rust: task: clarify comments on task UID accessors

Linux has separate subjective and objective task credentials, see the
comment above `struct cred`. Clarify which accessor functions operate on
which set of credentials.

Also document that Task::euid() is a very weird operation. You can see how
weird it is by grepping for task_euid() in the history - binder was its
only user. Task::euid() obtains the objective effective UID - it looks
at the credentials of the task for purposes of acting on it as an
object, but then accesses the effective UID (which the credentials.7 man
page describes as "[...] used by the kernel to determine the permissions
that the process will have when accessing shared resources [...]").

For context:
Arguably, binder's use of task_euid() is a theoretical security problem,
which only has no impact on Android because Android has no setuid binaries
executable by apps.
commit 29bc22ac5e5b ("binder: use euid from cred instead of using task")
originally fixed that by removing that only user of task_euid(), but the
fix got reverted in commit c21a80ca0684 ("binder: fix test regression
due to sender_euid change") because some Android test started failing.
It was since fixed again by commit 65b672152289 ("binder: use
current_euid() for transaction sender identity"), which uses
current_euid() instead.

Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 6 insertions, 3 deletions

diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index 38273f4eedb5..eabd65bfde12 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -210,14 +210,17 @@ impl Task {
         unsafe { *ptr::addr_of!((*self.as_ptr()).pid) }
     }
 
-    /// Returns the UID of the given task.
+    /// Returns the objective real UID of the given task.
     #[inline]
     pub fn uid(&self) -> Kuid {
         // SAFETY: It's always safe to call `task_uid` on a valid task.
         Kuid::from_raw(unsafe { bindings::task_uid(self.as_ptr()) })
     }
 
-    /// Returns the effective UID of the given task.
+    /// Returns the objective effective UID of the given task.
+    ///
+    /// You should probably not be using this; the effective UID is normally
+    /// only relevant in subjective credentials.
     #[inline]
     pub fn euid(&self) -> Kuid {
         // SAFETY: It's always safe to call `task_euid` on a valid task.
@@ -371,7 +374,7 @@ impl PartialEq for Task {
 impl Eq for Task {}
 
 impl Kuid {
-    /// Get the current euid.
+    /// Get the current subjective effective UID.
     #[inline]
     pub fn current_euid() -> Kuid {
         // SAFETY: Just an FFI call.