From: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
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.
This Rust abstraction makes the following assumptions about the C side:
* `struct cred` is refcounted with `get_cred`/`put_cred`.
* It's okay to transfer a `struct cred` across threads, that is, you do
not need to call `put_cred` on the same thread as where you called
`get_cred`.
* The `euid` field of a `struct cred` never changes after
initialization.
* The `f_cred` field of a `struct file` never changes after
initialization.
Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
Co-developed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Reviewed-by: Trevor Gross <tmgross@xxxxxxxxx>
Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>
Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>
Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
rust/bindings/bindings_helper.h | 1 +
rust/helpers/cred.c | 13 +++++++
rust/helpers/helpers.c | 1 +
rust/kernel/cred.rs | 76 +++++++++++++++++++++++++++++++++++++++++
rust/kernel/fs/file.rs | 13 +++++++
rust/kernel/lib.rs | 1 +
6 files changed, 105 insertions(+)
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 4a400a954979..f74247205cb5 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -10,6 +10,7 @@
#include <linux/blk-mq.h>
#include <linux/blk_types.h>
#include <linux/blkdev.h>
+#include <linux/cred.h>
#include <linux/errname.h>
#include <linux/ethtool.h>
#include <linux/file.h>
diff --git a/rust/helpers/cred.c b/rust/helpers/cred.c
new file mode 100644
index 000000000000..fde7ae20cdd1
--- /dev/null
+++ b/rust/helpers/cred.c
@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/cred.h>
+
+const struct cred *rust_helper_get_cred(const struct cred *cred)
+{
+ return get_cred(cred);
+}
+
+void rust_helper_put_cred(const struct cred *cred)
+{
+ put_cred(cred);
+}
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 3f2d0d0c8017..16e5de352dab 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -11,6 +11,7 @@
#include "bug.c"
#include "build_assert.c"
#include "build_bug.c"
+#include "cred.c"
#include "err.c"
#include "fs.c"
#include "kunit.c"
diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
new file mode 100644
index 000000000000..acee04768927
--- /dev/null
+++ b/rust/kernel/cred.rs
@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! 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`.
+///
+/// Credentials are used for various security checks in the kernel.
+///
+/// Most fields of credentials are immutable. When things have their credentials changed, that
+/// happens by replacing the credential instead 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/fs/file.rs b/rust/kernel/fs/file.rs
index 6adb7a7199ec..3c1f51719804 100644
--- a/rust/kernel/fs/file.rs
+++ b/rust/kernel/fs/file.rs
@@ -9,6 +9,7 @@
use crate::{
bindings,
+ cred::Credential,
error::{code::*, Error, Result},
types::{ARef, AlwaysRefCounted, Opaque},
};
@@ -308,6 +309,18 @@ pub fn as_ptr(&self) -> *mut bindings::file {
self.inner.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 c7d50f245f58..c537d17c6db9 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -30,6 +30,7 @@
#[cfg(CONFIG_BLOCK)]
pub mod block;
mod build_assert;
+pub mod cred;
pub mod device;
pub mod error;
#[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
--
2.46.0.662.g92d0881bb0-goog
