From: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx> Make it possible to access credentials from Rust drivers. In particular, this patch makes it possible to get the id for the security context of a given file. Signed-off-by: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx> Co-Developed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx> --- rust/bindings/bindings_helper.h | 2 + rust/helpers.c | 22 +++++++++++ rust/kernel/cred.rs | 66 +++++++++++++++++++++++++++++++++ rust/kernel/file.rs | 15 ++++++++ rust/kernel/lib.rs | 1 + 5 files changed, 106 insertions(+) create mode 100644 rust/kernel/cred.rs diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs new file mode 100644 index 000000000000..ca3fac4851a2 --- /dev/null +++ b/rust/kernel/cred.rs @@ -0,0 +1,66 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Credentials management. +//! +//! C header: [`include/linux/cred.h`](../../../../include/linux/cred.h) +//! +//! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html> + +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<bindings::cred>); + +// 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() } + } + + /// Get the id for this security context. + pub fn get_secid(&self) -> u32 { + let mut secid = 0; + // SAFETY: The invariants of this type ensures that the pointer is valid. + unsafe { bindings::security_cred_getsecid(self.0.get(), &mut secid) }; + secid + } +} + +// 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<Self>) { + // 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 99657adf2472..d379ae2906d9 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}, }; @@ -138,6 +139,20 @@ pub unsafe fn from_ptr<'a>(ptr: *const bindings::file) -> &'a File { unsafe { &*ptr.cast() } } + /// Returns the credentials of the task that originally opened the file. + pub fn cred(&self) -> &Credential { + // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount. + // + // This uses a volatile read because C code may be modifying this field in parallel using + // non-atomic unsynchronized writes. This corresponds to how the C macro READ_ONCE is + // implemented. + let ptr = unsafe { core::ptr::addr_of!((*self.0.get()).f_cred).read_volatile() }; + // SAFETY: The lifetimes of `self` and `Credential` are tied, so it is guaranteed that + // the credential pointer remains valid (because the file is still alive, and it doesn't + // change over the lifetime of a 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 650bfffc1e6f..07258bfa8960 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -31,6 +31,7 @@ #[cfg(not(testlib))] mod allocator; mod build_assert; +pub mod cred; pub mod error; pub mod file; pub mod init; diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h index c5b2cfd02bac..d89f0df93615 100644 --- a/rust/bindings/bindings_helper.h +++ b/rust/bindings/bindings_helper.h @@ -6,9 +6,11 @@ * Sorted alphabetically. */ +#include <linux/cred.h> #include <linux/errname.h> #include <linux/file.h> #include <linux/fs.h> +#include <linux/security.h> #include <linux/slab.h> #include <linux/refcount.h> #include <linux/wait.h> diff --git a/rust/helpers.c b/rust/helpers.c index 072f7ef80ea5..e13a7da430b1 100644 --- a/rust/helpers.c +++ b/rust/helpers.c @@ -22,12 +22,14 @@ #include <linux/bug.h> #include <linux/build_bug.h> +#include <linux/cred.h> #include <linux/err.h> #include <linux/errname.h> #include <linux/fs.h> #include <linux/mutex.h> #include <linux/refcount.h> #include <linux/sched/signal.h> +#include <linux/security.h> #include <linux/spinlock.h> #include <linux/wait.h> @@ -144,6 +146,26 @@ 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); + +#ifndef CONFIG_SECURITY +void rust_helper_security_cred_getsecid(const struct cred *c, u32 *secid) +{ + security_cred_getsecid(c, secid); +} +EXPORT_SYMBOL_GPL(rust_helper_security_cred_getsecid); +#endif + /* * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type * as the Rust `usize` type, so we can use it in contexts where Rust -- 2.41.0.255.g8b1d071c50-goog
