commoncap: Refactor to remove bprm_secureexec hook

The commoncap implementation of the bprm_secureexec hook is the only LSM
that depends on the final call to its bprm_set_creds hook (since it may
be called for multiple files, it ignores bprm->called_set_creds). As a
result, it cannot safely _clear_ bprm->secureexec since other LSMs may
have set it.  Instead, remove the bprm_secureexec hook by introducing a
new flag to bprm specific to commoncap: cap_elevated. This is similar to
cap_effective, but that is used for a specific subset of elevated
privileges, and exists solely to track state from bprm_set_creds to
bprm_secureexec. As such, it will be removed in the next patch.

Here, set the new bprm->cap_elevated flag when setuid/setgid has happened
from bprm_fill_uid() or fscapabilities have been prepared. This temporarily
moves the bprm_secureexec hook to a static inline. The helper will be
removed in the next patch; this makes the step easier to review and bisect,
since this does not introduce any changes to inputs nor outputs to the
"elevated privileges" calculation.

The new flag is merged with the bprm->secureexec flag in setup_new_exec()
since this marks the end of any further prepare_binprm() calls.

Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Acked-by: James Morris <james.l.morris@oracle.com>
Acked-by: Serge Hallyn <serge@hallyn.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 16838ba7ee75..213c61fa3780 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -35,6 +35,13 @@ struct linux_binprm {
 				 * false if not; except for init which inherits
 				 * its parent's caps anyway */
 		/*
+		 * True if most recent call to the commoncaps bprm_set_creds
+		 * hook (due to multiple prepare_binprm() calls from the
+		 * binfmt_script/misc handlers) resulted in elevated
+		 * privileges.
+		 */
+		cap_elevated:1,
+		/*
 		 * Set by bprm_set_creds hook to indicate a privilege-gaining
 		 * exec has happened. Used to sanitize execution environment
 		 * and to set AT_SECURE auxv for glibc.