starnix_core/security/selinux_hooks/
audit.rs

1// Copyright 2024 The Fuchsia Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5use crate::task::{CurrentTask, Task};
6use crate::vfs::{
7    DirEntry, DirEntryHandle, FileObject, FileSystem, FsNode, FsStr, NamespaceNode,
8    PathWithReachability,
9};
10use bstr::BStr;
11use fuchsia_rcu::RcuReadScope;
12use hex;
13use linux_uapi::AUDIT_AVC;
14use selinux::permission_check::{PermissionCheck, PermissionCheckResult};
15use selinux::{ClassPermission, KernelClass, KernelPermission, SecurityId};
16use starnix_logging::CATEGORY_STARNIX_SECURITY;
17use starnix_sync::{AuditDenyCountsLock, LockDepMutex};
18use std::collections::HashMap;
19use std::fmt::{Display, Error};
20use std::num::NonZeroU32;
21use std::sync::LazyLock;
22
23/// Represents a unique auditable instance, for rate limiting purposes.
24#[derive(Clone, Eq, Hash, PartialEq)]
25struct AuditableInstance {
26    source_sid: SecurityId,
27    target_sid: SecurityId,
28    class: KernelClass,
29    bug: NonZeroU32,
30}
31
32/// Stores count of todo_deny logged per auditable instance.
33static TODO_DENY_COUNTS: LazyLock<
34    LockDepMutex<HashMap<AuditableInstance, u32>, AuditDenyCountsLock>,
35> = LazyLock::new(|| Default::default());
36
37/// Checks whether an audit log entry should still be emitted for this audit instance.
38fn should_audit(
39    source_sid: SecurityId,
40    target_sid: SecurityId,
41    class: KernelClass,
42    bug: NonZeroU32,
43) -> bool {
44    // Audit-log the first few denials, but skip further denials to avoid logspamming.
45    const MAX_TODO_AUDIT_DENIALS: u32 = 5;
46
47    let mut counts = TODO_DENY_COUNTS.lock();
48    let count = counts.entry(AuditableInstance { source_sid, target_sid, class, bug }).or_default();
49    *count += 1;
50    *count <= MAX_TODO_AUDIT_DENIALS
51}
52
53/// Container for a reference to kernel state from which to include details when emitting audit
54/// logging.  [`Auditable`] instances are created from references to objects via `into()`, e.g:
55///
56///   fn my_lovely_hook(current_task: &CurrentTask, ...) {
57///     let audit_context = current_task.into();
58///     check_permission(..., audit_context)
59///   }
60///
61/// Call-sites which need to include context from multiple sources into audit logs can do so by
62/// creating an array of [`Auditable`] instances from those sources, and using `into()` to create
63/// an [`Auditable`] from a reference to that array, e.g:
64///
65///   fn my_lovelier_hook(current_task: &CurrentTask,..., audit_context: Auditable<'_>) {
66///     let audit_context = [audit_context, current_task.into()];
67///     check_permission(..., (&audit_context).into())
68///   }
69///
70/// [`Auditable`] instances are parameterized with the lifetime of the references they contain,
71/// which will be automagically derived by Rust. Since they only consist of a type discriminator and
72/// reference they are cheap to copy, avoiding the need to pass them by-reference if the same
73/// context is to be applied to multiple permission checks.
74#[derive(Debug, Clone, Copy)]
75pub enum Auditable<'a> {
76    // keep-sorted start
77    AuditContext(&'a [Auditable<'a>]),
78    Bug(u32),
79    CurrentTask,
80    DirEntry(&'a DirEntry),
81    FileObject(&'a FileObject),
82    FileSystem(&'a FileSystem),
83    FsNode(&'a FsNode),
84    IoctlCommand(u16),
85    Location(&'a std::panic::Location<'a>),
86    Name(&'a FsStr),
87    NamespaceNode(&'a NamespaceNode),
88    NlMsgtype(u16),
89    None,
90    SockOptArguments(u32, u32),
91    Task(&'a Task),
92    // keep-sorted end
93}
94
95impl Auditable<'_> {
96    fn from_bug(bug_id: u32) -> Self {
97        Auditable::Bug(bug_id)
98    }
99}
100
101impl<'a> From<&'a CurrentTask> for Auditable<'a> {
102    fn from(_value: &'a CurrentTask) -> Self {
103        // This case is vestigal and will be removed.
104        Auditable::CurrentTask
105    }
106}
107
108impl<'a> From<&'a Task> for Auditable<'a> {
109    fn from(value: &'a Task) -> Self {
110        Auditable::Task(value)
111    }
112}
113
114impl<'a> From<&'a DirEntry> for Auditable<'a> {
115    fn from(value: &'a DirEntry) -> Self {
116        Auditable::DirEntry(value)
117    }
118}
119
120impl<'a> From<&'a DirEntryHandle> for Auditable<'a> {
121    fn from(value: &'a DirEntryHandle) -> Self {
122        Auditable::DirEntry(&*value)
123    }
124}
125
126impl<'a> From<&'a FileObject> for Auditable<'a> {
127    fn from(value: &'a FileObject) -> Self {
128        Auditable::FileObject(value)
129    }
130}
131
132impl<'a> From<&'a FsNode> for Auditable<'a> {
133    fn from(value: &'a FsNode) -> Self {
134        Auditable::FsNode(value)
135    }
136}
137
138impl<'a> From<&'a FileSystem> for Auditable<'a> {
139    fn from(value: &'a FileSystem) -> Self {
140        Auditable::FileSystem(value)
141    }
142}
143
144impl<'a> From<&'a std::panic::Location<'a>> for Auditable<'a> {
145    fn from(value: &'a std::panic::Location<'a>) -> Self {
146        Auditable::Location(value)
147    }
148}
149
150impl<'a> From<&'a NamespaceNode> for Auditable<'a> {
151    fn from(value: &'a NamespaceNode) -> Self {
152        Auditable::NamespaceNode(value)
153    }
154}
155
156impl<'a, const N: usize> From<&'a [Auditable<'a>; N]> for Auditable<'a> {
157    fn from(value: &'a [Auditable<'a>; N]) -> Self {
158        Auditable::AuditContext(value)
159    }
160}
161
162/// Emits an audit log entry with the supplied details. See the SELinux Project's "AVC Audit Events"
163/// description (at https://selinuxproject.org/page/NB_AL) for details of the format and fields in
164/// audit logs.
165///
166/// The supplied `permission_check` is used to serialize the `source_sid` and `target_sid` into
167/// their string forms.
168///
169/// If the `result` has a `todo_bug` then the audit entry's decision will be "todo_deny", instead of
170/// the standard "granted" or "denied" decisions, to indicate that the check failed, but was granted
171/// nonetheless, via the todo-deny exceptions configuration.
172///
173/// Callers must supply an [`Auditable`] with context for the check (e.g. the calling task, target
174/// file object or filesystem node, etc.).
175pub(super) fn audit_decision(
176    current_task: &CurrentTask,
177    permission_check: &PermissionCheck<'_>,
178    result: PermissionCheckResult,
179    source_sid: SecurityId,
180    target_sid: SecurityId,
181    permission: KernelPermission,
182    audit_data: Auditable<'_>,
183) {
184    fuchsia_trace::instant!(
185        CATEGORY_STARNIX_SECURITY,
186        match (result.granted, result.todo_bug) {
187            (true, None) => c"audit.granted",
188            (true, Some(_)) => c"audit.todo_deny",
189            _ => c"audit.denied",
190        },
191        fuchsia_trace::Scope::Thread
192    );
193
194    let decision = if let Some(todo_bug) = result.todo_bug {
195        // If `todo_bug` is set then this check is being granted to accommodate errata, rather than
196        // the denial being enforced.
197
198        // Re-using the `track_stub!()` internals to track the denial, and determine whether
199        // too many denial audit logs have already been emit for this case.
200        if !should_audit(source_sid, target_sid, permission.class(), todo_bug) {
201            return;
202        }
203
204        // The first few of each `todo_bug` are logged as "todo_deny", and the denial tracked.
205        "todo_deny"
206    } else {
207        if result.granted { "granted" } else { "denied" }
208    };
209
210    // If there is an associated bug then add it to the audit context.
211    let audit_data_with_bug =
212        [Auditable::from_bug(result.todo_bug.map(NonZeroU32::get).unwrap_or(0)), audit_data];
213    let audit_data =
214        if result.todo_bug.is_some() { (&audit_data_with_bug).into() } else { audit_data };
215
216    let audit_logger = current_task.kernel().audit_logger();
217    audit_logger.audit_log(
218        AUDIT_AVC as u16,
219        || {
220            let tclass = permission.class().name();
221            let permission_name = permission.name();
222
223            // The source and target SIDs are by definition allocated to Security Contexts, so there is no
224            // need to handle `sid_to_security_context()` failure.
225            let security_server = permission_check.security_server();
226            let scontext = security_server.sid_to_security_context(source_sid).unwrap();
227            let scontext = BStr::new(&scontext);
228            let tcontext = security_server.sid_to_security_context(target_sid).unwrap();
229            let tcontext = BStr::new(&tcontext);
230
231            // Gather details about the calling task.
232            let pid = current_task.get_pid();
233            let command = current_task.command();
234
235            let is_permissive = result.permissive as u8;
236
237            format!("avc: {decision} {{ {permission_name} }} for pid={pid} comm=\"{command}\"{audit_data} scontext={scontext} tcontext={tcontext} tclass={tclass} permissive={is_permissive}")
238        }
239    );
240}
241
242impl Display for Auditable<'_> {
243    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), Error> {
244        match self {
245            Auditable::AuditContext(audit_context) => {
246                for item in *audit_context {
247                    item.fmt(f)?;
248                }
249                Ok(())
250            }
251            Auditable::Bug(bug_id) => {
252                write!(f, " bug={}", bug_id)
253            }
254            Auditable::CurrentTask => Ok(()),
255            Auditable::DirEntry(entry) => {
256                let scope = RcuReadScope::new();
257                write!(f, " name={}", hex_escape(entry.local_name(&scope)))
258            }
259            Auditable::FileObject(file) => {
260                write!(f, " path={}", hex_escape(&file.name.path_escaping_chroot()))
261            }
262            Auditable::FileSystem(fs) => {
263                write!(f, " dev={}", hex_escape(&fs.options.source))
264            }
265            Auditable::FsNode(node) => {
266                write!(f, " ino={}", node.ino)
267            }
268            Auditable::IoctlCommand(ioctl) => {
269                write!(f, " ioctlcmd={:#x}", ioctl)
270            }
271            Auditable::NlMsgtype(message_type) => {
272                write!(f, " nl-msgtype={}", message_type)
273            }
274            Auditable::Location(location) => {
275                write!(f, " caller={:?}", location)
276            }
277            Auditable::Name(name) => {
278                write!(f, " name={}", hex_escape(name))
279            }
280            Auditable::NamespaceNode(node) => {
281                let PathWithReachability::Reachable(path) = node.path_from_root(None) else {
282                    return Ok(());
283                };
284                write!(f, " path={}", hex_escape(&path))
285            }
286            Auditable::SockOptArguments(level, optname) => {
287                write!(f, " level={}, optname={}", level, optname)
288            }
289            Auditable::None => Ok(()),
290            Auditable::Task(task) => {
291                write!(f, " pid={}, comm={}", task.get_pid(), task.command())
292            }
293        }
294    }
295}
296
297struct EscapedString<'a> {
298    value: &'a [u8],
299}
300
301impl<'a> Display for EscapedString<'a> {
302    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), Error> {
303        // SELinux escapes strings containing spaces or control characters, to prevent userspace
304        // being able to construct names that confuse audit-log parsing tooling.
305        // Additionally enforcing that strings are valid UTF-8 encoded allows non-UTF-8 strings to
306        // be expressed losslessy (via hex escaping) rather than being formatted lossily by `bstr`.
307        let maybe_utf8 = str::from_utf8(self.value).ok();
308        if let Some(utf8) = maybe_utf8 {
309            if utf8.find(|c| c <= ' ').is_none() {
310                return write!(f, "\"{}\"", BStr::new(self.value));
311            }
312        }
313        hex::encode_upper(self.value).fmt(f)
314    }
315}
316
317fn hex_escape<'a>(value: &'a [u8]) -> EscapedString<'a> {
318    EscapedString { value }
319}
starnix_core/security/selinux_hooks/audit.rs

starnix_core/security/selinux_hooks/
audit.rs
