[PATCH v4 4/9] rust: cred: add Rust abstraction for `struct cred`

Alice Ryhl
2024-02-02T10:55:58Z

From: Wedson Almeida Filho <wedsonaf@gmail.com>

Add a wrapper around `struct cred` called `Credential`, and provide
functionality to get the `Credential` associated with a `File`.

Rust Binder must check the credentials of processes when they attempt to
perform various operations, and these checks usually take a
`&Credential` as parameter. The security_binder_set_context_mgr function
would be one example. This patch is necessary to access these security_*
methods from Rust.

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/bindings/bindings_helper.h | 1 +
rust/helpers.c | 13 ++++++
rust/kernel/cred.rs | 72 +++++++++++++++++++++++++++++++++
rust/kernel/file.rs | 13 ++++++
rust/kernel/lib.rs | 1 +
5 files changed, 100 insertions(+)
create mode 100644 rust/kernel/cred.rs

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 41fcd2905ed4..84a56e8b6b67 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -7,6 +7,7 @@
*/

#include <kunit/test.h>
+#include <linux/cred.h>
#include <linux/errname.h>
#include <linux/ethtool.h>
#include <linux/file.h>
diff --git a/rust/helpers.c b/rust/helpers.c
index 03141a3608a4..10ed69f76424 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -23,6 +23,7 @@
#include <kunit/test-bug.h>
#include <linux/bug.h>
#include <linux/build_bug.h>
+#include <linux/cred.h>
#include <linux/err.h>
#include <linux/errname.h>
#include <linux/fs.h>
@@ -164,6 +165,18 @@ struct file *rust_helper_get_file(struct file *f)
}
EXPORT_SYMBOL_GPL(rust_helper_get_file);

+const struct cred *rust_helper_get_cred(const struct cred *cred)
+{
+ return get_cred(cred);
+}
+EXPORT_SYMBOL_GPL(rust_helper_get_cred);
+
+void rust_helper_put_cred(const struct cred *cred)
+{
+ put_cred(cred);
+}
+EXPORT_SYMBOL_GPL(rust_helper_put_cred);
+
/*
* `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/cred.rs b/rust/kernel/cred.rs
new file mode 100644
index 000000000000..fabc50e48c9e
--- /dev/null
+++ b/rust/kernel/cred.rs
@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Credentials management.
+//!
+//! C header: [`include/linux/cred.h`](srctree/include/linux/cred.h).
+//!
+//! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html>
+
+use crate::{
+ bindings,
+ types::{AlwaysRefCounted, Opaque},
+};
+
+/// Wraps the kernel's `struct cred`.
+///
+/// Most fields of credentials are immutable. When things have their credentials changed, that
+/// happens by replacing the credential instad of changing an existing credential. See the [kernel
+/// documentation][ref] for more info on this.
+///
+/// # Invariants
+///
+/// Instances of this type are always ref-counted, that is, a call to `get_cred` ensures that the
+/// allocation remains valid at least until the matching call to `put_cred`.
+///
+/// [ref]: https://www.kernel.org/doc/html/latest/security/credentials.html
+#[repr(transparent)]
+pub struct Credential(Opaque<bindings::cred>);
+
+// SAFETY:
+// - `Credential::dec_ref` can be called from any thread.
+// - It is okay to send ownership of `Credential` across thread boundaries.
+unsafe impl Send for Credential {}
+
+// SAFETY: It's OK to access `Credential` through shared references from other threads because
+// we're either accessing properties that don't change or that are properly synchronised by C code.
+unsafe impl Sync for Credential {}
+
+impl Credential {
+ /// Creates a reference to a [`Credential`] from a valid pointer.
+ ///
+ /// # Safety
+ ///
+ /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
+ /// returned [`Credential`] reference.
+ pub unsafe fn from_ptr<'a>(ptr: *const bindings::cred) -> &'a Credential {
+ // SAFETY: The safety requirements guarantee the validity of the dereference, while the
+ // `Credential` type being transparent makes the cast ok.
+ unsafe { &*ptr.cast() }
+ }
+
+ /// Returns the effective UID of the given credential.
+ pub fn euid(&self) -> bindings::kuid_t {
+ // SAFETY: By the type invariant, we know that `self.0` is valid. Furthermore, the `euid`
+ // field of a credential is never changed after initialization, so there is no potential
+ // for data races.
+ unsafe { (*self.0.get()).euid }
+ }
+}
+
+// SAFETY: The type invariants guarantee that `Credential` is always ref-counted.
+unsafe impl AlwaysRefCounted for Credential {
+ fn inc_ref(&self) {
+ // SAFETY: The existence of a shared reference means that the refcount is nonzero.
+ unsafe { bindings::get_cred(self.0.get()) };
+ }
+
+ unsafe fn dec_ref(obj: core::ptr::NonNull<Credential>) {
+ // SAFETY: The safety requirements guarantee that the refcount is nonzero. The cast is okay
+ // because `Credential` has the same representation as `struct cred`.
+ unsafe { bindings::put_cred(obj.cast().as_ptr()) };
+ }
+}
diff --git a/rust/kernel/file.rs b/rust/kernel/file.rs
index 0d6ef32009c6..095775411979 100644
--- a/rust/kernel/file.rs
+++ b/rust/kernel/file.rs
@@ -7,6 +7,7 @@

use crate::{
bindings,
+ cred::Credential,
error::{code::*, Error, Result},
types::{ARef, AlwaysRefCounted, Opaque},
};
@@ -202,6 +203,18 @@ pub fn as_ptr(&self) -> *mut bindings::file {
self.0.get()
}

+ /// Returns the credentials of the task that originally opened the file.
+ pub fn cred(&self) -> &Credential {
+ // SAFETY: It's okay to read the `f_cred` field without synchronization because `f_cred` is
+ // never changed after initialization of the file.
+ let ptr = unsafe { (*self.as_ptr()).f_cred };
+
+ // SAFETY: The signature of this function ensures that the caller will only access the
+ // returned credential while the file is still valid, and the C side ensures that the
+ // credential stays valid at least as long as the file.
+ unsafe { Credential::from_ptr(ptr) }
+ }
+
/// Returns the flags associated with the file.
///
/// The flags are a combination of the constants in [`flags`].
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 9353dd713a20..f65e5978f807 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -33,6 +33,7 @@
#[cfg(not(testlib))]
mod allocator;
mod build_assert;
+pub mod cred;
pub mod error;
pub mod file;
pub mod init;
--
2.43.0.594.gd9cf4e227d-goog