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a=openpgp-sha256; l=7305; i=aliceryhl@google.com; h=from:subject:message-id; bh=LUr04YqufJkZXc462uNrCvjGvd9bTLfFksx5H6DRsjc=; b=owEBbQKS/ZANAwAKAQRYvu5YxjlGAcsmYgBmtO9XJ+/XGpnowA/6HESbnFbLf8XNjj3z4IPwG 852GLKcSRyJAjMEAAEKAB0WIQSDkqKUTWQHCvFIvbIEWL7uWMY5RgUCZrTvVwAKCRAEWL7uWMY5 RnfBD/wK2KV0yCbgPozS1Ka4SfARilLOmaMqqgnFbD8qaY8oJufJH4BVMwN/MncTAn8VZNWX4Ig NDKykgJhOuLEkPOlVtyHAa8dsSH9pndmMrEe2uN8IAT6IMXGR3jqro/Hougw3xLTLC1Jwhcdu66 3PmMBYaC9mJrNvBx5MKR63o0yhEwFuSwQcgXTbegheHPTpemh/cr5ziSOyUABrz+ajOR7UT5nJy ILOF3Ftx3n5/DiCmx7Dt5tjuh0tliuaAym4xrYzyO7dl5k13r/547HWe9bRgqSV2ySPYL069D44 o312tiOKOZm7gE4Gh1rfpgNh/lpOa95kfezu1gvZN2AWeEq7Tcl3ijlaosWZgWG07aqivuogEbi Yp1kJjGQt0XTU5k5PcXKKsYjwt/menjwJJWKnIeiLcv/IUHsgWrHlLXwXIXfb2hbDnGeyhCSqf4 Vrik7QzBFaLiMsQCcBcGsoTq5vhWmx5M1qPyNFkbicBLlyCBapAKISYDQ/nGGEvw5AC5sAdy5vP nyR8r0/nJ2L1gocRwsY3ViWvYXGLgiQpb0WZiZdTW/fDUtiTGqI00qHnCRfMZbZTMzCCo09xf0z aO5SLG/zZEbkLkHDYugUACORzSeZCIxN29nsemjs5qmqvM5OSpSlRxkYYG+ltSG1b2VBnSazdtK EM5jrAxBnRT8ZOA== X-Mailer: b4 0.13.0 Message-ID: <20240808-alice-file-v9-4-2cb7b934e0e1@google.com> Subject: [PATCH v9 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=C3=B8nnev=C3=A5g?=" , Todd Kjos , Martijn Coenen , Joel Fernandes , Carlos Llamas , Suren Baghdasaryan Cc: Dan Williams , Matthew Wilcox , Thomas Gleixner , Daniel Xu , Martin Rodriguez Reboredo , Trevor Gross , linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org, Alice Ryhl , Kees Cook Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable 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 Reviewed-by: Gary Guo Signed-off-by: Alice Ryhl --- rust/bindings/bindings_helper.h | 1 + rust/helpers.c | 13 +++++++ rust/kernel/cred.rs | 76 +++++++++++++++++++++++++++++++++++++= ++++ rust/kernel/fs/file.rs | 13 +++++++ rust/kernel/lib.rs | 1 + 5 files changed, 104 insertions(+) diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helpe= r.h index 550a4a46d413..81bd1c2db7c9 100644 --- a/rust/bindings/bindings_helper.h +++ b/rust/bindings/bindings_helper.h @@ -10,6 +10,7 @@ #include #include #include +#include #include #include #include diff --git a/rust/helpers.c b/rust/helpers.c index 80021b0a7c63..a63f6b614725 100644 --- a/rust/helpers.c +++ b/rust/helpers.c @@ -23,6 +23,7 @@ #include #include #include +#include #include #include #include @@ -207,6 +208,18 @@ struct file *rust_helper_get_file(struct file *f) } EXPORT_SYMBOL_GPL(rust_helper_get_file); =20 +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) indi= ces. 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: + +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 crede= ntials changed, that +/// happens by replacing the credential instead of changing an existing cr= edential. 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 o= ther threads because +// we're either accessing properties that don't change or that are properl= y 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 t= he lifetime of the + /// returned [`Credential`] reference. + pub unsafe fn from_ptr<'a>(ptr: *const bindings::cred) -> &'a Credenti= al { + // SAFETY: The safety requirements guarantee the validity of the d= ereference, 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-c= ounted. +unsafe impl AlwaysRefCounted for Credential { + fn inc_ref(&self) { + // SAFETY: The existence of a shared reference means that the refc= ount 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 cre= d`. + 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 @@ =20 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() } =20 + /// Returns the credentials of the task that originally opened the fil= e. + pub fn cred(&self) -> &Credential { + // SAFETY: It's okay to read the `f_cred` field without synchroniz= ation because `f_cred` is + // never changed after initialization of the file. + let ptr =3D 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 si= de 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 61e0dd4ded78..c9ce44812d21 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)] --=20 2.46.0.rc2.264.g509ed76dc8-goog