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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}