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Wed, 06 Dec 2023 04:00:06 -0800 (PST) Date: Wed, 06 Dec 2023 11:59:47 +0000 In-Reply-To: <20231206-alice-file-v2-0-af617c0d9d94@google.com> Mime-Version: 1.0 References: <20231206-alice-file-v2-0-af617c0d9d94@google.com> X-Developer-Key: i=aliceryhl@google.com; a=openpgp; fpr=49F6C1FAA74960F43A5B86A1EE7A392FDE96209F X-Developer-Signature: v=1; a=openpgp-sha256; l=6664; i=aliceryhl@google.com; h=from:subject:message-id; bh=Vm1W1WVMEXarXNOCcD4vjJx3cnrZOnPrxh0AiYWusOY=; b=owEBbQKS/ZANAwAKAQRYvu5YxjlGAcsmYgBlcGI5vSsFMnG38B3rWjePZ20b8+wpfFKfcctgA AqrR4pi9vaJAjMEAAEKAB0WIQSDkqKUTWQHCvFIvbIEWL7uWMY5RgUCZXBiOQAKCRAEWL7uWMY5 RhjUD/9DPZhmptqt8Yroautfb0PINF75GwF4YPTEeGm0XoT5DCH1WcBy03aPyiSnRkYxIr7fYai GLxLHjowK7BpyV+WajJyrBODTVO68sorORLofCHjoVHJEEDc/iczDs1NEunjShzU8dw19Kw2MG2 G77RmtsMjIbTJW9xhEqyHkRCJQ50qeaqOcao+NMpWFmiN4MIeW4i1TzQjkj8v/RW4FrxSHybv6g 4qOckgZNBsTKix4WqoxtuImFWZkBflfoC+54yPEsgpZUU6cyNP34Ur3LJA04s4wygkvyB4LZ2bI 8zaU1SO9ByC8rYGIjbi5uZVjoZ0Gm9GMHCxeJEZgR45bN8tXut1WnpiQ0WhSzmG86HPmr/ZqqAv SiBt85Qfmhu5+f5vRry13WuhAmThCT4RZ0R7lvjocrhJAlgM5jY/hSfL/aqiyAmtWFJeelNbd8K /SXoyZ9WNFsalnzfYMYmlKrN+7ZE8Pb6J/cxcHYBClVUeIEmoGLdSZLr8FwJZZJqc8EihN4tTeH 1E0n9BRU6OgkI4i19R/Mq2P3n1tyDExnRVgWwiE71sidFDqWWNLzeEz7lHb4GjJWzqsrQoD2xHY +PIerDizftBdGkiP+KWRYeSvAxUO91VfeQyOIuv+grXGLBsFUwc06+0D/9FMn+llXafWX/SYbOR s3mvrbcakKZ+eyw== X-Mailer: b4 0.13-dev-26615 Message-ID: <20231206-alice-file-v2-2-af617c0d9d94@google.com> Subject: [PATCH v2 2/7] 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 , 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 Content-Type: text/plain; charset="utf-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- rust/bindings/bindings_helper.h | 1 + rust/helpers.c | 13 +++++++++ rust/kernel/cred.rs | 64 +++++++++++++++++++++++++++++++++++++++++ rust/kernel/file.rs | 16 +++++++++++ rust/kernel/lib.rs | 1 + 5 files changed, 95 insertions(+) diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h index beed3ef1fbc3..6d1bd2229aab 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..497058ec89bb --- /dev/null +++ b/rust/kernel/cred.rs @@ -0,0 +1,64 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Credentials management. +//! +//! C header: [`include/linux/cred.h`](../../../../include/linux/cred.h) +//! +//! Reference: + +use crate::{ + bindings, + types::{AlwaysRefCounted, Opaque}, +}; + +/// Wraps the kernel's `struct cred`. +/// +/// # 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`. +#[repr(transparent)] +pub struct Credential(pub(crate) Opaque); + +// SAFETY: By design, the only way to access a `Credential` is via an immutable reference or an +// `ARef`. This means that the only situation in which a `Credential` can be accessed mutably is +// when the refcount drops to zero and the destructor runs. It is safe for that to happen on any +// thread, so it is ok for this type to be `Send`. +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. + 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. + unsafe { bindings::put_cred(obj.cast().as_ptr()) }; + } +} diff --git a/rust/kernel/file.rs b/rust/kernel/file.rs index 29e1aacacd06..a88140794a8d 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}, }; @@ -151,6 +152,21 @@ 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: Since the caller holds a reference to the file, it is guaranteed that its + // refcount does not hit zero during this function call. + // + // It's okay to read the `f_cred` field without synchronization as `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 ce9abceab784..097fe9bb93ed 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.rc2.451.g8631bc7472-goog