[PATCH] Documentation/no_new_privs.rst: document dropping effective ids

[Max Kellermann <max.kellermann@ionos.com>]

Usually, execve() preserves the effective ids.  Many programs rely on
this to detect setuid/setgid execution and will disable certain
features (such as rejecting certain user input / environment
variables).

However, if NO_NEW_PRIVS is set, effective ids are always reset by
cap_bprm_creds_from_file(), but capabilities are not revoked.  That
means the process looks like it's not setuid/setgid, but has full
capabilities, and is effectively a superuser process.  This breaks
userspace assumptions.

It was argued [1] that this surprising behavior must not change
because programs might rely on it:

Of course, this leaves many programs vulnerable, but if we decide the
behavior must remain, we should at least document it with a warning.

[1] https://lore.kernel.org/lkml/87h61t7siv.fsf@email.froward.int.ebiederm.org/

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 Documentation/userspace-api/no_new_privs.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/userspace-api/no_new_privs.rst b/Documentation/userspace-api/no_new_privs.rst
index d060ea217ea1..89b0884991e9 100644
--- a/Documentation/userspace-api/no_new_privs.rst
+++ b/Documentation/userspace-api/no_new_privs.rst
@@ -29,6 +29,12 @@ bits will no longer change the uid or gid; file capabilities will not
 add to the permitted set, and LSMs will not relax constraints after
 execve.
 
+A successful execve call with ``no_new_privs`` will reset the
+effective uid/gid to the real uid/gid, but does not drop capabilities.
+This means that comparing effective and real ids is not a valid method
+to detect setuid/setgid execution; the proper way to do that is
+getauxval(AT_SECURE).
+
 To set ``no_new_privs``, use::
 
     prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
-- 
2.47.2