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 fuchsia_sync::Mutex;
13use hex;
14use linux_uapi::AUDIT_AVC;
15use selinux::permission_check::{PermissionCheck, PermissionCheckResult};
16use selinux::{ClassPermission, KernelClass, KernelPermission, SecurityId};
17use starnix_logging::{BugRef, CATEGORY_STARNIX_SECURITY, trace_instant};
18use std::collections::HashMap;
19use std::fmt::{Display, Error};
20use std::num::NonZeroU64;
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: NonZeroU64,
30}
31
32/// Stores count of todo_deny logged per auditable instance.
33static TODO_DENY_COUNTS: LazyLock<Mutex<HashMap<AuditableInstance, u64>>> =
34    LazyLock::new(|| Mutex::new(HashMap::new()));
35
36/// Checks whether an audit log entry should still be emitted for this audit instance.
37fn should_audit(
38    source_sid: SecurityId,
39    target_sid: SecurityId,
40    class: KernelClass,
41    bug: NonZeroU64,
42) -> bool {
43    // Audit-log the first few denials, but skip further denials to avoid logspamming.
44    const MAX_TODO_AUDIT_DENIALS: u64 = 5;
45
46    let mut counts = TODO_DENY_COUNTS.lock();
47    let count = counts.entry(AuditableInstance { source_sid, target_sid, class, bug }).or_default();
48    *count += 1;
49    *count <= MAX_TODO_AUDIT_DENIALS
50}
51
52/// Container for a reference to kernel state from which to include details when emitting audit
53/// logging.  [`Auditable`] instances are created from references to objects via `into()`, e.g:
54///
55///   fn my_lovely_hook(current_task: &CurrentTask, ...) {
56///     let audit_context = current_task.into();
57///     check_permission(..., audit_context)
58///   }
59///
60/// Call-sites which need to include context from multiple sources into audit logs can do so by
61/// creating an array of [`Auditable`] instances from those sources, and using `into()` to create
62/// an [`Auditable`] from a reference to that array, e.g:
63///
64///   fn my_lovelier_hook(current_task: &CurrentTask,..., audit_context: Auditable<'_>) {
65///     let audit_context = [audit_context, current_task.into()];
66///     check_permission(..., (&audit_context).into())
67///   }
68///
69/// [`Auditable`] instances are parameterized with the lifetime of the references they contain,
70/// which will be automagically derived by Rust. Since they only consist of a type discriminator and
71/// reference they are cheap to copy, avoiding the need to pass them by-reference if the same
72/// context is to be applied to multiple permission checks.
73#[derive(Debug, Clone, Copy)]
74pub enum Auditable<'a> {
75    // keep-sorted start
76    AuditContext(&'a [Auditable<'a>]),
77    Bug(u64),
78    CurrentTask,
79    DirEntry(&'a DirEntry),
80    FileObject(&'a FileObject),
81    FileSystem(&'a FileSystem),
82    FsNode(&'a FsNode),
83    IoctlCommand(u16),
84    Location(&'a std::panic::Location<'a>),
85    Name(&'a FsStr),
86    NamespaceNode(&'a NamespaceNode),
87    NlMsgtype(u16),
88    None,
89    SockOptArguments(u32, u32),
90    Task(&'a Task),
91    TodoCheck,
92    // keep-sorted end
93}
94
95impl Auditable<'_> {
96    fn from_bug(bug_id: u64) -> 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 [`super::todo_check_permission`] or 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    trace_instant!(
185        CATEGORY_STARNIX_SECURITY,
186        match (result.permit, 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.permit { "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(NonZeroU64::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            format!("avc: {decision} {{ {permission_name} }} for pid={pid} comm=\"{command}\"{audit_data} scontext={scontext} tcontext={tcontext} tclass={tclass}")
235        }
236    );
237}
238
239/// Emits an audit log entry for a check that failed, but will still be granted because it was made
240/// with the [`super::todo_check_permission()`] API.
241pub(super) fn audit_todo_decision(
242    current_task: &CurrentTask,
243    bug: BugRef,
244    permission_check: &PermissionCheck<'_>,
245    mut result: PermissionCheckResult,
246    source_sid: SecurityId,
247    target_sid: SecurityId,
248    permission: KernelPermission,
249    audit_context: Auditable<'_>,
250) {
251    if result.todo_bug.is_none() {
252        result.todo_bug = Some(bug.into());
253        audit_decision(
254            current_task,
255            permission_check,
256            result,
257            source_sid,
258            target_sid,
259            permission,
260            (&[Auditable::TodoCheck, audit_context]).into(),
261        )
262    } else {
263        audit_decision(
264            current_task,
265            permission_check,
266            result,
267            source_sid,
268            target_sid,
269            permission,
270            audit_context,
271        )
272    }
273}
274
275impl Display for Auditable<'_> {
276    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), Error> {
277        match self {
278            Auditable::AuditContext(audit_context) => {
279                for item in *audit_context {
280                    item.fmt(f)?;
281                }
282                Ok(())
283            }
284            Auditable::Bug(bug_id) => {
285                write!(f, " bug={}", bug_id)
286            }
287            Auditable::CurrentTask => Ok(()),
288            Auditable::DirEntry(entry) => {
289                let scope = RcuReadScope::new();
290                write!(f, " name={}", hex_escape(entry.local_name(&scope)))
291            }
292            Auditable::FileObject(file) => {
293                write!(f, " path={}", hex_escape(&file.name.path_escaping_chroot()))
294            }
295            Auditable::FileSystem(fs) => {
296                write!(f, " dev={}", hex_escape(&fs.options.source))
297            }
298            Auditable::FsNode(node) => {
299                write!(f, " ino={}", node.ino)
300            }
301            Auditable::IoctlCommand(ioctl) => {
302                write!(f, " ioctlcmd={:#x}", ioctl)
303            }
304            Auditable::NlMsgtype(message_type) => {
305                write!(f, " nl-msgtype={}", message_type)
306            }
307            Auditable::Location(location) => {
308                write!(f, " caller={:?}", location)
309            }
310            Auditable::Name(name) => {
311                write!(f, " name={}", hex_escape(name))
312            }
313            Auditable::NamespaceNode(node) => {
314                let PathWithReachability::Reachable(path) = node.path_from_root(None) else {
315                    return Ok(());
316                };
317                write!(f, " path={}", hex_escape(&path))
318            }
319            Auditable::SockOptArguments(level, optname) => {
320                write!(f, " level={}, optname={}", level, optname)
321            }
322            Auditable::None => Ok(()),
323            Auditable::Task(task) => {
324                write!(f, " pid={}, comm={}", task.get_pid(), task.command())
325            }
326            Auditable::TodoCheck => {
327                write!(f, " todo_check")
328            }
329        }
330    }
331}
332
333struct EscapedString<'a> {
334    value: &'a [u8],
335}
336
337impl<'a> Display for EscapedString<'a> {
338    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), Error> {
339        // SELinux escapes strings containing spaces or control characters, to prevent userspace
340        // being able to construct names that confuse audit-log parsing tooling.
341        // Additionally enforcing that strings are valid UTF-8 encoded allows non-UTF-8 strings to
342        // be expressed losslessy (via hex escaping) rather than being formatted lossily by `bstr`.
343        let maybe_utf8 = str::from_utf8(self.value).ok();
344        if let Some(utf8) = maybe_utf8 {
345            if utf8.find(|c| c <= ' ').is_none() {
346                return write!(f, "\"{}\"", BStr::new(self.value));
347            }
348        }
349        hex::encode_upper(self.value).fmt(f)
350    }
351}
352
353fn hex_escape<'a>(value: &'a [u8]) -> EscapedString<'a> {
354    EscapedString { value }
355}
starnix_core/security/selinux_hooks/audit.rs

starnix_core/security/selinux_hooks/
audit.rs
