kernel/
security.rs

1// SPDX-License-Identifier: GPL-2.0
2
3// Copyright (C) 2024 Google LLC.
4
5//! Linux Security Modules (LSM).
6//!
7//! C header: [`include/linux/security.h`](srctree/include/linux/security.h).
8
9use crate::{
10    bindings,
11    error::{to_result, Result},
12};
13
14/// A security context string.
15///
16/// # Invariants
17///
18/// The `ctx` field corresponds to a valid security context as returned by a successful call to
19/// `security_secid_to_secctx`, that has not yet been destroyed by `security_release_secctx`.
20pub struct SecurityCtx {
21    ctx: bindings::lsm_context,
22}
23
24impl SecurityCtx {
25    /// Get the security context given its id.
26    pub fn from_secid(secid: u32) -> Result<Self> {
27        // SAFETY: `struct lsm_context` can be initialized to all zeros.
28        let mut ctx: bindings::lsm_context = unsafe { core::mem::zeroed() };
29
30        // SAFETY: Just a C FFI call. The pointer is valid for writes.
31        to_result(unsafe { bindings::security_secid_to_secctx(secid, &mut ctx) })?;
32
33        // INVARIANT: If the above call did not fail, then we have a valid security context.
34        Ok(Self { ctx })
35    }
36
37    /// Returns whether the security context is empty.
38    pub fn is_empty(&self) -> bool {
39        self.ctx.len == 0
40    }
41
42    /// Returns the length of this security context.
43    pub fn len(&self) -> usize {
44        self.ctx.len as usize
45    }
46
47    /// Returns the bytes for this security context.
48    pub fn as_bytes(&self) -> &[u8] {
49        let ptr = self.ctx.context;
50        if ptr.is_null() {
51            debug_assert_eq!(self.len(), 0);
52            // We can't pass a null pointer to `slice::from_raw_parts` even if the length is zero.
53            return &[];
54        }
55
56        // SAFETY: The call to `security_secid_to_secctx` guarantees that the pointer is valid for
57        // `self.len()` bytes. Furthermore, if the length is zero, then we have ensured that the
58        // pointer is not null.
59        unsafe { core::slice::from_raw_parts(ptr.cast(), self.len()) }
60    }
61}
62
63impl Drop for SecurityCtx {
64    fn drop(&mut self) {
65        // SAFETY: By the invariant of `Self`, this frees a context that came from a successful
66        // call to `security_secid_to_secctx` and has not yet been destroyed by
67        // `security_release_secctx`.
68        unsafe { bindings::security_release_secctx(&mut self.ctx) };
69    }
70}