1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
//! Utilities for working with `/proc`, where Linux's `procfs` is typically
//! mounted.
//!
//! `/proc` serves as an adjunct to Linux's main syscall surface area,
//! providing additional features with an awkward interface.
//!
//! This module does a considerable amount of work to determine whether `/proc`
//! is mounted, with actual `procfs`, and without any additional mount points
//! on top of the paths we open.
//!
//! Why all the effort to detect bind mount points? People are doing all kinds
//! of things with Linux containers these days, with many different privilege
//! schemes, and we want to avoid making any unnecessary assumptions. Rustix
//! and its users will sometimes use procfs *implicitly* (when Linux gives them
//! no better options), in ways that aren't obvious from their public APIs.
//! These filesystem accesses might not be visible to someone auditing the main
//! code of an application for places which may be influenced by the filesystem
//! namespace. So with the checking here, they may fail, but they won't be able
//! to succeed with bogus results.

use crate::fd::{AsFd, BorrowedFd, OwnedFd};
use crate::ffi::CStr;
use crate::fs::{
    fstat, fstatfs, major, openat, renameat, seek, FileType, FsWord, Mode, OFlags, RawDir,
    SeekFrom, Stat, CWD, PROC_SUPER_MAGIC,
};
use crate::io;
use crate::path::DecInt;
#[cfg(feature = "rustc-dep-of-std")]
use core::lazy::OnceCell;
use core::mem::MaybeUninit;
#[cfg(not(feature = "rustc-dep-of-std"))]
use once_cell::sync::OnceCell;

/// Linux's procfs always uses inode 1 for its root directory.
const PROC_ROOT_INO: u64 = 1;

// Identify an entry within "/proc", to determine which anomalies to check for.
#[derive(Copy, Clone, Debug)]
enum Kind {
    Proc,
    Pid,
    Fd,
    File,
    Symlink,
}

/// Check a subdirectory of "/proc" for anomalies.
fn check_proc_entry(
    kind: Kind,
    entry: BorrowedFd<'_>,
    proc_stat: Option<&Stat>,
) -> io::Result<Stat> {
    let entry_stat = fstat(entry)?;
    check_proc_entry_with_stat(kind, entry, entry_stat, proc_stat)
}

/// Check a subdirectory of "/proc" for anomalies, using the provided `Stat`.
fn check_proc_entry_with_stat(
    kind: Kind,
    entry: BorrowedFd<'_>,
    entry_stat: Stat,
    proc_stat: Option<&Stat>,
) -> io::Result<Stat> {
    // Check the filesystem magic.
    check_procfs(entry)?;

    match kind {
        Kind::Proc => check_proc_root(entry, &entry_stat)?,
        Kind::Pid | Kind::Fd => check_proc_subdir(entry, &entry_stat, proc_stat)?,
        Kind::File => check_proc_file(&entry_stat, proc_stat)?,
        Kind::Symlink => check_proc_symlink(&entry_stat, proc_stat)?,
    }

    // "/proc" directories are typically mounted r-xr-xr-x.
    // "/proc/self/fd" is r-x------. Allow them to have fewer permissions, but
    // not more.
    match kind {
        Kind::Symlink => {
            // On Linux, symlinks don't have their own permissions.
        }
        _ => {
            let expected_mode = if let Kind::Fd = kind { 0o500 } else { 0o555 };
            if entry_stat.st_mode & 0o777 & !expected_mode != 0 {
                return Err(io::Errno::NOTSUP);
            }
        }
    }

    match kind {
        Kind::Fd => {
            // Check that the "/proc/self/fd" directory doesn't have any
            // extraneous links into it (which might include unexpected
            // subdirectories).
            if entry_stat.st_nlink != 2 {
                return Err(io::Errno::NOTSUP);
            }
        }
        Kind::Pid | Kind::Proc => {
            // Check that the "/proc" and "/proc/self" directories aren't
            // empty.
            if entry_stat.st_nlink <= 2 {
                return Err(io::Errno::NOTSUP);
            }
        }
        Kind::File => {
            // Check that files in procfs don't have extraneous hard links to
            // them (which might indicate hard links to other things).
            if entry_stat.st_nlink != 1 {
                return Err(io::Errno::NOTSUP);
            }
        }
        Kind::Symlink => {
            // Check that symlinks in procfs don't have extraneous hard links
            // to them (which might indicate hard links to other things).
            if entry_stat.st_nlink != 1 {
                return Err(io::Errno::NOTSUP);
            }
        }
    }

    Ok(entry_stat)
}

fn check_proc_root(entry: BorrowedFd<'_>, stat: &Stat) -> io::Result<()> {
    // We use `O_DIRECTORY` for proc directories, so open should fail if we
    // don't get a directory when we expect one.
    assert_eq!(FileType::from_raw_mode(stat.st_mode), FileType::Directory);

    // Check the root inode number.
    if stat.st_ino != PROC_ROOT_INO {
        return Err(io::Errno::NOTSUP);
    }

    // Proc is a non-device filesystem, so check for major number 0.
    // <https://www.kernel.org/doc/Documentation/admin-guide/devices.txt>
    if major(stat.st_dev) != 0 {
        return Err(io::Errno::NOTSUP);
    }

    // Check that "/proc" is a mountpoint.
    if !is_mountpoint(entry) {
        return Err(io::Errno::NOTSUP);
    }

    Ok(())
}

fn check_proc_subdir(
    entry: BorrowedFd<'_>,
    stat: &Stat,
    proc_stat: Option<&Stat>,
) -> io::Result<()> {
    // We use `O_DIRECTORY` for proc directories, so open should fail if we
    // don't get a directory when we expect one.
    assert_eq!(FileType::from_raw_mode(stat.st_mode), FileType::Directory);

    check_proc_nonroot(stat, proc_stat)?;

    // Check that subdirectories of "/proc" are not mount points.
    if is_mountpoint(entry) {
        return Err(io::Errno::NOTSUP);
    }

    Ok(())
}

fn check_proc_file(stat: &Stat, proc_stat: Option<&Stat>) -> io::Result<()> {
    // Check that we have a regular file.
    if FileType::from_raw_mode(stat.st_mode) != FileType::RegularFile {
        return Err(io::Errno::NOTSUP);
    }

    check_proc_nonroot(stat, proc_stat)?;

    Ok(())
}

fn check_proc_symlink(stat: &Stat, proc_stat: Option<&Stat>) -> io::Result<()> {
    // Check that we have a symbolic link.
    if FileType::from_raw_mode(stat.st_mode) != FileType::Symlink {
        return Err(io::Errno::NOTSUP);
    }

    check_proc_nonroot(stat, proc_stat)?;

    Ok(())
}

fn check_proc_nonroot(stat: &Stat, proc_stat: Option<&Stat>) -> io::Result<()> {
    // Check that we haven't been linked back to the root of "/proc".
    if stat.st_ino == PROC_ROOT_INO {
        return Err(io::Errno::NOTSUP);
    }

    // Check that we're still in procfs.
    if stat.st_dev != proc_stat.unwrap().st_dev {
        return Err(io::Errno::NOTSUP);
    }

    Ok(())
}

/// Check that `file` is opened on a `procfs` filesystem.
fn check_procfs(file: BorrowedFd<'_>) -> io::Result<()> {
    let statfs = fstatfs(file)?;
    let f_type = statfs.f_type;
    if f_type != FsWord::from(PROC_SUPER_MAGIC) {
        return Err(io::Errno::NOTSUP);
    }

    Ok(())
}

/// Check whether the given directory handle is a mount point.
fn is_mountpoint(file: BorrowedFd<'_>) -> bool {
    // We use a `renameat` call that would otherwise fail, but which fails with
    // `XDEV` first if it would cross a mount point.
    let err = renameat(file, cstr!("../."), file, cstr!(".")).unwrap_err();
    match err {
        io::Errno::XDEV => true,  // the rename failed due to crossing a mount point
        io::Errno::BUSY => false, // the rename failed normally
        _ => panic!("Unexpected error from `renameat`: {:?}", err),
    }
}

/// Open a directory in `/proc`, mapping all errors to `io::Errno::NOTSUP`.
fn proc_opendirat<P: crate::path::Arg, Fd: AsFd>(dirfd: Fd, path: P) -> io::Result<OwnedFd> {
    // We don't add `PATH` here because that disables `DIRECTORY`. And we don't
    // add `NOATIME` for the same reason as the comment in `open_and_check_file`.
    let oflags =
        OFlags::RDONLY | OFlags::NOFOLLOW | OFlags::DIRECTORY | OFlags::CLOEXEC | OFlags::NOCTTY;
    openat(dirfd, path, oflags, Mode::empty()).map_err(|_err| io::Errno::NOTSUP)
}

/// Returns a handle to Linux's `/proc` directory.
///
/// This ensures that `/proc` is procfs, that nothing is mounted on top of it,
/// and that it looks normal. It also returns the `Stat` of `/proc`.
///
/// # References
///  - [Linux]
///
/// [Linux]: https://man7.org/linux/man-pages/man5/proc.5.html
fn proc() -> io::Result<(BorrowedFd<'static>, &'static Stat)> {
    static PROC: StaticFd = StaticFd::new();

    // `OnceBox` is “racy” in that the initialization function may run
    // multiple times. We're ok with that, since the initialization function
    // has no side effects.
    PROC.get_or_try_init(|| {
        // Open "/proc".
        let proc = proc_opendirat(CWD, cstr!("/proc"))?;
        let proc_stat =
            check_proc_entry(Kind::Proc, proc.as_fd(), None).map_err(|_err| io::Errno::NOTSUP)?;

        Ok(new_static_fd(proc, proc_stat))
    })
    .map(|(fd, stat)| (fd.as_fd(), stat))
}

/// Returns a handle to Linux's `/proc/self` directory.
///
/// This ensures that `/proc/self` is procfs, that nothing is mounted on top of
/// it, and that it looks normal. It also returns the `Stat` of `/proc/self`.
///
/// # References
///  - [Linux]
///
/// [Linux]: https://man7.org/linux/man-pages/man5/proc.5.html
#[allow(unsafe_code)]
fn proc_self() -> io::Result<(BorrowedFd<'static>, &'static Stat)> {
    static PROC_SELF: StaticFd = StaticFd::new();

    // The init function here may run multiple times; see above.
    PROC_SELF
        .get_or_try_init(|| {
            let (proc, proc_stat) = proc()?;

            // `getpid` would return our pid in our own pid namespace, so
            // instead use `readlink` on the `self` symlink to learn our pid in
            // the procfs namespace.
            let self_symlink = open_and_check_file(proc, proc_stat, cstr!("self"), Kind::Symlink)?;
            let mut buf = [MaybeUninit::<u8>::uninit(); 20];
            let len = crate::backend::fs::syscalls::readlinkat(
                self_symlink.as_fd(),
                cstr!(""),
                &mut buf,
            )?;
            let pid: &[u8] = unsafe { core::mem::transmute(&buf[..len]) };

            // Open "/proc/self". Use our pid to compute the name rather than
            // literally using "self", as "self" is a symlink.
            let proc_self = proc_opendirat(proc, pid)?;
            let proc_self_stat = check_proc_entry(Kind::Pid, proc_self.as_fd(), Some(proc_stat))
                .map_err(|_err| io::Errno::NOTSUP)?;

            Ok(new_static_fd(proc_self, proc_self_stat))
        })
        .map(|(owned, stat)| (owned.as_fd(), stat))
}

/// Returns a handle to Linux's `/proc/self/fd` directory.
///
/// This ensures that `/proc/self/fd` is `procfs`, that nothing is mounted on
/// top of it, and that it looks normal.
///
/// # References
///  - [Linux]
///
/// [Linux]: https://man7.org/linux/man-pages/man5/proc.5.html
#[cfg_attr(docsrs, doc(cfg(feature = "procfs")))]
pub fn proc_self_fd() -> io::Result<BorrowedFd<'static>> {
    static PROC_SELF_FD: StaticFd = StaticFd::new();

    // The init function here may run multiple times; see above.
    PROC_SELF_FD
        .get_or_try_init(|| {
            let (_, proc_stat) = proc()?;

            let (proc_self, _proc_self_stat) = proc_self()?;

            // Open "/proc/self/fd".
            let proc_self_fd = proc_opendirat(proc_self, cstr!("fd"))?;
            let proc_self_fd_stat =
                check_proc_entry(Kind::Fd, proc_self_fd.as_fd(), Some(proc_stat))
                    .map_err(|_err| io::Errno::NOTSUP)?;

            Ok(new_static_fd(proc_self_fd, proc_self_fd_stat))
        })
        .map(|(owned, _stat)| owned.as_fd())
}

type StaticFd = OnceCell<(OwnedFd, Stat)>;

#[inline]
fn new_static_fd(fd: OwnedFd, stat: Stat) -> (OwnedFd, Stat) {
    (fd, stat)
}

/// Returns a handle to Linux's `/proc/self/fdinfo` directory.
///
/// This ensures that `/proc/self/fdinfo` is `procfs`, that nothing is mounted
/// on top of it, and that it looks normal. It also returns the `Stat` of
/// `/proc/self/fd`.
///
/// # References
///  - [Linux]
///
/// [Linux]: https://man7.org/linux/man-pages/man5/proc.5.html
fn proc_self_fdinfo() -> io::Result<(BorrowedFd<'static>, &'static Stat)> {
    static PROC_SELF_FDINFO: StaticFd = StaticFd::new();

    PROC_SELF_FDINFO
        .get_or_try_init(|| {
            let (_, proc_stat) = proc()?;

            let (proc_self, _proc_self_stat) = proc_self()?;

            // Open "/proc/self/fdinfo".
            let proc_self_fdinfo = proc_opendirat(proc_self, cstr!("fdinfo"))?;
            let proc_self_fdinfo_stat =
                check_proc_entry(Kind::Fd, proc_self_fdinfo.as_fd(), Some(proc_stat))
                    .map_err(|_err| io::Errno::NOTSUP)?;

            Ok((proc_self_fdinfo, proc_self_fdinfo_stat))
        })
        .map(|(owned, stat)| (owned.as_fd(), stat))
}

/// Returns a handle to a Linux `/proc/self/fdinfo/<fd>` file.
///
/// This ensures that `/proc/self/fdinfo/<fd>` is `procfs`, that nothing is
/// mounted on top of it, and that it looks normal.
///
/// # References
///  - [Linux]
///
/// [Linux]: https://man7.org/linux/man-pages/man5/proc.5.html
#[inline]
#[cfg_attr(docsrs, doc(cfg(feature = "procfs")))]
pub fn proc_self_fdinfo_fd<Fd: AsFd>(fd: Fd) -> io::Result<OwnedFd> {
    _proc_self_fdinfo(fd.as_fd())
}

fn _proc_self_fdinfo(fd: BorrowedFd<'_>) -> io::Result<OwnedFd> {
    let (proc_self_fdinfo, proc_self_fdinfo_stat) = proc_self_fdinfo()?;
    let fd_str = DecInt::from_fd(fd);
    open_and_check_file(
        proc_self_fdinfo,
        proc_self_fdinfo_stat,
        fd_str.as_c_str(),
        Kind::File,
    )
}

/// Returns a handle to a Linux `/proc/self/pagemap` file.
///
/// This ensures that `/proc/self/pagemap` is `procfs`, that nothing is
/// mounted on top of it, and that it looks normal.
///
/// # References
///  - [Linux]
///  - [Linux pagemap]
///
/// [Linux]: https://man7.org/linux/man-pages/man5/proc.5.html
/// [Linux pagemap]: https://www.kernel.org/doc/Documentation/vm/pagemap.txt
#[inline]
#[cfg_attr(docsrs, doc(cfg(feature = "procfs")))]
pub fn proc_self_pagemap() -> io::Result<OwnedFd> {
    proc_self_file(cstr!("pagemap"))
}

/// Returns a handle to a Linux `/proc/self/maps` file.
///
/// This ensures that `/proc/self/maps` is `procfs`, that nothing is
/// mounted on top of it, and that it looks normal.
///
/// # References
///  - [Linux]
///
/// [Linux]: https://man7.org/linux/man-pages/man5/proc.5.html
#[inline]
#[cfg_attr(docsrs, doc(cfg(feature = "procfs")))]
pub fn proc_self_maps() -> io::Result<OwnedFd> {
    proc_self_file(cstr!("maps"))
}

/// Returns a handle to a Linux `/proc/self/status` file.
///
/// This ensures that `/proc/self/status` is `procfs`, that nothing is
/// mounted on top of it, and that it looks normal.
///
/// # References
///  - [Linux]
///
/// [Linux]: https://man7.org/linux/man-pages/man5/proc.5.html
#[inline]
#[cfg_attr(docsrs, doc(cfg(feature = "procfs")))]
pub fn proc_self_status() -> io::Result<OwnedFd> {
    proc_self_file(cstr!("status"))
}

/// Open a file under `/proc/self`.
fn proc_self_file(name: &CStr) -> io::Result<OwnedFd> {
    let (proc_self, proc_self_stat) = proc_self()?;
    open_and_check_file(proc_self, proc_self_stat, name, Kind::File)
}

/// Open a procfs file within in `dir` and check it for bind mounts.
fn open_and_check_file(
    dir: BorrowedFd<'_>,
    dir_stat: &Stat,
    name: &CStr,
    kind: Kind,
) -> io::Result<OwnedFd> {
    let (_, proc_stat) = proc()?;

    // Don't use `NOATIME`, because it [requires us to own the file], and when
    // a process sets itself non-dumpable Linux changes the user:group of its
    // `/proc/<pid>` files [to root:root].
    //
    // [requires us to own the file]: https://man7.org/linux/man-pages/man2/openat.2.html
    // [to root:root]: https://man7.org/linux/man-pages/man5/proc.5.html
    let mut oflags = OFlags::RDONLY | OFlags::CLOEXEC | OFlags::NOFOLLOW | OFlags::NOCTTY;
    if let Kind::Symlink = kind {
        // Open symlinks with `O_PATH`.
        oflags |= OFlags::PATH;
    }
    let file = openat(dir, name, oflags, Mode::empty()).map_err(|_err| io::Errno::NOTSUP)?;
    let file_stat = fstat(&file)?;

    // `is_mountpoint` only works on directory mount points, not file mount
    // points. To detect file mount points, scan the parent directory to see if
    // we can find a regular file with an inode and name that matches the file
    // we just opened. If we can't find it, there could be a file bind mount on
    // top of the file we want.
    //
    // TODO: With Linux 5.8 we might be able to use `statx` and
    // `STATX_ATTR_MOUNT_ROOT` to detect mountpoints directly instead of doing
    // this scanning.

    let expected_type = match kind {
        Kind::File => FileType::RegularFile,
        Kind::Symlink => FileType::Symlink,
        _ => unreachable!(),
    };

    let mut found_file = false;
    let mut found_dot = false;

    // Open a new fd, so that if we're called on multiple threads, they don't
    // share a seek position.
    let oflags =
        OFlags::RDONLY | OFlags::CLOEXEC | OFlags::NOFOLLOW | OFlags::NOCTTY | OFlags::DIRECTORY;
    let dir = openat(dir, cstr!("."), oflags, Mode::empty()).map_err(|_err| io::Errno::NOTSUP)?;
    let check_dir_stat = fstat(&dir)?;
    if check_dir_stat.st_dev != dir_stat.st_dev || check_dir_stat.st_ino != dir_stat.st_ino {
        return Err(io::Errno::NOTSUP);
    }

    // Position the directory iteration at the start.
    seek(&dir, SeekFrom::Start(0))?;

    let mut buf = [MaybeUninit::uninit(); 2048];
    let mut iter = RawDir::new(dir, &mut buf);
    while let Some(entry) = iter.next() {
        let entry = entry.map_err(|_err| io::Errno::NOTSUP)?;
        if entry.ino() == file_stat.st_ino
            && entry.file_type() == expected_type
            && entry.file_name() == name
        {
            // We found the file. Proceed to check the file handle.
            let _ = check_proc_entry_with_stat(kind, file.as_fd(), file_stat, Some(proc_stat))?;

            found_file = true;
        } else if entry.ino() == dir_stat.st_ino
            && entry.file_type() == FileType::Directory
            && entry.file_name() == cstr!(".")
        {
            // We found ".", and it's the right ".".
            found_dot = true;
        }
    }

    if found_file && found_dot {
        Ok(file)
    } else {
        Err(io::Errno::NOTSUP)
    }
}