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a=openpgp-sha256; l=7648; i=aliceryhl@google.com; h=from:subject:message-id; bh=pZluqOYDewoyL/Pw33wzo0+9mXuz9XshwBqpSa+QO2E=; b=owEBbQKS/ZANAwAKAQRYvu5YxjlGAcsmYgBm5u/CPUwRpX0HXjIIOfY6uZ0KzDr9daxhGzzRG bLR0Gy+y5yJAjMEAAEKAB0WIQSDkqKUTWQHCvFIvbIEWL7uWMY5RgUCZubvwgAKCRAEWL7uWMY5 Rh6LEACEeOGnZkjswcGTbhfEd6qUCjTH4OdIecXnKr4sOCs7PU104TJpThMdfWV3TYbvI3IG0LL wTzbzGvY/O+my2pyHifEIwFfCgyvRQJYHm2lmFIQcKjgpX8sDzTn4/w9vdHj1+/PdrMKWN8U9rL kNjGyuCbMQaiJAT450K0+aUNS9IsBnfr59hLjWT39M5efj4tPqVrCHOHppf7YnRFsPF/A66tsOG aKiXmkyNzP3UB0hFZjO7olLiEFUGf83JMc1mE+GzQniYC3C2jdGli9WThGINMD22TwPMEMo7rJE dtDEnpEEOSJV+O97MwlEnk0bZ7OmE/9/fWFaLxCX5yBqtDVTLz5meC/y+ah5vpYGs7XPY40XTaF +4Q2TX0e2wf9GIIvw1ZAffnRrx6RbQUKGHdpOOTHvtaV84x1eDqCj3XaKCr5qbGgGvZCsuIqvc4 Lxnr+N2iSCIn+1KzrnBXD6VL3j1CZzBGOQDu0hWWEG2oaACYCFB1DdRU13GyK9fpv6Zlsj91okN 0Appvfc0WBft83SP3O6mwHQfoGqIQ1UPE7O30BC5Ujph75CZ+EioKc4DEKXlrqrgHyb1XYIvHJt XtFthh5wjYwUn23UfKJ/JcXEUI0RlGoRSk3vMNbGrl0IEGtQzJOdzhZGVoDR7qhcAHxwvh3RSXQ cXh9DO23N2Inxgw== X-Mailer: b4 0.13.0 Message-ID: <20240915-alice-file-v10-4-88484f7a3dcf@google.com> Subject: [PATCH v10 4/8] rust: cred: add Rust abstraction for `struct cred` From: Alice Ryhl To: Paul Moore , James Morris , "Serge E. Hallyn" , Miguel Ojeda , Christian Brauner Cc: 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 , Greg Kroah-Hartman , "=?utf-8?q?Arve_Hj=C3=B8nnev=C3=A5g?=" , Todd Kjos , Martijn Coenen , Joel Fernandes , Carlos Llamas , Suren Baghdasaryan , Dan Williams , Matthew Wilcox , Thomas Gleixner , Daniel Xu , Martin Rodriguez Reboredo , Trevor Gross , linux-kernel@vger.kernel.org, linux-security-module@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. 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 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 Reviewed-by: Kees Cook Reviewed-by: Paul Moore --- 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_helpe= r.h index 4a400a954979..f74247205cb5 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/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 + +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: + +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 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)] --=20 2.46.0.662.g92d0881bb0-goog