodes are created with correct major/minor numbers and permissions.
let dev_infos: Vec<(&[u8], u32, u32)> = vec![
(b"/dev/net/tun-test\0", DEV_NET_TUN_MAJOR, DEV_NET_TUN_MINOR),
(b"/dev/kvm-test\0", DEV_KVM_MAJOR, DEV_KVM_MINOR),
let dev_infos: Vec<(&str, u32, u32)> = vec![
("/dev/net/tun-test", DEV_NET_TUN_MAJOR, DEV_NET_TUN_MINOR),
("/dev/kvm-test", DEV_KVM_MAJOR, DEV_KVM_MINOR),
];

for (dev, major, minor) in dev_infos {
let dev_str = CStr::from_bytes_with_nul(dev).unwrap().to_str().unwrap();

// Checking this just to be super sure there's no file at `dev_str` path (though
// it shouldn't be as we deleted it at the end of the previous test run).
if Path::new(dev_str).exists() {
fs::remove_file(dev_str).unwrap();
if Path::new(dev).exists() {
fs::remove_file(dev).unwrap();
}

// Create a new device node.
env.mknod_and_own_dev(dev, major, minor).unwrap();

// Ensure device's properties.
let metadata = fs::metadata(dev_str).unwrap();
let metadata = fs::metadata(dev).unwrap();
assert!(metadata.file_type().is_char_device());
assert_eq!(get_major(metadata.st_rdev()), major);
assert_eq!(get_minor(metadata.st_rdev()), minor);
Expand All @@ -1050,12 +1028,12 @@ mod tests {
assert_eq!(
format!("{}", env.mknod_and_own_dev(dev, major, minor).unwrap_err()),
format!(
"Failed to create {}\u{0} via mknod inside the jail: File exists (os error 17)",
dev_str
"Failed to create {} via mknod inside the jail: File exists (os error 17)",
dev
)
);
// Remove the device node.
fs::remove_file(dev_str).expect("Could not remove file.");
fs::remove_file(dev).expect("Could not remove file.");
}
}

Expand Down
2 changes: 1 addition & 1 deletion src/jailer/src/main.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -91,7 +91,7 @@ pub enum JailerError {
#[error("Failed to create the jail root directory before pivoting root: {0}")]
MkdirOldRoot(io::Error),
#[error("Failed to create {1} via mknod inside the jail: {0}")]
MknodDev(io::Error, &'static str),
MknodDev(io::Error, String),
#[error("Failed to bind mount the jail root directory: {0}")]
MountBind(io::Error),
#[error("Failed to change the propagation type to slave: {0}")]
Expand Down

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