cred.rs

 1// SPDX-License-Identifier: GPL-2.0
 2
 3// Copyright (C) 2024 Google LLC.
 4
 5//! Credentials management.
 6//!
 7//! C header: [`include/linux/cred.h`](srctree/include/linux/cred.h).
 8//!
 9//! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html>
10
11use crate::{bindings, sync::aref::AlwaysRefCounted, task::Kuid, types::Opaque};
12
13/// Wraps the kernel's `struct cred`.
14///
15/// Credentials are used for various security checks in the kernel.
16///
17/// Most fields of credentials are immutable. When things have their credentials changed, that
18/// happens by replacing the credential instead of changing an existing credential. See the [kernel
19/// documentation][ref] for more info on this.
20///
21/// # Invariants
22///
23/// Instances of this type are always ref-counted, that is, a call to `get_cred` ensures that the
24/// allocation remains valid at least until the matching call to `put_cred`.
25///
26/// [ref]: https://www.kernel.org/doc/html/latest/security/credentials.html
27#[repr(transparent)]
28pub struct Credential(Opaque<bindings::cred>);
29
30// SAFETY:
31// - `Credential::dec_ref` can be called from any thread.
32// - It is okay to send ownership of `Credential` across thread boundaries.
33unsafe impl Send for Credential {}
34
35// SAFETY: It's OK to access `Credential` through shared references from other threads because
36// we're either accessing properties that don't change or that are properly synchronised by C code.
37unsafe impl Sync for Credential {}
38
39impl Credential {
40    /// Creates a reference to a [`Credential`] from a valid pointer.
41    ///
42    /// # Safety
43    ///
44    /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
45    /// returned [`Credential`] reference.
46    #[inline]
47    pub unsafe fn from_ptr<'a>(ptr: *const bindings::cred) -> &'a Credential {
48        // SAFETY: The safety requirements guarantee the validity of the dereference, while the
49        // `Credential` type being transparent makes the cast ok.
50        unsafe { &*ptr.cast() }
51    }
52
53    /// Returns a raw pointer to the inner credential.
54    #[inline]
55    pub fn as_ptr(&self) -> *const bindings::cred {
56        self.0.get()
57    }
58
59    /// Get the id for this security context.
60    #[inline]
61    pub fn get_secid(&self) -> u32 {
62        let mut secid = 0;
63        // SAFETY: The invariants of this type ensures that the pointer is valid.
64        unsafe { bindings::security_cred_getsecid(self.0.get(), &mut secid) };
65        secid
66    }
67
68    /// Returns the effective UID of the given credential.
69    #[inline]
70    pub fn euid(&self) -> Kuid {
71        // SAFETY: By the type invariant, we know that `self.0` is valid. Furthermore, the `euid`
72        // field of a credential is never changed after initialization, so there is no potential
73        // for data races.
74        Kuid::from_raw(unsafe { (*self.0.get()).euid })
75    }
76}
77
78// SAFETY: The type invariants guarantee that `Credential` is always ref-counted.
79unsafe impl AlwaysRefCounted for Credential {
80    #[inline]
81    fn inc_ref(&self) {
82        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
83        unsafe { bindings::get_cred(self.0.get()) };
84    }
85
86    #[inline]
87    unsafe fn dec_ref(obj: core::ptr::NonNull<Credential>) {
88        // SAFETY: The safety requirements guarantee that the refcount is nonzero. The cast is okay
89        // because `Credential` has the same representation as `struct cred`.
90        unsafe { bindings::put_cred(obj.cast().as_ptr()) };
91    }
92}