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a=openpgp-sha256; l=7101; i=aliceryhl@google.com; h=from:subject:message-id; bh=RLxJnIFmmThp3fETtbijGb8ieDu222aEk+QMIYVNKMM=; b=owEBbQKS/ZANAwAKAQRYvu5YxjlGAcsmYgBmfs9kLchzWCZxKvmZirniEjMYg6x6IpyWbkVDt Gc2J6HlVjGJAjMEAAEKAB0WIQSDkqKUTWQHCvFIvbIEWL7uWMY5RgUCZn7PZAAKCRAEWL7uWMY5 RrdbEACmlKuKhZnKfdtJeHmMsHCFUz/SVUYD6sxWsLjXbGQpriNwGnqQkUUT/IJxr0Csp4XBCLr VKcZ09jPKO73ykP5uPfbsOn7E2qu2anYkFBpcCDpsqMJXMOIE+b2ZKV3tGw4Upkw2lnur2qn08N iV2MAA8nGD7rna2bsf8DyM0re2MCU1Ux4jhT8Vz7blpwi60ezAUBJ4t7XIUXSesWjbrgbLzjdBq eNOV5op4NjcpKqenS5oEsbOUhq53e3y0l25ic1cWBHEOwDggXrlA4ZmPwwSTfitrsv1uGm6abO2 NqYzjx/lzcBTUFSMCdNaba7iddy0dqwWFbpTuPgPPKcTkHuvovpiwimGJWjPLM9Mu/PwABhmtwT R6qEsjgPK0Cui3794fLBnPNfltSrNY9AnM19rtYqvgd5/PSArwKR0zLB2D+mDoX1e0sTC9QW52/ wkS6IUiGXF44qkJ9n5kzY4hUjz0HQmLw7ldgVSKjhFnPniJTfOJs8Pq70RGB66kTbLCRgaOIxW2 x/RSaJqhOneYL3vWG/yJT0SF+ZZYw4bbaN8mmg6+e8A43/6CiR2Xgh31UCTcjrxS26s8CxEms0r zBpJqZcPItoUM15UGH+1/onjUR+W2Qj/dts/+8yS7rkFbpDz7mtfYlcBz0u5v0TAlTHQkcbyAOm DUHj8DuGOhYQGQg== X-Mailer: b4 0.13-dev-26615 Message-ID: <20240628-alice-file-v7-4-4d701f6335f3@google.com> Subject: [PATCH v7 4/8] rust: cred: add Rust abstraction for `struct cred` From: Alice Ryhl To: Miguel Ojeda , Alex Gaynor , Wedson Almeida Filho , Boqun Feng , Gary Guo , " =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= " , Benno Lossin , Andreas Hindborg , Peter Zijlstra , Alexander Viro , Christian Brauner , Greg Kroah-Hartman , " =?utf-8?q?Arve_Hj?= =?utf-8?q?=C3=B8nnev=C3=A5g?= " , Todd Kjos , Martijn Coenen , Joel Fernandes , Carlos Llamas , Suren Baghdasaryan Cc: Dan Williams , Kees Cook , Matthew Wilcox , Thomas Gleixner , Daniel Xu , linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org, Alice Ryhl , Trevor Gross , Martin Rodriguez Reboredo From: Wedson Almeida Filho 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 Co-developed-by: Alice Ryhl Reviewed-by: Trevor Gross Reviewed-by: Benno Lossin Reviewed-by: Martin Rodriguez Reboredo Signed-off-by: Alice Ryhl --- rust/bindings/bindings_helper.h | 1 + rust/helpers.c | 13 ++++++++ rust/kernel/cred.rs | 74 +++++++++++++++++++++++++++++++++++++++++ rust/kernel/file.rs | 13 ++++++++ rust/kernel/lib.rs | 1 + 5 files changed, 102 insertions(+) diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h index 541afef7ddc4..94091cb337e9 100644 --- a/rust/bindings/bindings_helper.h +++ b/rust/bindings/bindings_helper.h @@ -7,6 +7,7 @@ */ #include +#include #include #include #include diff --git a/rust/helpers.c b/rust/helpers.c index e68025b53342..9cf25e5324a4 100644 --- a/rust/helpers.c +++ b/rust/helpers.c @@ -23,6 +23,7 @@ #include #include #include +#include #include #include #include @@ -172,6 +173,18 @@ 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..360d6fdbe5e7 --- /dev/null +++ b/rust/kernel/cred.rs @@ -0,0 +1,74 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Credentials management. +//! +//! C header: [`include/linux/cred.h`](srctree/include/linux/cred.h). +//! +//! Reference: + +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); + +// 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) { + // 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 08551bf5c625..91113f844981 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}, }; @@ -272,6 +273,18 @@ 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 dba3415c1cee..ea7d07f0e83d 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -28,6 +28,7 @@ pub mod alloc; mod build_assert; +pub mod cred; pub mod error; pub mod file; pub mod init;