On Fri, Feb 09, 2024 at 11:18:21AM +0000, Alice Ryhl wrote:
> To close an fd from kernel space, we could call `ksys_close`. However,
> if we do this to an fd that is held using `fdget`, then we may trigger a
> use-after-free. Introduce a helper that can be used to close an fd even
> if the fd is currently held with `fdget`. This is done by grabbing an
> extra refcount to the file and dropping it in a task work once we return
> to userspace.
>
> This is necessary for Rust Binder because otherwise the user might try
> to have Binder close its fd for /dev/binder, which would cause problems
> as this happens inside an ioctl on /dev/binder, and ioctls hold the fd
> using `fdget`.
>
> Additional motivation can be found in commit 80cd795630d6 ("binder: fix
> use-after-free due to ksys_close() during fdget()") and in the comments
> on `binder_do_fd_close`.
>
> If there is some way to detect whether an fd is currently held with
> `fdget`, then this could be optimized to skip the allocation and task
> work when this is not the case. Another possible optimization would be
> to combine several fds into a single task work, since this is used with
> fd arrays that might hold several fds.
>
> That said, it might not be necessary to optimize it, because Rust Binder
> has two ways to send fds: BINDER_TYPE_FD and BINDER_TYPE_FDA. With
> BINDER_TYPE_FD, it is userspace's responsibility to close the fd, so
> this mechanism is used only by BINDER_TYPE_FDA, but fd arrays are used
> rarely these days.
>
> Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> ---
> rust/bindings/bindings_helper.h | 2 +
> rust/helpers.c | 8 ++
> rust/kernel/file.rs | 184 +++++++++++++++++++++++++++++++++++++++-
> rust/kernel/task.rs | 14 +++
> 4 files changed, 207 insertions(+), 1 deletion(-)
>
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 4194b057ef6b..f4d9d04333c0 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -10,6 +10,7 @@
> #include <linux/cred.h>
> #include <linux/errname.h>
> #include <linux/ethtool.h>
> +#include <linux/fdtable.h>
> #include <linux/file.h>
> #include <linux/fs.h>
> #include <linux/jiffies.h>
> @@ -21,6 +22,7 @@
> #include <linux/refcount.h>
> #include <linux/wait.h>
> #include <linux/sched.h>
> +#include <linux/task_work.h>
> #include <linux/workqueue.h>
>
> /* `bindgen` gets confused at certain things. */
> diff --git a/rust/helpers.c b/rust/helpers.c
> index 58e3a9dff349..d146bbf25aec 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -32,6 +32,7 @@
> #include <linux/sched/signal.h>
> #include <linux/security.h>
> #include <linux/spinlock.h>
> +#include <linux/task_work.h>
> #include <linux/wait.h>
> #include <linux/workqueue.h>
>
> @@ -243,6 +244,13 @@ void rust_helper_security_release_secctx(char *secdata, u32 seclen)
> EXPORT_SYMBOL_GPL(rust_helper_security_release_secctx);
> #endif
>
> +void rust_helper_init_task_work(struct callback_head *twork,
> + task_work_func_t func)
> +{
> + init_task_work(twork, func);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_init_task_work);
> +
> /*
> * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
> * use it in contexts where Rust expects a `usize` like slice (array) indices.
> diff --git a/rust/kernel/file.rs b/rust/kernel/file.rs
> index fb903b7f23fe..8902f490ccc8 100644
> --- a/rust/kernel/file.rs
> +++ b/rust/kernel/file.rs
> @@ -11,7 +11,8 @@
> error::{code::*, Error, Result},
> types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
> };
> -use core::ptr;
> +use alloc::boxed::Box;
> +use core::{alloc::AllocError, mem, ptr};
>
> /// Flags associated with a [`File`].
> pub mod flags {
> @@ -318,6 +319,187 @@ fn drop(&mut self) {
> }
> }
>
> +/// Helper used for closing file descriptors in a way that is safe even if the file is currently
> +/// held using `fdget`.
> +///
> +/// Additional motivation can be found in commit 80cd795630d6 ("binder: fix use-after-free due to
> +/// ksys_close() during fdget()") and in the comments on `binder_do_fd_close`.
> +pub struct DeferredFdCloser {
> + inner: Box<DeferredFdCloserInner>,
> +}
> +
> +/// SAFETY: This just holds an allocation with no real content, so there's no safety issue with
> +/// moving it across threads.
> +unsafe impl Send for DeferredFdCloser {}
> +unsafe impl Sync for DeferredFdCloser {}
> +
> +/// # Invariants
> +///
> +/// If the `file` pointer is non-null, then it points at a `struct file` and owns a refcount to
> +/// that file.
> +#[repr(C)]
> +struct DeferredFdCloserInner {
> + twork: mem::MaybeUninit<bindings::callback_head>,
> + file: *mut bindings::file,
> +}
> +
> +impl DeferredFdCloser {
So the explicitly deferred close is due to how binder works so it's not
much of a general purpose interface as I don't recall having other
codepaths with similar problems. So this should live in the binder
specific rust code imo.
> + /// Create a new [`DeferredFdCloser`].
> + pub fn new() -> Result<Self, AllocError> {
> + Ok(Self {
> + // INVARIANT: The `file` pointer is null, so the type invariant does not apply.
> + inner: Box::try_new(DeferredFdCloserInner {
> + twork: mem::MaybeUninit::uninit(),
> + file: core::ptr::null_mut(),
> + })?,
> + })
> + }
> +
> + /// Schedule a task work that closes the file descriptor when this task returns to userspace.
> + ///
> + /// Fails if this is called from a context where we cannot run work when returning to
> + /// userspace. (E.g., from a kthread.)
> + pub fn close_fd(self, fd: u32) -> Result<(), DeferredFdCloseError> {
> + use bindings::task_work_notify_mode_TWA_RESUME as TWA_RESUME;
> +
> + // In this method, we schedule the task work before closing the file. This is because
> + // scheduling a task work is fallible, and we need to know whether it will fail before we
> + // attempt to close the file.
> +
> + // Task works are not available on kthreads.
> + let current = crate::current!();
> + if current.is_kthread() {
> + return Err(DeferredFdCloseError::TaskWorkUnavailable);
> + }
> +
> + // Transfer ownership of the box's allocation to a raw pointer. This disables the
> + // destructor, so we must manually convert it back to a Box to drop it.
> + //
> + // Until we convert it back to a `Box`, there are no aliasing requirements on this
> + // pointer.
> + let inner = Box::into_raw(self.inner);
> +
> + // The `callback_head` field is first in the struct, so this cast correctly gives us a
> + // pointer to the field.
> + let callback_head = inner.cast::<bindings::callback_head>();
> + // SAFETY: This pointer offset operation does not go out-of-bounds.
> + let file_field = unsafe { core::ptr::addr_of_mut!((*inner).file) };
> +
> + let current = current.as_raw();
> +
> + // SAFETY: This function currently has exclusive access to the `DeferredFdCloserInner`, so
> + // it is okay for us to perform unsynchronized writes to its `callback_head` field.
> + unsafe { bindings::init_task_work(callback_head, Some(Self::do_close_fd)) };
> +
> + // SAFETY: This inserts the `DeferredFdCloserInner` into the task workqueue for the current
> + // task. If this operation is successful, then this transfers exclusive ownership of the
> + // `callback_head` field to the C side until it calls `do_close_fd`, and we don't touch or
> + // invalidate the field during that time.
> + //
> + // When the C side calls `do_close_fd`, the safety requirements of that method are
> + // satisfied because when a task work is executed, the callback is given ownership of the
> + // pointer.
> + //
> + // The file pointer is currently null. If it is changed to be non-null before `do_close_fd`
> + // is called, then that change happens due to the write at the end of this function, and
> + // that write has a safety comment that explains why the refcount can be dropped when
> + // `do_close_fd` runs.
> + let res = unsafe { bindings::task_work_add(current, callback_head, TWA_RESUME) };
> +
> + if res != 0 {
> + // SAFETY: Scheduling the task work failed, so we still have ownership of the box, so
> + // we may destroy it.
> + unsafe { drop(Box::from_raw(inner)) };
> +
> + return Err(DeferredFdCloseError::TaskWorkUnavailable);
> + }
> +
> + // This removes the fd from the fd table in `current`. The file is not fully closed until
> + // `filp_close` is called. We are given ownership of one refcount to the file.
> + //
> + // SAFETY: This is safe no matter what `fd` is. If the `fd` is valid (that is, if the
> + // pointer is non-null), then we call `filp_close` on the returned pointer as required by
> + // `file_close_fd`.
> + let file = unsafe { bindings::file_close_fd(fd) };
> + if file.is_null() {
> + // We don't clean up the task work since that might be expensive if the task work queue
> + // is long. Just let it execute and let it clean up for itself.
> + return Err(DeferredFdCloseError::BadFd);
> + }
> +
> + // Acquire a second refcount to the file.
> + //
> + // SAFETY: The `file` pointer points at a file with a non-zero refcount.
> + unsafe { bindings::get_file(file) };
> +
> + // This method closes the fd, consuming one of our two refcounts. There could be active
> + // light refcounts created from that fd, so we must ensure that the file has a positive
> + // refcount for the duration of those active light refcounts. We do that by holding on to
> + // the second refcount until the current task returns to userspace.
> + //
> + // SAFETY: The `file` pointer is valid. Passing `current->files` as the file table to close
> + // it in is correct, since we just got the `fd` from `file_close_fd` which also uses
> + // `current->files`.
> + //
> + // Note: fl_owner_t is currently a void pointer.
> + unsafe { bindings::filp_close(file, (*current).files as bindings::fl_owner_t) };
> +
> + // We update the file pointer that the task work is supposed to fput. This transfers
> + // ownership of our last refcount.
> + //
> + // INVARIANT: This changes the `file` field of a `DeferredFdCloserInner` from null to
> + // non-null. This doesn't break the type invariant for `DeferredFdCloserInner` because we
> + // still own a refcount to the file, so we can pass ownership of that refcount to the
> + // `DeferredFdCloserInner`.
> + //
> + // When `do_close_fd` runs, it must be safe for it to `fput` the refcount. However, this is
> + // the case because all light refcounts that are associated with the fd we closed
> + // previously must be dropped when `do_close_fd`, since light refcounts must be dropped
> + // before returning to userspace.
> + //
> + // SAFETY: Task works are executed on the current thread right before we return to
> + // userspace, so this write is guaranteed to happen before `do_close_fd` is called, which
> + // means that a race is not possible here.
> + unsafe { *file_field = file };
> +
> + Ok(())
> + }
> +
> + /// # Safety
> + ///
> + /// The provided pointer must point at the `twork` field of a `DeferredFdCloserInner` stored in
> + /// a `Box`, and the caller must pass exclusive ownership of that `Box`. Furthermore, if the
> + /// file pointer is non-null, then it must be okay to release the refcount by calling `fput`.
> + unsafe extern "C" fn do_close_fd(inner: *mut bindings::callback_head) {
> + // SAFETY: The caller just passed us ownership of this box.
> + let inner = unsafe { Box::from_raw(inner.cast::<DeferredFdCloserInner>()) };
> + if !inner.file.is_null() {
> + // SAFETY: By the type invariants, we own a refcount to this file, and the caller
> + // guarantees that dropping the refcount now is okay.
> + unsafe { bindings::fput(inner.file) };
> + }
> + // The allocation is freed when `inner` goes out of scope.
> + }
> +}
> +
> +/// Represents a failure to close an fd in a deferred manner.
> +#[derive(Copy, Clone, Debug, Eq, PartialEq)]
> +pub enum DeferredFdCloseError {
> + /// Closing the fd failed because we were unable to schedule a task work.
> + TaskWorkUnavailable,
> + /// Closing the fd failed because the fd does not exist.
> + BadFd,
> +}
> +
> +impl From<DeferredFdCloseError> for Error {
> + fn from(err: DeferredFdCloseError) -> Error {
> + match err {
> + DeferredFdCloseError::TaskWorkUnavailable => ESRCH,
> + DeferredFdCloseError::BadFd => EBADF,
> + }
> + }
> +}
> +
> /// Represents the `EBADF` error code.
> ///
> /// Used for methods that can only fail with `EBADF`.
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index f46ea3ba9e8e..6adcd8ffcfde 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -145,6 +145,12 @@ fn deref(&self) -> &Self::Target {
> }
> }
>
> + /// Returns a raw pointer to the task.
> + #[inline]
> + pub fn as_raw(&self) -> *mut bindings::task_struct {
> + self.0.get()
> + }
> +
> /// Returns the group leader of the given task.
> pub fn group_leader(&self) -> &Task {
> // SAFETY: By the type invariant, we know that `self.0` is a valid task. Valid tasks always
> @@ -189,6 +195,14 @@ pub fn pid_in_current_ns(&self) -> Pid {
> unsafe { bindings::task_tgid_nr_ns(self.0.get(), ptr::null_mut()) }
> }
>
> + /// Returns whether this task corresponds to a kernel thread.
> + pub fn is_kthread(&self) -> bool {
> + // SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid. There
> + // are no further requirements to read the task's flags.
> + let flags = unsafe { (*self.0.get()).flags };
> + (flags & bindings::PF_KTHREAD) != 0
> + }
> +
> /// Wakes up the task.
> pub fn wake_up(&self) {
> // SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid.
>
> --
> 2.43.0.687.g38aa6559b0-goog
>
