selinux/

permission_check.rs

// Copyright 2023 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use crate::access_vector_cache::{AccessVectorCache, Query};
use crate::policy::{AccessVector, KernelAccessDecision, SELINUX_AVD_FLAGS_PERMISSIVE, XpermsKind};
use crate::security_server::SecurityServer;
use crate::{
    ClassPermission, FsNodeClass, KernelClass, KernelPermission, NullessByteStr, SecurityId,
};

use std::num::NonZeroU32;

/// Describes the result of a permission lookup between two Security Contexts.
#[derive(Clone, Debug, PartialEq)]
pub struct PermissionCheckResult {
    /// True if the specified permissions are granted by policy.
    pub granted: bool,

    /// True if details of the check should be audit logged. Audit logs are by default only output
    /// when the policy defines that the permissions should be denied (whether or not the check is
    /// "permissive"), but may be suppressed for some denials ("dontaudit"), or for some allowed
    /// permissions ("auditallow").
    pub audit: bool,

    /// True if the access should be granted because either the security server is running in
    /// permissive mode, or the subject domain is marked as permissive.
    pub permissive: bool,

    /// If the `AccessDecision` indicates that permission denials should not be enforced then `permit`
    /// will be true, and this field will hold the Id of the bug to reference in audit logging.
    pub todo_bug: Option<NonZeroU32>,
}

impl PermissionCheckResult {
    /// Returns true if the request was granted, or was made in permissive mode.
    pub fn permit(&self) -> bool {
        self.granted || self.permissive
    }
}

/// Implements the `has_permission()` API, based on supplied `SecurityServer` and
/// `AccessVectorCache` implementations.
// TODO: https://fxbug.dev/362699811 - Revise the traits to avoid direct dependencies on `SecurityServer`.
pub struct PermissionCheck<'a> {
    security_server: &'a SecurityServer,
    access_vector_cache: &'a AccessVectorCache,
}

impl<'a> PermissionCheck<'a> {
    pub(crate) fn new(
        security_server: &'a SecurityServer,
        access_vector_cache: &'a AccessVectorCache,
    ) -> Self {
        Self { security_server, access_vector_cache }
    }

    /// Returns whether the `source_sid` has the specified `permission` on `target_sid`.
    /// The result indicates both whether `permission` is `permit`ted, and whether the caller
    /// should `audit` log the query.
    pub fn has_permission<P: ClassPermission + Into<KernelPermission> + Clone + 'static>(
        &self,
        source_sid: SecurityId,
        target_sid: SecurityId,
        permission: P,
    ) -> PermissionCheckResult {
        has_permission(
            self.security_server.is_enforcing(),
            self.access_vector_cache,
            source_sid,
            target_sid,
            permission.into(),
        )
    }

    /// Returns whether the `source_sid` has both a base permission (i.e. `ioctl` or `nlmsg`) and
    /// the specified extended permission on `target_sid`, and whether the decision should be
    /// audited.
    ///
    /// A request is allowed if the base permission is `allow`ed and either the numeric extended
    /// permission of this `xperms_kind` is included in an `allowxperm` statement, or extended
    /// permissions of this kind are not filtered for this domain.
    ///
    /// A granted request is audited if the base permission is `auditallow` and the extended
    /// permission is `auditallowxperm`.
    ///
    /// A denied request is audited if the base permission is `dontaudit` or the extended
    /// permission is `dontauditxperm`.
    pub fn has_extended_permission<
        P: ClassPermission + Into<KernelPermission> + Clone + 'static,
    >(
        &self,
        xperms_kind: XpermsKind,
        source_sid: SecurityId,
        target_sid: SecurityId,
        permission: P,
        xperm: u16,
    ) -> PermissionCheckResult {
        has_extended_permission(
            self.security_server.is_enforcing(),
            self.access_vector_cache,
            xperms_kind,
            source_sid,
            target_sid,
            permission.into(),
            xperm,
        )
    }

    // TODO: https://fxbug.dev/362699811 - Remove this once `SecurityServer` APIs such as `sid_to_security_context()`
    // are exposed via a trait rather than directly by that implementation.
    pub fn security_server(&self) -> &SecurityServer {
        self.security_server
    }

    /// Returns the SID with which to label a new `file_class` instance created by `subject_sid`, with `target_sid`
    /// as its parent, taking into account role & type transition rules, and filename-transition rules.
    /// If a filename-transition rule matches the `fs_node_name` then that will be used, otherwise the
    /// filename-independent computation will be applied.
    pub fn compute_new_fs_node_sid(
        &self,
        source_sid: SecurityId,
        target_sid: SecurityId,
        fs_node_class: FsNodeClass,
        fs_node_name: NullessByteStr<'_>,
    ) -> Result<SecurityId, anyhow::Error> {
        // TODO: https://fxbug.dev/385075470 - Stop skipping empty name lookups once by-name lookup is better optimized.
        if !fs_node_name.as_bytes().is_empty() {
            if let Some(sid) = self.access_vector_cache.compute_new_fs_node_sid_with_name(
                source_sid,
                target_sid,
                fs_node_class,
                fs_node_name,
            ) {
                return Ok(sid);
            }
        }
        self.access_vector_cache.compute_create_sid(source_sid, target_sid, fs_node_class.into())
    }

    /// Returns the SID with which to label a new `target_class` instance created by `subject_sid`, with `target_sid`
    /// as its parent, taking into account role & type transition rules.
    pub fn compute_create_sid(
        &self,
        source_sid: SecurityId,
        target_sid: SecurityId,
        target_class: KernelClass,
    ) -> Result<SecurityId, anyhow::Error> {
        self.access_vector_cache.compute_create_sid(source_sid, target_sid, target_class)
    }

    /// Returns the raw `AccessDecision` for a specified source, target and class.
    pub fn compute_access_decision(
        &self,
        source_sid: SecurityId,
        target_sid: SecurityId,
        target_class: KernelClass,
    ) -> KernelAccessDecision {
        self.access_vector_cache.compute_access_decision(source_sid, target_sid, target_class)
    }
}

/// Internal implementation of the `has_permission()` API, in terms of the `Query` trait.
fn has_permission(
    is_enforcing: bool,
    query: &impl Query,
    source_sid: SecurityId,
    target_sid: SecurityId,
    permission: KernelPermission,
) -> PermissionCheckResult {
    let target_class = permission.class();
    let permission_access_vector = permission.as_access_vector();

    let decision = query.compute_access_decision(source_sid, target_sid, target_class.into());
    let permissive = decision.flags & SELINUX_AVD_FLAGS_PERMISSIVE != 0;
    let granted = permission_access_vector & decision.allow == permission_access_vector;
    let audit = permission_access_vector & decision.audit != AccessVector::NONE;
    let mut result = PermissionCheckResult { granted, audit, permissive, todo_bug: None };

    if !result.granted {
        if !is_enforcing {
            // If the security server is not currently enforcing then permit all access.
            result.permissive = true;
        } else if decision.todo_bug.is_some() {
            // If the access decision includes a `todo_bug` then permit the access and return the
            // bug id to the caller, for audit logging.
            //
            // This is checked before the "permissive" settings because exceptions work-around
            // issues in our implementation, or policy builds, so permissive treatment can lead
            // to logspam as well as losing bug-tracking information.
            result.granted = true;
            result.todo_bug = decision.todo_bug;
        }
    }

    result
}

/// Internal implementation of the `has_extended_permission()` API, in terms of the `Query` trait.
fn has_extended_permission(
    is_enforcing: bool,
    query: &impl Query,
    xperms_kind: XpermsKind,
    source_sid: SecurityId,
    target_sid: SecurityId,
    permission: KernelPermission,
    xperm: u16,
) -> PermissionCheckResult {
    let [xperms_postfix, xperms_prefix] = xperm.to_le_bytes();
    let xperms_decision = query.compute_xperms_access_decision(
        xperms_kind,
        source_sid,
        target_sid,
        permission,
        xperms_prefix,
    );

    let granted = xperms_decision.allow.contains(xperms_postfix);
    let audit = xperms_decision.audit.contains(xperms_postfix);
    let permissive = xperms_decision.permissive;
    let mut result = PermissionCheckResult { granted, audit, permissive, todo_bug: None };

    if !result.granted {
        if !is_enforcing {
            result.permissive = true;
        } else if xperms_decision.has_todo {
            // A todo_bug applies to this entry. Look up the base decision for details.
            // This will re-compute the base decision if it is not cached.
            let base_decision =
                query.compute_access_decision(source_sid, target_sid, permission.class());
            result.todo_bug = base_decision.todo_bug;
        }
    }

    result
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::access_vector_cache::KernelXpermsAccessDecision;
    use crate::policy::{
        AccessDecision, AccessVector, AccessVectorComputer, KernelAccessDecision, XpermsBitmap,
    };
    use crate::{CommonFsNodePermission, FileClass, ForClass, KernelClass, ProcessPermission};

    use std::num::NonZeroU32;
    use std::sync::LazyLock;
    use std::sync::atomic::{AtomicU32, Ordering};

    /// SID to use where any value will do.
    static A_TEST_SID: LazyLock<SecurityId> = LazyLock::new(unique_sid);

    /// Returns a new `SecurityId` with unique id.
    fn unique_sid() -> SecurityId {
        static NEXT_ID: AtomicU32 = AtomicU32::new(1000);
        SecurityId(NonZeroU32::new(NEXT_ID.fetch_add(1, Ordering::AcqRel)).unwrap())
    }

    // Assume permissions are mapped one to one.
    fn access_decision_to_kernel_access_decision(
        _class: KernelClass,
        decision: AccessDecision,
    ) -> KernelAccessDecision {
        KernelAccessDecision {
            allow: decision.allow,
            audit: (decision.allow & decision.auditallow) | (!decision.allow & decision.auditdeny),
            todo_bug: decision.todo_bug,
            flags: decision.flags,
        }
    }

    #[derive(Default)]
    pub struct DenyAllPermissions;

    impl Query for DenyAllPermissions {
        fn compute_access_decision(
            &self,
            _source_sid: SecurityId,
            _target_sid: SecurityId,
            _target_class: KernelClass,
        ) -> KernelAccessDecision {
            KernelAccessDecision {
                allow: AccessVector::NONE,
                audit: AccessVector::ALL,
                flags: 0,
                todo_bug: None,
            }
        }

        fn compute_create_sid(
            &self,
            _source_sid: SecurityId,
            _target_sid: SecurityId,
            _target_class: KernelClass,
        ) -> Result<SecurityId, anyhow::Error> {
            unreachable!();
        }

        fn compute_new_fs_node_sid_with_name(
            &self,
            _source_sid: SecurityId,
            _target_sid: SecurityId,
            _fs_node_class: FsNodeClass,
            _fs_node_name: NullessByteStr<'_>,
        ) -> Option<SecurityId> {
            unreachable!();
        }

        fn compute_xperms_access_decision(
            &self,
            _xperms_kind: XpermsKind,
            _source_sid: SecurityId,
            _target_sid: SecurityId,
            _permission: KernelPermission,
            _xperms_prefix: u8,
        ) -> KernelXpermsAccessDecision {
            KernelXpermsAccessDecision {
                allow: XpermsBitmap::NONE,
                audit: XpermsBitmap::ALL,
                permissive: false,
                has_todo: false,
            }
        }
    }

    impl AccessVectorComputer for DenyAllPermissions {
        fn access_decision_to_kernel_access_decision(
            &self,
            class: KernelClass,
            av: AccessDecision,
        ) -> KernelAccessDecision {
            access_decision_to_kernel_access_decision(class, av)
        }
    }

    /// A [`Query`] that permits all [`AccessVector`].
    #[derive(Default)]
    struct AllowAllPermissions;

    impl Query for AllowAllPermissions {
        fn compute_access_decision(
            &self,
            _source_sid: SecurityId,
            _target_sid: SecurityId,
            _target_class: KernelClass,
        ) -> KernelAccessDecision {
            KernelAccessDecision {
                allow: AccessVector::ALL,
                audit: AccessVector::NONE,
                flags: 0,
                todo_bug: None,
            }
        }

        fn compute_create_sid(
            &self,
            _source_sid: SecurityId,
            _target_sid: SecurityId,
            _target_class: KernelClass,
        ) -> Result<SecurityId, anyhow::Error> {
            unreachable!();
        }

        fn compute_new_fs_node_sid_with_name(
            &self,
            _source_sid: SecurityId,
            _target_sid: SecurityId,
            _fs_node_class: FsNodeClass,
            _fs_node_name: NullessByteStr<'_>,
        ) -> Option<SecurityId> {
            unreachable!();
        }

        fn compute_xperms_access_decision(
            &self,
            _xperms_kind: XpermsKind,
            _source_sid: SecurityId,
            _target_sid: SecurityId,
            _permission: KernelPermission,
            _xperms_prefix: u8,
        ) -> KernelXpermsAccessDecision {
            KernelXpermsAccessDecision {
                allow: XpermsBitmap::ALL,
                audit: XpermsBitmap::NONE,
                permissive: false,
                has_todo: false,
            }
        }
    }

    impl AccessVectorComputer for AllowAllPermissions {
        fn access_decision_to_kernel_access_decision(
            &self,
            class: KernelClass,
            av: AccessDecision,
        ) -> KernelAccessDecision {
            access_decision_to_kernel_access_decision(class, av)
        }
    }

    #[test]
    fn has_permission_both() {
        let deny_all = DenyAllPermissions::default();
        let allow_all = AllowAllPermissions::default();

        // Use permissions that are mapped to access vector bits in
        // `access_vector_from_permission`.
        let permissions = [ProcessPermission::Fork, ProcessPermission::Transition];
        for permission in permissions {
            // DenyAllPermissions denies.
            let result = has_permission(
                /*is_enforcing=*/ true,
                &deny_all,
                *A_TEST_SID,
                *A_TEST_SID,
                permission.into(),
            );
            assert_eq!(
                result,
                PermissionCheckResult {
                    granted: false,
                    audit: true,
                    permissive: false,
                    todo_bug: None
                }
            );
            assert!(!result.permit());

            // AllowAllPermissions allows.
            let result = has_permission(
                /*is_enforcing=*/ true,
                &allow_all,
                *A_TEST_SID,
                *A_TEST_SID,
                permission.into(),
            );
            assert_eq!(
                result,
                PermissionCheckResult {
                    granted: true,
                    audit: false,
                    permissive: false,
                    todo_bug: None
                }
            );
            assert!(result.permit());
        }
    }

    #[test]
    fn has_ioctl_permission_enforcing() {
        let deny_all = DenyAllPermissions::default();
        let allow_all = AllowAllPermissions::default();
        let permission = CommonFsNodePermission::Ioctl.for_class(FileClass::File);

        // DenyAllPermissions denies.
        let result = has_extended_permission(
            /*is_enforcing=*/ true,
            &deny_all,
            XpermsKind::Ioctl,
            *A_TEST_SID,
            *A_TEST_SID,
            permission.into(),
            0xabcd,
        );
        assert_eq!(
            result,
            PermissionCheckResult {
                granted: false,
                audit: true,
                permissive: false,
                todo_bug: None
            }
        );
        assert!(!result.permit());

        // AllowAllPermissions allows.
        let result = has_extended_permission(
            /*is_enforcing=*/ true,
            &allow_all,
            XpermsKind::Ioctl,
            *A_TEST_SID,
            *A_TEST_SID,
            permission.into(),
            0xabcd,
        );
        assert_eq!(
            result,
            PermissionCheckResult {
                granted: true,
                audit: false,
                permissive: false,
                todo_bug: None
            }
        );
        assert!(result.permit());
    }

    #[test]
    fn has_ioctl_permission_not_enforcing() {
        let deny_all = DenyAllPermissions::default();
        let permission = CommonFsNodePermission::Ioctl.for_class(FileClass::File);

        // DenyAllPermissions denies, but the permission is allowed when the security server
        // is not in enforcing mode. The decision should still be audited.
        let result = has_extended_permission(
            /*is_enforcing=*/ false,
            &deny_all,
            XpermsKind::Ioctl,
            *A_TEST_SID,
            *A_TEST_SID,
            permission,
            0xabcd,
        );
        assert_eq!(
            result,
            PermissionCheckResult { granted: false, audit: true, permissive: true, todo_bug: None }
        );
        assert!(result.permit());
    }
}