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Fri, 02 Feb 2024 02:55:57 -0800 (PST) Date: Fri, 2 Feb 2024 10:55:38 +0000 In-Reply-To: <20240202-alice-file-v4-0-fc9c2080663b@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240202-alice-file-v4-0-fc9c2080663b@google.com> X-Developer-Key: i=aliceryhl@google.com; a=openpgp; fpr=49F6C1FAA74960F43A5B86A1EE7A392FDE96209F X-Developer-Signature: v=1; a=openpgp-sha256; l=6907; i=aliceryhl@google.com; h=from:subject:message-id; bh=2vVeNVuu/5fFy4n9rU91a6ylN+gEl4mdw/ZMMDxAU7M=; b=owEBbQKS/ZANAwAKAQRYvu5YxjlGAcsmYgBlvMjJdh5WgrbgvuCcDMOcJWBvJ31HrlBrqBMtS /dzR3QR8VqJAjMEAAEKAB0WIQSDkqKUTWQHCvFIvbIEWL7uWMY5RgUCZbzIyQAKCRAEWL7uWMY5 RhthD/0bD1c2e2WGe2h61KiU83ljczWqb3+bILZhB64qEg9UlTt63TNfGwR9QGT+RIDVrZPzLV9 1ZM7UsVbYuzKvfJYloFIwFO4pJxR8EJjXV+Dto5AnDpOVOs9fJDQtQEIbBC4KLK/FsnrGQxgljM 2CDtyixe35XiYQ3MRMXE3/OV2m4+ST6kdJMOic6v7WVjy0Ry2uxTZte8FsQCLA+jElL1KAAsGSX AVuscLUH5cgMv+gARKHD64flxCjtTPIuCyA7GF1DoxeBGx9xmap6ueEjm1o037WOTQLsNaNwNO+ mLFJl/qUTfaFrRTf7vK/r4l/xYl4oLItqBzZUGJM94T5NTc3vnrjKD13UtP6d7gRaDTNZ2+2QBq zWI/shFSPuPqJG+t5ukjZqpJU4opkpwFM/FNtfzpQI3jWecOh7fhiK3MhOlGvtPm2FM7MfvMP2a ty5TKPi4IAG2GFfF8mZ7mG/aljSHkc7uKOCNTRUIXF8bx/cczvr32jFCxugdlNygaN2WhKQvykn ezvsqLuVCH347pWbgDNsjOQCH/ZtttJweKtkljqN0//5j+g7zqUhK6mPiZGwz8e3TBJEaXC2MCq bV1V+mOeUgKvDUjJ/rk8tuLyu3RpNaBVHPFOpPvTzY1Q/+6cUnzScaAbOs9IosJEuP54fTRKdk8 dn2xJV158IEQIeA== X-Mailer: git-send-email 2.43.0.594.gd9cf4e227d-goog Message-ID: <20240202-alice-file-v4-4-fc9c2080663b@google.com> Subject: [PATCH v4 4/9] 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=B8n?= =?utf-8?q?nev=C3=A5g?= " , Todd Kjos , Martijn Coenen , Joel Fernandes , Carlos Llamas , Suren Baghdasaryan , Dan Williams , Kees Cook , Matthew Wilcox , Thomas Gleixner Cc: Daniel Xu , Alice Ryhl , linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1789784438202588557 X-GMAIL-MSGID: 1789784438202588557 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 Signed-off-by: Alice Ryhl Reviewed-by: Benno Lossin Reviewed-by: Trevor Gross Reviewed-by: Martin Rodriguez Reboredo --- 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 +#include #include #include #include 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 #include #include +#include #include #include #include @@ -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: + +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); + +// 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 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;