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rust: task: clarify comments on task UID accessors

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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 29bc22ac5e ("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 c21a80ca06 ("binder: fix test regression
due to sender_euid change") because some Android test started failing.
It was since fixed again by commit 65b6721522 ("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>
This commit is contained in:
Jann Horn
2026-07-07 15:10:48 -04:00
committed by Paul Moore
parent 149b192e37
commit ef2d3e4635
1 changed files with 6 additions and 3 deletions
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rust/kernel/task.rs
+6 -3
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@@ -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.