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Jann Horn
ef2d3e4635
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. commit29bc22ac5e("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 commitc21a80ca06("binder: fix test regression due to sender_euid change") because some Android test started failing. It was since fixed again by commit65b6721522("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>