linux.git/fs/file.c, branch v5.10

io_uring: don't rely on weak ->files references

2020-10-01T02:32:32+00:00

Grab actual references to the files_struct. To avoid circular references
issues due to this, we add a per-task note that keeps track of what
io_uring contexts a task has used. When the tasks execs or exits its
assigned files, we cancel requests based on this tracking.

With that, we can grab proper references to the files table, and no
longer need to rely on stashing away ring_fd and ring_file to check
if the ring_fd may have been closed.

Cc: stable@vger.kernel.org # v5.5+
Reviewed-by: Pavel Begunkov 
Signed-off-by: Jens Axboe

Merge branch 'hch.init_path' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs

2020-08-07T16:40:34+00:00

Pull init and set_fs() cleanups from Al Viro:
 "Christoph's 'getting rid of ksys_...() uses under KERNEL_DS' series"

* 'hch.init_path' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs: (50 commits)
  init: add an init_dup helper
  init: add an init_utimes helper
  init: add an init_stat helper
  init: add an init_mknod helper
  init: add an init_mkdir helper
  init: add an init_symlink helper
  init: add an init_link helper
  init: add an init_eaccess helper
  init: add an init_chmod helper
  init: add an init_chown helper
  init: add an init_chroot helper
  init: add an init_chdir helper
  init: add an init_rmdir helper
  init: add an init_unlink helper
  init: add an init_umount helper
  init: add an init_mount helper
  init: mark create_dev as __init
  init: mark console_on_rootfs as __init
  init: initialize ramdisk_execute_command at compile time
  devtmpfs: refactor devtmpfsd()
  ...

Merge tag 'close-range-v5.9' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux

2020-08-04T22:12:02+00:00

Pull close_range() implementation from Christian Brauner:
 "This adds the close_range() syscall. It allows to efficiently close a
  range of file descriptors up to all file descriptors of a calling
  task.

  This is coordinated with the FreeBSD folks which have copied our
  version of this syscall and in the meantime have already merged it in
  April 2019:

    https://reviews.freebsd.org/D21627
    https://svnweb.freebsd.org/base?view=revision&revision=359836

  The syscall originally came up in a discussion around the new mount
  API and making new file descriptor types cloexec by default. During
  this discussion, Al suggested the close_range() syscall.

  First, it helps to close all file descriptors of an exec()ing task.
  This can be done safely via (quoting Al's example from [1] verbatim):

        /* that exec is sensitive */
        unshare(CLONE_FILES);
        /* we don't want anything past stderr here */
        close_range(3, ~0U);
        execve(....);

  The code snippet above is one way of working around the problem that
  file descriptors are not cloexec by default. This is aggravated by the
  fact that we can't just switch them over without massively regressing
  userspace. For a whole class of programs having an in-kernel method of
  closing all file descriptors is very helpful (e.g. demons, service
  managers, programming language standard libraries, container managers
  etc.).

  Second, it allows userspace to avoid implementing closing all file
  descriptors by parsing through /proc//fd/* and calling close() on
  each file descriptor and other hacks. From looking at various
  large(ish) userspace code bases this or similar patterns are very
  common in service managers, container runtimes, and programming
  language runtimes/standard libraries such as Python or Rust.

  In addition, the syscall will also work for tasks that do not have
  procfs mounted and on kernels that do not have procfs support compiled
  in. In such situations the only way to make sure that all file
  descriptors are closed is to call close() on each file descriptor up
  to UINT_MAX or RLIMIT_NOFILE, OPEN_MAX trickery.

  Based on Linus' suggestion close_range() also comes with a new flag
  CLOSE_RANGE_UNSHARE to more elegantly handle file descriptor dropping
  right before exec. This would usually be expressed in the sequence:

        unshare(CLONE_FILES);
        close_range(3, ~0U);

  as pointed out by Linus it might be desirable to have this be a part
  of close_range() itself under a new flag CLOSE_RANGE_UNSHARE which
  gets especially handy when we're closing all file descriptors above a
  certain threshold.

  Test-suite as always included"

* tag 'close-range-v5.9' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux:
  tests: add CLOSE_RANGE_UNSHARE tests
  close_range: add CLOSE_RANGE_UNSHARE
  tests: add close_range() tests
  arch: wire-up close_range()
  open: add close_range()

fs: remove ksys_dup

2020-07-31T06:16:00+00:00

Fold it into the only remaining caller.

Signed-off-by: Christoph Hellwig 
Acked-by: Linus Torvalds

fs: Expand __receive_fd() to accept existing fd

2020-07-13T18:03:45+00:00

Expand __receive_fd() with support for replace_fd() for the coming seccomp
"addfd" ioctl(). Add new wrapper receive_fd_replace() for the new behavior
and update existing wrappers to retain old behavior.

Thanks to Colin Ian King  for pointing out an
uninitialized variable exposure in an earlier version of this patch.

Cc: Alexander Viro 
Cc: Dmitry Kadashev 
Cc: Jens Axboe 
Cc: Arnd Bergmann 
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Sargun Dhillon 
Acked-by: Christian Brauner 
Signed-off-by: Kees Cook

fs: Add receive_fd() wrapper for __receive_fd()

2020-07-13T18:03:44+00:00

For both pidfd and seccomp, the __user pointer is not used. Update
__receive_fd() to make writing to ufd optional via a NULL check. However,
for the receive_fd_user() wrapper, ufd is NULL checked so an -EFAULT
can be returned to avoid changing the SCM_RIGHTS interface behavior. Add
new wrapper receive_fd() for pidfd and seccomp that does not use the ufd
argument. For the new helper, the allocated fd needs to be returned on
success. Update the existing callers to handle it.

Cc: Alexander Viro 
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Sargun Dhillon 
Acked-by: Christian Brauner 
Signed-off-by: Kees Cook

fs: Move __scm_install_fd() to __receive_fd()

2020-07-13T18:03:44+00:00

In preparation for users of the "install a received file" logic outside
of net/ (pidfd and seccomp), relocate and rename __scm_install_fd() from
net/core/scm.c to __receive_fd() in fs/file.c, and provide a wrapper
named receive_fd_user(), as future patches will change the interface
to __receive_fd().

Additionally add a comment to fd_install() as a counterpoint to how
__receive_fd() interacts with fput().

Cc: Alexander Viro 
Cc: "David S. Miller" 
Cc: Jakub Kicinski 
Cc: Dmitry Kadashev 
Cc: Jens Axboe 
Cc: Arnd Bergmann 
Cc: Sargun Dhillon 
Cc: Ido Schimmel 
Cc: Ioana Ciornei 
Cc: linux-fsdevel@vger.kernel.org
Cc: netdev@vger.kernel.org
Reviewed-by: Sargun Dhillon 
Acked-by: Christian Brauner 
Signed-off-by: Kees Cook

close_range: add CLOSE_RANGE_UNSHARE

2020-06-16T22:07:38+00:00

One of the use-cases of close_range() is to drop file descriptors just before
execve(). This would usually be expressed in the sequence:

unshare(CLONE_FILES);
close_range(3, ~0U);

as pointed out by Linus it might be desirable to have this be a part of
close_range() itself under a new flag CLOSE_RANGE_UNSHARE.

This expands {dup,unshare)_fd() to take a max_fds argument that indicates the
maximum number of file descriptors to copy from the old struct files. When the
user requests that all file descriptors are supposed to be closed via
close_range(min, max) then we can cap via unshare_fd(min) and hence don't need
to do any of the heavy fput() work for everything above min.

The patch makes it so that if CLOSE_RANGE_UNSHARE is requested and we do in
fact currently share our file descriptor table we create a new private copy.
We then close all fds in the requested range and finally after we're done we
install the new fd table.

Suggested-by: Linus Torvalds 
Signed-off-by: Christian Brauner

open: add close_range()

2020-06-16T22:05:19+00:00

This adds the close_range() syscall. It allows to efficiently close a range
of file descriptors up to all file descriptors of a calling task.

I was contacted by FreeBSD as they wanted to have the same close_range()
syscall as we proposed here. We've coordinated this and in the meantime, Kyle
was fast enough to merge close_range() into FreeBSD already in April:
https://reviews.freebsd.org/D21627
https://svnweb.freebsd.org/base?view=revision&revision=359836
and the current plan is to backport close_range() to FreeBSD 12.2 (cf. [2])
once its merged in Linux too. Python is in the process of switching to
close_range() on FreeBSD and they are waiting on us to merge this to switch on
Linux as well: https://bugs.python.org/issue38061

The syscall came up in a recent discussion around the new mount API and
making new file descriptor types cloexec by default. During this
discussion, Al suggested the close_range() syscall (cf. [1]). Note, a
syscall in this manner has been requested by various people over time.

First, it helps to close all file descriptors of an exec()ing task. This
can be done safely via (quoting Al's example from [1] verbatim):

        /* that exec is sensitive */
        unshare(CLONE_FILES);
        /* we don't want anything past stderr here */
        close_range(3, ~0U);
        execve(....);

The code snippet above is one way of working around the problem that file
descriptors are not cloexec by default. This is aggravated by the fact that
we can't just switch them over without massively regressing userspace. For
a whole class of programs having an in-kernel method of closing all file
descriptors is very helpful (e.g. demons, service managers, programming
language standard libraries, container managers etc.).
(Please note, unshare(CLONE_FILES) should only be needed if the calling
task is multi-threaded and shares the file descriptor table with another
thread in which case two threads could race with one thread allocating file
descriptors and the other one closing them via close_range(). For the
general case close_range() before the execve() is sufficient.)

Second, it allows userspace to avoid implementing closing all file
descriptors by parsing through /proc//fd/* and calling close() on each
file descriptor. From looking at various large(ish) userspace code bases
this or similar patterns are very common in:
- service managers (cf. [4])
- libcs (cf. [6])
- container runtimes (cf. [5])
- programming language runtimes/standard libraries
  - Python (cf. [2])
  - Rust (cf. [7], [8])
As Dmitry pointed out there's even a long-standing glibc bug about missing
kernel support for this task (cf. [3]).
In addition, the syscall will also work for tasks that do not have procfs
mounted and on kernels that do not have procfs support compiled in. In such
situations the only way to make sure that all file descriptors are closed
is to call close() on each file descriptor up to UINT_MAX or RLIMIT_NOFILE,
OPEN_MAX trickery (cf. comment [8] on Rust).

The performance is striking. For good measure, comparing the following
simple close_all_fds() userspace implementation that is essentially just
glibc's version in [6]:

static int close_all_fds(void)
{
        int dir_fd;
        DIR *dir;
        struct dirent *direntp;

        dir = opendir("/proc/self/fd");
        if (!dir)
                return -1;
        dir_fd = dirfd(dir);
        while ((direntp = readdir(dir))) {
                int fd;
                if (strcmp(direntp->d_name, ".") == 0)
                        continue;
                if (strcmp(direntp->d_name, "..") == 0)
                        continue;
                fd = atoi(direntp->d_name);
                if (fd == dir_fd || fd == 0 || fd == 1 || fd == 2)
                        continue;
                close(fd);
        }
        closedir(dir);
        return 0;
}

to close_range() yields:
1. closing 4 open files:
   - close_all_fds(): ~280 us
   - close_range():    ~24 us

2. closing 1000 open files:
   - close_all_fds(): ~5000 us
   - close_range():   ~800 us

close_range() is designed to allow for some flexibility. Specifically, it
does not simply always close all open file descriptors of a task. Instead,
callers can specify an upper bound.
This is e.g. useful for scenarios where specific file descriptors are
created with well-known numbers that are supposed to be excluded from
getting closed.
For extra paranoia close_range() comes with a flags argument. This can e.g.
be used to implement extension. Once can imagine userspace wanting to stop
at the first error instead of ignoring errors under certain circumstances.
There might be other valid ideas in the future. In any case, a flag
argument doesn't hurt and keeps us on the safe side.

From an implementation side this is kept rather dumb. It saw some input
from David and Jann but all nonsense is obviously my own!
- Errors to close file descriptors are currently ignored. (Could be changed
  by setting a flag in the future if needed.)
- __close_range() is a rather simplistic wrapper around __close_fd().
  My reasoning behind this is based on the nature of how __close_fd() needs
  to release an fd. But maybe I misunderstood specifics:
  We take the files_lock and rcu-dereference the fdtable of the calling
  task, we find the entry in the fdtable, get the file and need to release
  files_lock before calling filp_close().
  In the meantime the fdtable might have been altered so we can't just
  retake the spinlock and keep the old rcu-reference of the fdtable
  around. Instead we need to grab a fresh reference to the fdtable.
  If my reasoning is correct then there's really no point in fancyfying
  __close_range(): We just need to rcu-dereference the fdtable of the
  calling task once to cap the max_fd value correctly and then go on
  calling __close_fd() in a loop.

/* References */
[1]: https://lore.kernel.org/lkml/20190516165021.GD17978@ZenIV.linux.org.uk/
[2]: https://github.com/python/cpython/blob/9e4f2f3a6b8ee995c365e86d976937c141d867f8/Modules/_posixsubprocess.c#L220
[3]: https://sourceware.org/bugzilla/show_bug.cgi?id=10353#c7
[4]: https://github.com/systemd/systemd/blob/5238e9575906297608ff802a27e2ff9effa3b338/src/basic/fd-util.c#L217
[5]: https://github.com/lxc/lxc/blob/ddf4b77e11a4d08f09b7b9cd13e593f8c047edc5/src/lxc/start.c#L236
[6]: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/grantpt.c;h=2030e07fa6e652aac32c775b8c6e005844c3c4eb;hb=HEAD#l17
     Note that this is an internal implementation that is not exported.
     Currently, libc seems to not provide an exported version of this
     because of missing kernel support to do this.
     Note, in a recent patch series Florian made grantpt() a nop thereby
     removing the code referenced here.
[7]: https://github.com/rust-lang/rust/issues/12148
[8]: https://github.com/rust-lang/rust/blob/5f47c0613ed4eb46fca3633c1297364c09e5e451/src/libstd/sys/unix/process2.rs#L303-L308
     Rust's solution is slightly different but is equally unperformant.
     Rust calls getdtablesize() which is a glibc library function that
     simply returns the current RLIMIT_NOFILE or OPEN_MAX values. Rust then
     goes on to call close() on each fd. That's obviously overkill for most
     tasks. Rarely, tasks - especially non-demons - hit RLIMIT_NOFILE or
     OPEN_MAX.
     Let's be nice and assume an unprivileged user with RLIMIT_NOFILE set
     to 1024. Even in this case, there's a very high chance that in the
     common case Rust is calling the close() syscall 1021 times pointlessly
     if the task just has 0, 1, and 2 open.

Suggested-by: Al Viro 
Signed-off-by: Christian Brauner 
Cc: Arnd Bergmann 
Cc: Kyle Evans 
Cc: Jann Horn 
Cc: David Howells 
Cc: Dmitry V. Levin 
Cc: Oleg Nesterov 
Cc: Linus Torvalds 
Cc: Florian Weimer 
Cc: linux-api@vger.kernel.org

fix multiplication overflow in copy_fdtable()

2020-05-19T22:29:36+00:00

cpy and set really should be size_t; we won't get an overflow on that,
since sysctl_nr_open can't be set above ~(size_t)0 / sizeof(void *),
so nr that would've managed to overflow size_t on that multiplication
won't get anywhere near copy_fdtable() - we'll fail with EMFILE
before that.

Cc: stable@kernel.org # v2.6.25+
Fixes: 9cfe015aa424 (get rid of NR_OPEN and introduce a sysctl_nr_open)
Reported-by: Thiago Macieira 
Signed-off-by: Al Viro