rust: file: add `Kuid` wrapper

Adds a wrapper around `kuid_t` called `Kuid`. This allows us to define
various operations on kuids such as equality and current_euid. It also
lets us provide conversions from kuid into userspace values.

Rust Binder needs these operations because it needs to compare kuids for
equality, and it needs to tell userspace about the pid and uid of
incoming transactions.

To read kuids from a `struct task_struct`, you must currently use
various #defines that perform the appropriate field access under an RCU
read lock. Currently, we do not have a Rust wrapper for rcu_read_lock,
which means that for this patch, there are two ways forward:

 1. Inline the methods into Rust code, and use __rcu_read_lock directly
    rather than the rcu_read_lock wrapper. This gives up lockdep for
    these usages of RCU.

 2. Wrap the various #defines in helpers and call the helpers from Rust.

This patch uses the second option. One possible disadvantage of the
second option is the possible introduction of speculation gadgets, but
as discussed in [1], the risk appears to be acceptable.

Of course, once a wrapper for rcu_read_lock is available, it is
preferable to use that over either of the two above approaches.

Link: https://lore.kernel.org/all/202312080947.674CD2DC7@keescook/
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240915-alice-file-v10-7-88484f7a3dcf@google.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 51ec78c355c05eeab69803ab13e41ac2f723e3e3..e854ccddeceeb5d98ba7b9bfd10cdbde67983067 100644 (file)
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -19,6 +19,7 @@
 #include <linux/jiffies.h>
 #include <linux/mdio.h>
 #include <linux/phy.h>
+#include <linux/pid_namespace.h>
 #include <linux/refcount.h>
 #include <linux/sched.h>
 #include <linux/security.h>

diff --git a/rust/helpers/task.c b/rust/helpers/task.c
index 7ac789232d11cd1e200bf686b6d4e43335c7958a..7d66487db8312fca46c94684108d5ef921f4b30b 100644 (file)
--- a/rust/helpers/task.c
+++ b/rust/helpers/task.c
@@ -17,3 +17,41 @@ void rust_helper_put_task_struct(struct task_struct *t)
 {
        put_task_struct(t);
 }
+
+kuid_t rust_helper_task_uid(struct task_struct *task)
+{
+       return task_uid(task);
+}
+
+kuid_t rust_helper_task_euid(struct task_struct *task)
+{
+       return task_euid(task);
+}
+
+#ifndef CONFIG_USER_NS
+uid_t rust_helper_from_kuid(struct user_namespace *to, kuid_t uid)
+{
+       return from_kuid(to, uid);
+}
+#endif /* CONFIG_USER_NS */
+
+bool rust_helper_uid_eq(kuid_t left, kuid_t right)
+{
+       return uid_eq(left, right);
+}
+
+kuid_t rust_helper_current_euid(void)
+{
+       return current_euid();
+}
+
+struct user_namespace *rust_helper_current_user_ns(void)
+{
+       return current_user_ns();
+}
+
+pid_t rust_helper_task_tgid_nr_ns(struct task_struct *tsk,
+                                 struct pid_namespace *ns)
+{
+       return task_tgid_nr_ns(tsk, ns);
+}

diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
index 92659649e932265ed7c0b6b73fcc701c77ac09e5..81d67789b16f243e7832ff3b2e5e479a1ab2bf9e 100644 (file)
--- a/rust/kernel/cred.rs
+++ b/rust/kernel/cred.rs
@@ -10,6 +10,7 @@
 
 use crate::{
     bindings,
+    task::Kuid,
     types::{AlwaysRefCounted, Opaque},
 };
 
@@ -61,11 +62,11 @@ impl Credential {
     }
 
     /// Returns the effective UID of the given credential.
-    pub fn euid(&self) -> bindings::kuid_t {
+    pub fn euid(&self) -> Kuid {
         // SAFETY: By the type invariant, we know that `self.0` is valid. Furthermore, the `euid`
         // field of a credential is never changed after initialization, so there is no potential
         // for data races.
-        unsafe { (*self.0.get()).euid }
+        Kuid::from_raw(unsafe { (*self.0.get()).euid })
     }
 }
 

diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index 367b4bbddd9fab146540cd16109aaaa26a50a2fe..1a36a9f193685393e7211793b6e6dd7576af8bfd 100644 (file)
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -9,6 +9,7 @@ use crate::{
     types::{NotThreadSafe, Opaque},
 };
 use core::{
+    cmp::{Eq, PartialEq},
     ffi::{c_int, c_long, c_uint},
     ops::Deref,
     ptr,
@@ -96,6 +97,12 @@ unsafe impl Sync for Task {}
 /// The type of process identifiers (PIDs).
 type Pid = bindings::pid_t;
 
+/// The type of user identifiers (UIDs).
+#[derive(Copy, Clone)]
+pub struct Kuid {
+    kuid: bindings::kuid_t,
+}
+
 impl Task {
     /// Returns a raw pointer to the current task.
     ///
@@ -157,12 +164,31 @@ impl Task {
         unsafe { *ptr::addr_of!((*self.0.get()).pid) }
     }
 
+    /// Returns the UID of the given task.
+    pub fn uid(&self) -> Kuid {
+        // SAFETY: By the type invariant, we know that `self.0` is valid.
+        Kuid::from_raw(unsafe { bindings::task_uid(self.0.get()) })
+    }
+
+    /// Returns the effective UID of the given task.
+    pub fn euid(&self) -> Kuid {
+        // SAFETY: By the type invariant, we know that `self.0` is valid.
+        Kuid::from_raw(unsafe { bindings::task_euid(self.0.get()) })
+    }
+
     /// Determines whether the given task has pending signals.
     pub fn signal_pending(&self) -> bool {
         // SAFETY: By the type invariant, we know that `self.0` is valid.
         unsafe { bindings::signal_pending(self.0.get()) != 0 }
     }
 
+    /// Returns the given task's pid in the current pid namespace.
+    pub fn pid_in_current_ns(&self) -> Pid {
+        // SAFETY: We know that `self.0.get()` is valid by the type invariant, and passing a null
+        // pointer as the namespace is correct for using the current namespace.
+        unsafe { bindings::task_tgid_nr_ns(self.0.get(), ptr::null_mut()) }
+    }
+
     /// Wakes up the task.
     pub fn wake_up(&self) {
         // SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid.
@@ -184,3 +210,43 @@ unsafe impl crate::types::AlwaysRefCounted for Task {
         unsafe { bindings::put_task_struct(obj.cast().as_ptr()) }
     }
 }
+
+impl Kuid {
+    /// Get the current euid.
+    #[inline]
+    pub fn current_euid() -> Kuid {
+        // SAFETY: Just an FFI call.
+        Self::from_raw(unsafe { bindings::current_euid() })
+    }
+
+    /// Create a `Kuid` given the raw C type.
+    #[inline]
+    pub fn from_raw(kuid: bindings::kuid_t) -> Self {
+        Self { kuid }
+    }
+
+    /// Turn this kuid into the raw C type.
+    #[inline]
+    pub fn into_raw(self) -> bindings::kuid_t {
+        self.kuid
+    }
+
+    /// Converts this kernel UID into a userspace UID.
+    ///
+    /// Uses the namespace of the current task.
+    #[inline]
+    pub fn into_uid_in_current_ns(self) -> bindings::uid_t {
+        // SAFETY: Just an FFI call.
+        unsafe { bindings::from_kuid(bindings::current_user_ns(), self.kuid) }
+    }
+}
+
+impl PartialEq for Kuid {
+    #[inline]
+    fn eq(&self, other: &Kuid) -> bool {
+        // SAFETY: Just an FFI call.
+        unsafe { bindings::uid_eq(self.kuid, other.kuid) }
+    }
+}
+
+impl Eq for Kuid {}